NFS Version 3 Protocol Specification
RFC 1813
| Document | Type | RFC - Informational (June 1995) | |
|---|---|---|---|
| Authors | Peter Staubach, Brian Pawlowski , Brent Callaghan | ||
| Last updated | 2013-03-02 | ||
| Stream | Legacy stream | ||
| Formats | plain text html pdf htmlized bibtex | ||
| Stream | Legacy state | (None) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | RFC 1813 (Informational) | |
| Telechat date | (None) | ||
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| Send notices to | (None) |
RFC 1813
Network Working Group B. Callaghan
Request for Comments: 1813 B. Pawlowski
Category: Informational P. Staubach
Sun Microsystems, Inc.
June 1995
NFS Version 3 Protocol Specification
Status of this Memo
This memo provides information for the Internet community.
This memo does not specify an Internet standard of any kind.
Distribution of this memo is unlimited.
IESG Note
Internet Engineering Steering Group comment: please note that
the IETF is not involved in creating or maintaining this
specification. This is the significance of the specification
not being on the standards track.
Abstract
This paper describes the NFS version 3 protocol. This paper is
provided so that people can write compatible implementations.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Scope of the NFS version 3 protocol . . . . . . . . . . 4
1.2 Useful terms . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Remote Procedure Call . . . . . . . . . . . . . . . . . 5
1.4 External Data Representation . . . . . . . . . . . . . . 5
1.5 Authentication and Permission Checking . . . . . . . . . 7
1.6 Philosophy . . . . . . . . . . . . . . . . . . . . . . . 8
1.7 Changes from the NFS version 2 protocol . . . . . . . . 11
2. RPC Information . . . . . . . . . . . . . . . . . . . . . 14
2.1 Authentication . . . . . . . . . . . . . . . . . . . . . 14
2.2 Constants . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Transport address . . . . . . . . . . . . . . . . . . . 14
2.4 Sizes . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5 Basic Data Types . . . . . . . . . . . . . . . . . . . . 15
2.6 Defined Error Numbers . . . . . . . . . . . . . . . . . 17
3. Server Procedures . . . . . . . . . . . . . . . . . . . . 27
3.1 General comments on attributes . . . . . . . . . . . . . 29
3.2 General comments on filenames . . . . . . . . . . . . . 30
3.3.0 NULL: Do nothing . . . . . . . . . . . . . . . . . . . . 31
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3.3.1 GETATTR: Get file attributes . . . . . . . . . . . . . . 32
3.3.2 SETATTR: Set file attributes . . . . . . . . . . . . . . 33
3.3.3 LOOKUP: Lookup filename . . . . . . . . . . . . . . . . 37
3.3.4 ACCESS: Check access permission . . . . . . . . . . . . 40
3.3.5 READLINK: Read from symbolic link . . . . . . . . . . . 44
3.3.6 READ: Read from file . . . . . . . . . . . . . . . . . . 46
3.3.7 WRITE: Write to file . . . . . . . . . . . . . . . . . . 49
3.3.8 CREATE: Create a file . . . . . . . . . . . . . . . . . 54
3.3.9 MKDIR: Create a directory . . . . . . . . . . . . . . . 58
3.3.10 SYMLINK: Create a symbolic link . . . . . . . . . . . . 61
3.3.11 MKNOD: Create a special device . . . . . . . . . . . . . 63
3.3.12 REMOVE: Remove a file . . . . . . . . . . . . . . . . . 67
3.3.13 RMDIR: Remove a directory . . . . . . . . . . . . . . . 69
3.3.14 RENAME: Rename a file or directory . . . . . . . . . . . 71
3.3.15 LINK: Create link to an object . . . . . . . . . . . . . 74
3.3.16 READDIR: Read From directory . . . . . . . . . . . . . . 76
3.3.17 READDIRPLUS: Extended read from directory . . . . . . . 80
3.3.18 FSSTAT: Get dynamic file system information . . . . . . 84
3.3.19 FSINFO: Get static file system information . . . . . . . 86
3.3.20 PATHCONF: Retrieve POSIX information . . . . . . . . . . 90
3.3.21 COMMIT: Commit cached data on a server to stable storage 92
4. Implementation issues . . . . . . . . . . . . . . . . . . 96
4.1 Multiple version support . . . . . . . . . . . . . . . . 96
4.2 Server/client relationship . . . . . . . . . . . . . . . 96
4.3 Path name interpretation . . . . . . . . . . . . . . . . 97
4.4 Permission issues . . . . . . . . . . . . . . . . . . . 98
4.5 Duplicate request cache . . . . . . . . . . . . . . . . 99
4.6 File name component handling . . . . . . . . . . . . . . 101
4.7 Synchronous modifying operations . . . . . . . . . . . . 101
4.8 Stable storage . . . . . . . . . . . . . . . . . . . . . 101
4.9 Lookups and name resolution . . . . . . . . . . . . . . 102
4.10 Adaptive retransmission . . . . . . . . . . . . . . . . 102
4.11 Caching policies . . . . . . . . . . . . . . . . . . . . 102
4.12 Stable versus unstable writes. . . . . . . . . . . . . . 103
4.13 32 bit clients/servers and 64 bit clients/servers. . . . 104
5. Appendix I: Mount protocol . . . . . . . . . . . . . . . . 106
5.1 RPC Information . . . . . . . . . . . . . . . . . . . . 106
5.1.1 Authentication . . . . . . . . . . . . . . . . . . . . 106
5.1.2 Constants . . . . . . . . . . . . . . . . . . . . . . 106
5.1.3 Transport address . . . . . . . . . . . . . . . . . . 106
5.1.4 Sizes . . . . . . . . . . . . . . . . . . . . . . . . 106
5.1.5 Basic Data Types . . . . . . . . . . . . . . . . . . . 106
5.2 Server Procedures . . . . . . . . . . . . . . . . . . . 107
5.2.0 NULL: Do nothing . . . . . . . . . . . . . . . . . . . 108
5.2.1 MNT: Add mount entry . . . . . . . . . . . . . . . . . 109
5.2.2 DUMP: Return mount entries . . . . . . . . . . . . . . 110
5.2.3 UMNT: Remove mount entry . . . . . . . . . . . . . . . 111
5.2.4 UMNTALL: Remove all mount entries . . . . . . . . . . 112
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5.2.5 EXPORT: Return export list . . . . . . . . . . . . . . 113
6. Appendix II: Lock manager protocol . . . . . . . . . . . . 114
6.1 RPC Information . . . . . . . . . . . . . . . . . . . . 114
6.1.1 Authentication . . . . . . . . . . . . . . . . . . . . 114
6.1.2 Constants . . . . . . . . . . . . . . . . . . . . . . 114
6.1.3 Transport Address . . . . . . . . . . . . . . . . . . 115
6.1.4 Basic Data Types . . . . . . . . . . . . . . . . . . . 115
6.2 NLM Procedures . . . . . . . . . . . . . . . . . . . . . 118
6.2.0 NULL: Do nothing . . . . . . . . . . . . . . . . . . . 120
6.3 Implementation issues . . . . . . . . . . . . . . . . . 120
6.3.1 64-bit offsets and lengths . . . . . . . . . . . . . . 120
6.3.2 File handles . . . . . . . . . . . . . . . . . . . . . 120
7. Appendix III: Bibliography . . . . . . . . . . . . . . . . 122
8. Security Considerations . . . . . . . . . . . . . . . . . 125
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 125
10. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 126
1. Introduction
Sun's NFS protocol provides transparent remote access to shared
file systems across networks. The NFS protocol is designed to be
machine, operating system, network architecture, and transport
protocol independent. This independence is achieved through the
use of Remote Procedure Call (RPC) primitives built on top of an
eXternal Data Representation (XDR). Implementations of the NFS
version 2 protocol exist for a variety of machines, from personal
computers to supercomputers. The initial version of the NFS
protocol is specified in the Network File System Protocol
Specification [RFC1094]. A description of the initial
implementation can be found in [Sandberg].
The supporting MOUNT protocol performs the operating
system-specific functions that allow clients to attach remote
directory trees to a point within the local file system. The
mount process also allows the server to grant remote access
privileges to a restricted set of clients via export control.
The Lock Manager provides support for file locking when used in
the NFS environment. The Network Lock Manager (NLM) protocol
isolates the inherently stateful aspects of file locking into a
separate protocol.
A complete description of the above protocols and their
implementation is to be found in [X/OpenNFS].
The purpose of this document is to:
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o Specify the NFS version 3 protocol.
o Describe semantics of the protocol through annotation
and description of intended implementation.
o Specify the MOUNT version 3 protocol.
o Briefly describe the changes between the NLM version 3
protocol and the NLM version 4 protocol.
The normative text is the description of the RPC procedures and
arguments and results, which defines the over-the-wire protocol,
and the semantics of those procedures. The material describing
implementation practice aids the understanding of the protocol
specification and describes some possible implementation issues
and solutions. It is not possible to describe all implementations
and the UNIX operating system implementation of the NFS version 3
protocol is most often used to provide examples. Given that, the
implementation discussion does not bear the authority of the
description of the over-the-wire protocol itself.
1.1 Scope of the NFS version 3 protocol
This revision of the NFS protocol addresses new requirements.
The need to support larger files and file systems has prompted
extensions to allow 64 bit file sizes and offsets. The revision
enhances security by adding support for an access check to be
done on the server. Performance modifications are of three
types:
1. The number of over-the-wire packets for a given
set of file operations is reduced by returning file
attributes on every operation, thus decreasing the number
of calls to get modified attributes.
2. The write throughput bottleneck caused by the synchronous
definition of write in the NFS version 2 protocol has been
addressed by adding support so that the NFS server can do
unsafe writes. Unsafe writes are writes which have not
been committed to stable storage before the operation
returns. This specification defines a method for
committing these unsafe writes to stable storage in a
reliable way.
3. Limitations on transfer sizes have been relaxed.
The ability to support multiple versions of a protocol in RPC
will allow implementors of the NFS version 3 protocol to define
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clients and servers that provide backwards compatibility with
the existing installed base of NFS version 2 protocol
implementations.
The extensions described here represent an evolution of the
existing NFS protocol and most of the design features of the
NFS protocol described in [Sandberg] persist. See Changes
from the NFS version 2 protocol on page 11 for a more
detailed summary of the changes introduced by this revision.
1.2 Useful terms
In this specification, a "server" is a machine that provides
resources to the network; a "client" is a machine that accesses
resources over the network; a "user" is a person logged in on a
client; an "application" is a program that executes on a client.
1.3 Remote Procedure Call
The Sun Remote Procedure Call specification provides a
procedure-oriented interface to remote services. Each server
supplies a program, which is a set of procedures. The NFS
service is one such program. The combination of host address,
program number, version number, and procedure number specify one
remote service procedure. Servers can support multiple versions
of a program by using different protocol version numbers.
The NFS protocol was designed to not require any specific level
of reliability from its lower levels so it could potentially be
used on many underlying transport protocols. The NFS service is
based on RPC which provides the abstraction above lower level
network and transport protocols.
The rest of this document assumes the NFS environment is
implemented on top of Sun RPC, which is specified in [RFC1057].
A complete discussion is found in [Corbin].
1.4 External Data Representation
The eXternal Data Representation (XDR) specification provides a
standard way of representing a set of data types on a network.
This solves the problem of different byte orders, structure
alignment, and data type representation on different,
communicating machines.
In this document, the RPC Data Description Language is used to
specify the XDR format parameters and results to each of the RPC
service procedures that an NFS server provides. The RPC Data
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Description Language is similar to declarations in the C
programming language. A few new constructs have been added.
The notation:
string name[SIZE];
string data<DSIZE>;
defines name, which is a fixed size block of SIZE bytes, and
data, which is a variable sized block of up to DSIZE bytes. This
notation indicates fixed-length arrays and arrays with a
variable number of elements up to a fixed maximum. A
variable-length definition with no size specified means there is
no maximum size for the field.
The discriminated union definition:
union example switch (enum status) {
case OK:
struct {
filename file1;
filename file2;
integer count;
}
case ERROR:
struct {
errstat error;
integer errno;
}
default:
void;
}
defines a structure where the first thing over the network is an
enumeration type called status. If the value of status is OK,
the next thing on the network will be the structure containing
file1, file2, and count. Else, if the value of status is ERROR,
the next thing on the network will be a structure containing
error and errno. If the value of status is neither OK nor
ERROR, then there is no more data in the structure.
The XDR type, hyper, is an 8 byte (64 bit) quantity. It is used
in the same way as the integer type. For example:
hyper foo;
unsigned hyper bar;
foo is an 8 byte signed value, while bar is an 8 byte unsigned
value.
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Although RPC/XDR compilers exist to generate client and server
stubs from RPC Data Description Language input, NFS
implementations do not require their use. Any software that
provides equivalent encoding and decoding to the canonical
network order of data defined by XDR can be used to interoperate
with other NFS implementations.
XDR is described in [RFC1014].
1.5 Authentication and Permission Checking
The RPC protocol includes a slot for authentication parameters
on every call. The contents of the authentication parameters are
determined by the type of authentication used by the server and
client. A server may support several different flavors of
authentication at once. The AUTH_NONE flavor provides null
authentication, that is, no authentication information is
passed. The AUTH_UNIX flavor provides UNIX-style user ID, group
ID, and groups with each call. The AUTH_DES flavor provides
DES-encrypted authentication parameters based on a network-wide
name, with session keys exchanged via a public key scheme. The
AUTH_KERB flavor provides DES encrypted authentication
parameters based on a network-wide name with session keys
exchanged via Kerberos secret keys.
The NFS server checks permissions by taking the credentials from
the RPC authentication information in each remote request. For
example, using the AUTH_UNIX flavor of authentication, the
server gets the user's effective user ID, effective group ID and
groups on each call, and uses them to check access. Using user
ids and group ids implies that the client and server either
share the same ID list or do local user and group ID mapping.
Servers and clients must agree on the mapping from user to uid
and from group to gid, for those sites that do not implement a
consistent user ID and group ID space. In practice, such mapping
is typically performed on the server, following a static mapping
scheme or a mapping established by the user from a client at
mount time.
The AUTH_DES and AUTH_KERB style of authentication is based on a
network-wide name. It provides greater security through the use
of DES encryption and public keys in the case of AUTH_DES, and
DES encryption and Kerberos secret keys (and tickets) in the
AUTH_KERB case. Again, the server and client must agree on the
identity of a particular name on the network, but the name to
identity mapping is more operating system independent than the
uid and gid mapping in AUTH_UNIX. Also, because the
authentication parameters are encrypted, a malicious user must
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know another users network password or private key to masquerade
as that user. Similarly, the server returns a verifier that is
also encrypted so that masquerading as a server requires knowing
a network password.
The NULL procedure typically requires no authentication.
1.6 Philosophy
This specification defines the NFS version 3 protocol, that is
the over-the-wire protocol by which a client accesses a server.
The protocol provides a well-defined interface to a server's
file resources. A client or server implements the protocol and
provides a mapping of the local file system semantics and
actions into those defined in the NFS version 3 protocol.
Implementations may differ to varying degrees, depending on the
extent to which a given environment can support all the
operations and semantics defined in the NFS version 3 protocol.
Although implementations exist and are used to illustrate
various aspects of the NFS version 3 protocol, the protocol
specification itself is the final description of how clients
access server resources.
Because the NFS version 3 protocol is designed to be
operating-system independent, it does not necessarily match the
semantics of any existing system. Server implementations are
expected to make a best effort at supporting the protocol. If a
server cannot support a particular protocol procedure, it may
return the error, NFS3ERR_NOTSUP, that indicates that the
operation is not supported. For example, many operating systems
do not support the notion of a hard link. A server that cannot
support hard links should return NFS3ERR_NOTSUP in response to a
LINK request. FSINFO describes the most commonly unsupported
procedures in the properties bit map. Alternatively, a server
may not natively support a given operation, but can emulate it
in the NFS version 3 protocol implementation to provide greater
functionality.
In some cases, a server can support most of the semantics
described by the protocol but not all. For example, the ctime
field in the fattr structure gives the time that a file's
attributes were last modified. Many systems do not keep this
information. In this case, rather than not support the GETATTR
operation, a server could simulate it by returning the last
modified time in place of ctime. Servers must be careful when
simulating attribute information because of possible side
effects on clients. For example, many clients use file
modification times as a basis for their cache consistency
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scheme.
NFS servers are dumb and NFS clients are smart. It is the
clients that do the work required to convert the generalized
file access that servers provide into a file access method that
is useful to applications and users. In the LINK example given
above, a UNIX client that received an NFS3ERR_NOTSUP error from
a server would do the recovery necessary to either make it look
to the application like the link request had succeeded or return
a reasonable error. In general, it is the burden of the client
to recover.
The NFS version 3 protocol assumes a stateless server
implementation. Statelessness means that the server does not
need to maintain state about any of its clients in order to
function correctly. Stateless servers have a distinct advantage
over stateful servers in the event of a crash. With stateless
servers, a client need only retry a request until the server
responds; the client does not even need to know that the server
has crashed. See additional comments in Duplicate request cache
on page 99.
For a server to be useful, it holds nonvolatile state: data
stored in the file system. Design assumptions in the NFS version
3 protocol regarding flushing of modified data to stable storage
reduce the number of failure modes in which data loss can occur.
In this way, NFS version 3 protocol implementations can tolerate
transient failures, including transient failures of the network.
In general, server implementations of the NFS version 3 protocol
cannot tolerate a non-transient failure of the stable storage
itself. However, there exist fault tolerant implementations
which attempt to address such problems.
That is not to say that an NFS version 3 protocol server can't
maintain noncritical state. In many cases, servers will maintain
state (cache) about previous operations to increase performance.
For example, a client READ request might trigger a read-ahead of
the next block of the file into the server's data cache in the
anticipation that the client is doing a sequential read and the
next client READ request will be satisfied from the server's
data cache instead of from the disk. Read-ahead on the server
increases performance by overlapping server disk I/O with client
requests. The important point here is that the read-ahead block
is not necessary for correct server behavior. If the server
crashes and loses its memory cache of read buffers, recovery is
simple on reboot - clients will continue read operations
retrieving data from the server disk.
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Most data-modifying operations in the NFS protocol are
synchronous. That is, when a data modifying procedure returns
to the client, the client can assume that the operation has
completed and any modified data associated with the request is
now on stable storage. For example, a synchronous client WRITE
request may cause the server to update data blocks, file system
information blocks, and file attribute information - the latter
information is usually referred to as metadata. When the WRITE
operation completes, the client can assume that the write data
is safe and discard it. This is a very important part of the
stateless nature of the server. If the server did not flush
dirty data to stable storage before returning to the client, the
client would have no way of knowing when it was safe to discard
modified data. The following data modifying procedures are
synchronous: WRITE (with stable flag set to FILE_SYNC), CREATE,
MKDIR, SYMLINK, MKNOD, REMOVE, RMDIR, RENAME, LINK, and COMMIT.
The NFS version 3 protocol introduces safe asynchronous writes
on the server, when the WRITE procedure is used in conjunction
with the COMMIT procedure. The COMMIT procedure provides a way
for the client to flush data from previous asynchronous WRITE
requests on the server to stable storage and to detect whether
it is necessary to retransmit the data. See the procedure
descriptions of WRITE on page 49 and COMMIT on page 92.
The LOOKUP procedure is used by the client to traverse
multicomponent file names (pathnames). Each call to LOOKUP is
used to resolve one segment of a pathname. There are two reasons
for restricting LOOKUP to a single segment: it is hard to
standardize a common format for hierarchical file names and the
client and server may have different mappings of pathnames to
file systems. This would imply that either the client must break
the path name at file system attachment points, or the server
must know about the client's file system attachment points. In
NFS version 3 protocol implementations, it is the client that
constructs the hierarchical file name space using mounts to
build a hierarchy. Support utilities, such as the Automounter,
provide a way to manage a shared, consistent image of the file
name space while still being driven by the client mount
process.
Clients can perform caching in varied manner. The general
practice with the NFS version 2 protocol was to implement a
time-based client-server cache consistency mechanism. It is
expected NFS version 3 protocol implementations will use a
similar mechanism. The NFS version 3 protocol has some explicit
support, in the form of additional attribute information to
eliminate explicit attribute checks. However, caching is not
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required, nor is any caching policy defined by the protocol.
Neither the NFS version 2 protocol nor the NFS version 3
protocol provide a means of maintaining strict client-server
consistency (and, by implication, consistency across client
caches).
1.7 Changes from the NFS Version 2 Protocol
The ROOT and WRITECACHE procedures have been removed. A MKNOD
procedure has been defined to allow the creation of special
files, eliminating the overloading of CREATE. Caching on the
client is not defined nor dictated by the NFS version 3
protocol, but additional information and hints have been added
to the protocol to allow clients that implement caching to
manage their caches more effectively. Procedures that affect the
attributes of a file or directory may now return the new
attributes after the operation has completed to optimize out a
subsequent GETATTR used in validating attribute caches. In
addition, operations that modify the directory in which the
target object resides return the old and new attributes of the
directory to allow clients to implement more intelligent cache
invalidation procedures. The ACCESS procedure provides access
permission checking on the server, the FSSTAT procedure returns
dynamic information about a file system, the FSINFO procedure
returns static information about a file system and server, the
READDIRPLUS procedure returns file handles and attributes in
addition to directory entries, and the PATHCONF procedure
returns POSIX pathconf information about a file.
Below is a list of the important changes between the NFS version
2 protocol and the NFS version 3 protocol.
File handle size
The file handle has been increased to a variable-length
array of 64 bytes maximum from a fixed array of 32
bytes. This addresses some known requirements for a
slightly larger file handle size. The file handle was
converted from fixed length to variable length to
reduce local storage and network bandwidth requirements
for systems which do not utilize the full 64 bytes of
length.
Maximum data sizes
The maximum size of a data transfer used in the READ
and WRITE procedures is now set by values in the FSINFO
return structure. In addition, preferred transfer sizes
are returned by FSINFO. The protocol does not place any
artificial limits on the maximum transfer sizes.
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Filenames and pathnames are now specified as strings of
variable length. The actual length restrictions are
determined by the client and server implementations as
appropriate. The protocol does not place any
artificial limits on the length. The error,
NFS3ERR_NAMETOOLONG, is provided to allow the server to
return an indication to the client that it received a
pathname that was too long for it to handle.
Error return
Error returns in some instances now return data (for
example, attributes). nfsstat3 now defines the full set
of errors that can be returned by a server. No other
values are allowed.
File type
The file type now includes NF3CHR and NF3BLK for
special files. Attributes for these types include
subfields for UNIX major and minor devices numbers.
NF3SOCK and NF3FIFO are now defined for sockets and
fifos in the file system.
File attributes
The blocksize (the size in bytes of a block in the
file) field has been removed. The mode field no longer
contains file type information. The size and fileid
fields have been widened to eight-byte unsigned
integers from four-byte integers. Major and minor
device information is now presented in a distinct
structure. The blocks field name has been changed to
used and now contains the total number of bytes used by
the file. It is also an eight-byte unsigned integer.
Set file attributes
In the NFS version 2 protocol, the settable attributes
were represented by a subset of the file attributes
structure; the client indicated those attributes which
were not to be modified by setting the corresponding
field to -1, overloading some unsigned fields. The set
file attributes structure now uses a discriminated
union for each field to tell whether or how to set that
field. The atime and mtime fields can be set to either
the server's current time or a time supplied by the
client.
LOOKUP
The LOOKUP return structure now includes the attributes
for the directory searched.
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ACCESS
An ACCESS procedure has been added to allow an explicit
over-the-wire permissions check. This addresses known
problems with the superuser ID mapping feature in many
server implementations (where, due to mapping of root
user, unexpected permission denied errors could occur
while reading from or writing to a file). This also
removes the assumption which was made in the NFS
version 2 protocol that access to files was based
solely on UNIX style mode bits.
READ
The reply structure includes a Boolean that is TRUE if
the end-of-file was encountered during the READ. This
allows the client to correctly detect end-of-file.
WRITE
The beginoffset and totalcount fields were removed from
the WRITE arguments. The reply now includes a count so
that the server can write less than the requested
amount of data, if required. An indicator was added to
the arguments to instruct the server as to the level of
cache synchronization that is required by the client.
CREATE
An exclusive flag and a create verifier was added for
the exclusive creation of regular files.
MKNOD
This procedure was added to support the creation of
special files. This avoids overloading fields of CREATE
as was done in some NFS version 2 protocol
implementations.
READDIR
The READDIR arguments now include a verifier to allow
the server to validate the cookie. The cookie is now a
64 bit unsigned integer instead of the 4 byte array
which was used in the NFS version 2 protocol. This
will help to reduce interoperability problems.
READDIRPLUS
This procedure was added to return file handles and
attributes in an extended directory list.
FSINFO
FSINFO was added to provide nonvolatile information
about a file system. The reply includes preferred and
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maximum read transfer size, preferred and maximum write
transfer size, and flags stating whether links or
symbolic links are supported. Also returned are
preferred transfer size for READDIR procedure replies,
server time granularity, and whether times can be set
in a SETATTR request.
FSSTAT
FSSTAT was added to provide volatile information about
a file system, for use by utilities such as the Unix
system df command. The reply includes the total size
and free space in the file system specified in bytes,
the total number of files and number of free file slots
in the file system, and an estimate of time between
file system modifications (for use in cache consistency
checking algorithms).
COMMIT
The COMMIT procedure provides the synchronization
mechanism to be used with asynchronous WRITE
operations.
2. RPC Information
2.1 Authentication
The NFS service uses AUTH_NONE in the NULL procedure. AUTH_UNIX,
AUTH_DES, or AUTH_KERB are used for all other procedures. Other
authentication types may be supported in the future.
2.2 Constants
These are the RPC constants needed to call the NFS Version 3
service. They are given in decimal.
PROGRAM 100003
VERSION 3
2.3 Transport address
The NFS protocol is normally supported over the TCP and UDP
protocols. It uses port 2049, the same as the NFS version 2
protocol.
2.4 Sizes
These are the sizes, given in decimal bytes, of various XDR
structures used in the NFS version 3 protocol:
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NFS3_FHSIZE 64
The maximum size in bytes of the opaque file handle.
NFS3_COOKIEVERFSIZE 8
The size in bytes of the opaque cookie verifier passed by
READDIR and READDIRPLUS.
NFS3_CREATEVERFSIZE 8
The size in bytes of the opaque verifier used for
exclusive CREATE.
NFS3_WRITEVERFSIZE 8
The size in bytes of the opaque verifier used for
asynchronous WRITE.
2.5 Basic Data Types
The following XDR definitions are basic definitions that are
used in other structures.
uint64
typedef unsigned hyper uint64;
int64
typedef hyper int64;
uint32
typedef unsigned long uint32;
int32
typedef long int32;
filename3
typedef string filename3<>;
nfspath3
typedef string nfspath3<>;
fileid3
typedef uint64 fileid3;
cookie3
typedef uint64 cookie3;
cookieverf3
typedef opaque cookieverf3[NFS3_COOKIEVERFSIZE];
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createverf3
typedef opaque createverf3[NFS3_CREATEVERFSIZE];
writeverf3
typedef opaque writeverf3[NFS3_WRITEVERFSIZE];
uid3
typedef uint32 uid3;
gid3
typedef uint32 gid3;
size3
typedef uint64 size3;
offset3
typedef uint64 offset3;
mode3
typedef uint32 mode3;
count3
typedef uint32 count3;
nfsstat3
enum nfsstat3 {
NFS3_OK = 0,
NFS3ERR_PERM = 1,
NFS3ERR_NOENT = 2,
NFS3ERR_IO = 5,
NFS3ERR_NXIO = 6,
NFS3ERR_ACCES = 13,
NFS3ERR_EXIST = 17,
NFS3ERR_XDEV = 18,
NFS3ERR_NODEV = 19,
NFS3ERR_NOTDIR = 20,
NFS3ERR_ISDIR = 21,
NFS3ERR_INVAL = 22,
NFS3ERR_FBIG = 27,
NFS3ERR_NOSPC = 28,
NFS3ERR_ROFS = 30,
NFS3ERR_MLINK = 31,
NFS3ERR_NAMETOOLONG = 63,
NFS3ERR_NOTEMPTY = 66,
NFS3ERR_DQUOT = 69,
NFS3ERR_STALE = 70,
NFS3ERR_REMOTE = 71,
NFS3ERR_BADHANDLE = 10001,
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NFS3ERR_NOT_SYNC = 10002,
NFS3ERR_BAD_COOKIE = 10003,
NFS3ERR_NOTSUPP = 10004,
NFS3ERR_TOOSMALL = 10005,
NFS3ERR_SERVERFAULT = 10006,
NFS3ERR_BADTYPE = 10007,
NFS3ERR_JUKEBOX = 10008
};
The nfsstat3 type is returned with every procedure's results
except for the NULL procedure. A value of NFS3_OK indicates that
the call completed successfully. Any other value indicates that
some error occurred on the call, as identified by the error
code. Note that the precise numeric encoding must be followed.
No other values may be returned by a server. Servers are
expected to make a best effort mapping of error conditions to
the set of error codes defined. In addition, no error
precedences are specified by this specification. Error
precedences determine the error value that should be returned
when more than one error applies in a given situation. The error
precedence will be determined by the individual server
implementation. If the client requires specific error
precedences, it should check for the specific errors for
itself.
2.6 Defined Error Numbers
A description of each defined error follows:
NFS3_OK
Indicates the call completed successfully.
NFS3ERR_PERM
Not owner. The operation was not allowed because the
caller is either not a privileged user (root) or not the
owner of the target of the operation.
NFS3ERR_NOENT
No such file or directory. The file or directory name
specified does not exist.
NFS3ERR_IO
I/O error. A hard error (for example, a disk error)
occurred while processing the requested operation.
NFS3ERR_NXIO
I/O error. No such device or address.
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NFS3ERR_ACCES
Permission denied. The caller does not have the correct
permission to perform the requested operation. Contrast
this with NFS3ERR_PERM, which restricts itself to owner
or privileged user permission failures.
NFS3ERR_EXIST
File exists. The file specified already exists.
NFS3ERR_XDEV
Attempt to do a cross-device hard link.
NFS3ERR_NODEV
No such device.
NFS3ERR_NOTDIR
Not a directory. The caller specified a non-directory in
a directory operation.
NFS3ERR_ISDIR
Is a directory. The caller specified a directory in a
non-directory operation.
NFS3ERR_INVAL
Invalid argument or unsupported argument for an
operation. Two examples are attempting a READLINK on an
object other than a symbolic link or attempting to
SETATTR a time field on a server that does not support
this operation.
NFS3ERR_FBIG
File too large. The operation would have caused a file to
grow beyond the server's limit.
NFS3ERR_NOSPC
No space left on device. The operation would have caused
the server's file system to exceed its limit.
NFS3ERR_ROFS
Read-only file system. A modifying operation was
attempted on a read-only file system.
NFS3ERR_MLINK
Too many hard links.
NFS3ERR_NAMETOOLONG
The filename in an operation was too long.
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NFS3ERR_NOTEMPTY
An attempt was made to remove a directory that was not
empty.
NFS3ERR_DQUOT
Resource (quota) hard limit exceeded. The user's resource
limit on the server has been exceeded.
NFS3ERR_STALE
Invalid file handle. The file handle given in the
arguments was invalid. The file referred to by that file
handle no longer exists or access to it has been
revoked.
NFS3ERR_REMOTE
Too many levels of remote in path. The file handle given
in the arguments referred to a file on a non-local file
system on the server.
NFS3ERR_BADHANDLE
Illegal NFS file handle. The file handle failed internal
consistency checks.
NFS3ERR_NOT_SYNC
Update synchronization mismatch was detected during a
SETATTR operation.
NFS3ERR_BAD_COOKIE
READDIR or READDIRPLUS cookie is stale.
NFS3ERR_NOTSUPP
Operation is not supported.
NFS3ERR_TOOSMALL
Buffer or request is too small.
NFS3ERR_SERVERFAULT
An error occurred on the server which does not map to any
of the legal NFS version 3 protocol error values. The
client should translate this into an appropriate error.
UNIX clients may choose to translate this to EIO.
NFS3ERR_BADTYPE
An attempt was made to create an object of a type not
supported by the server.
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NFS3ERR_JUKEBOX
The server initiated the request, but was not able to
complete it in a timely fashion. The client should wait
and then try the request with a new RPC transaction ID.
For example, this error should be returned from a server
that supports hierarchical storage and receives a request
to process a file that has been migrated. In this case,
the server should start the immigration process and
respond to client with this error.
ftype3
enum ftype3 {
NF3REG = 1,
NF3DIR = 2,
NF3BLK = 3,
NF3CHR = 4,
NF3LNK = 5,
NF3SOCK = 6,
NF3FIFO = 7
};
The enumeration, ftype3, gives the type of a file. The type,
NF3REG, is a regular file, NF3DIR is a directory, NF3BLK is a
block special device file, NF3CHR is a character special device
file, NF3LNK is a symbolic link, NF3SOCK is a socket, and
NF3FIFO is a named pipe. Note that the precise enum encoding
must be followed.
specdata3
struct specdata3 {
uint32 specdata1;
uint32 specdata2;
};
The interpretation of the two words depends on the type of file
system object. For a block special (NF3BLK) or character special
(NF3CHR) file, specdata1 and specdata2 are the major and minor
device numbers, respectively. (This is obviously a
UNIX-specific interpretation.) For all other file types, these
two elements should either be set to 0 or the values should be
agreed upon by the client and server. If the client and server
do not agree upon the values, the client should treat these
fields as if they are set to 0. This data field is returned as
part of the fattr3 structure and so is available from all
replies returning attributes. Since these fields are otherwise
unused for objects which are not devices, out of band
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information can be passed from the server to the client.
However, once again, both the server and the client must agree
on the values passed.
nfs_fh3
struct nfs_fh3 {
opaque data<NFS3_FHSIZE>;
};
The nfs_fh3 is the variable-length opaque object returned by the
server on LOOKUP, CREATE, SYMLINK, MKNOD, LINK, or READDIRPLUS
operations, which is used by the client on subsequent operations
to reference the file. The file handle contains all the
information the server needs to distinguish an individual file.
To the client, the file handle is opaque. The client stores file
handles for use in a later request and can compare two file
handles from the same server for equality by doing a
byte-by-byte comparison, but cannot otherwise interpret the
contents of file handles. If two file handles from the same
server are equal, they must refer to the same file, but if they
are not equal, no conclusions can be drawn. Servers should try
to maintain a one-to-one correspondence between file handles and
files, but this is not required. Clients should use file handle
comparisons only to improve performance, not for correct
behavior.
Servers can revoke the access provided by a file handle at any
time. If the file handle passed in a call refers to a file
system object that no longer exists on the server or access for
that file handle has been revoked, the error, NFS3ERR_STALE,
should be returned.
nfstime3
struct nfstime3 {
uint32 seconds;
uint32 nseconds;
};
The nfstime3 structure gives the number of seconds and
nanoseconds since midnight January 1, 1970 Greenwich Mean Time.
It is used to pass time and date information. The times
associated with files are all server times except in the case of
a SETATTR operation where the client can explicitly set the file
time. A server converts to and from local time when processing
time values, preserving as much accuracy as possible. If the
precision of timestamps stored for a file is less than that
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defined by NFS version 3 protocol, loss of precision can occur.
An adjunct time maintenance protocol is recommended to reduce
client and server time skew.
fattr3
struct fattr3 {
ftype3 type;
mode3 mode;
uint32 nlink;
uid3 uid;
gid3 gid;
size3 size;
size3 used;
specdata3 rdev;
uint64 fsid;
fileid3 fileid;
nfstime3 atime;
nfstime3 mtime;
nfstime3 ctime;
};
This structure defines the attributes of a file system object.
It is returned by most operations on an object; in the case of
operations that affect two objects (for example, a MKDIR that
modifies the target directory attributes and defines new
attributes for the newly created directory), the attributes for
both may be returned. In some cases, the attributes are returned
in the structure, wcc_data, which is defined below; in other
cases the attributes are returned alone. The main changes from
the NFS version 2 protocol are that many of the fields have been
widened and the major/minor device information is now presented
in a distinct structure rather than being packed into a word.
The fattr3 structure contains the basic attributes of a file.
All servers should support this set of attributes even if they
have to simulate some of the fields. Type is the type of the
file. Mode is the protection mode bits. Nlink is the number of
hard links to the file - that is, the number of different names
for the same file. Uid is the user ID of the owner of the file.
Gid is the group ID of the group of the file. Size is the size
of the file in bytes. Used is the number of bytes of disk space
that the file actually uses (which can be smaller than the size
because the file may have holes or it may be larger due to
fragmentation). Rdev describes the device file if the file type
is NF3CHR or NF3BLK - see specdata3 on page 20. Fsid is the file
system identifier for the file system. Fileid is a number which
uniquely identifies the file within its file system (on UNIX
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this would be the inumber). Atime is the time when the file data
was last accessed. Mtime is the time when the file data was last
modified. Ctime is the time when the attributes of the file
were last changed. Writing to the file changes the ctime in
addition to the mtime.
The mode bits are defined as follows:
0x00800 Set user ID on execution.
0x00400 Set group ID on execution.
0x00200 Save swapped text (not defined in POSIX).
0x00100 Read permission for owner.
0x00080 Write permission for owner.
0x00040 Execute permission for owner on a file. Or lookup
(search) permission for owner in directory.
0x00020 Read permission for group.
0x00010 Write permission for group.
0x00008 Execute permission for group on a file. Or lookup
(search) permission for group in directory.
0x00004 Read permission for others.
0x00002 Write permission for others.
0x00001 Execute permission for others on a file. Or lookup
(search) permission for others in directory.
post_op_attr
union post_op_attr switch (bool attributes_follow) {
case TRUE:
fattr3 attributes;
case FALSE:
void;
};
This structure is used for returning attributes in those
operations that are not directly involved with manipulating
attributes. One of the principles of this revision of the NFS
protocol is to return the real value from the indicated
operation and not an error from an incidental operation. The
post_op_attr structure was designed to allow the server to
recover from errors encountered while getting attributes.
This appears to make returning attributes optional. However,
server implementors are strongly encouraged to make best effort
to return attributes whenever possible, even when returning an
error.
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wcc_attr
struct wcc_attr {
size3 size;
nfstime3 mtime;
nfstime3 ctime;
};
This is the subset of pre-operation attributes needed to better
support the weak cache consistency semantics. Size is the file
size in bytes of the object before the operation. Mtime is the
time of last modification of the object before the operation.
Ctime is the time of last change to the attributes of the object
before the operation. See discussion in wcc_attr on page 24.
The use of mtime by clients to detect changes to file system
objects residing on a server is dependent on the granularity of
the time base on the server.
pre_op_attr
union pre_op_attr switch (bool attributes_follow) {
case TRUE:
wcc_attr attributes;
case FALSE:
void;
};
wcc_data
struct wcc_data {
pre_op_attr before;
post_op_attr after;
};
When a client performs an operation that modifies the state of a
file or directory on the server, it cannot immediately determine
from the post-operation attributes whether the operation just
performed was the only operation on the object since the last
time the client received the attributes for the object. This is
important, since if an intervening operation has changed the
object, the client will need to invalidate any cached data for
the object (except for the data that it just wrote).
To deal with this, the notion of weak cache consistency data or
wcc_data is introduced. A wcc_data structure consists of certain
key fields from the object attributes before the operation,
together with the object attributes after the operation. This
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information allows the client to manage its cache more
accurately than in NFS version 2 protocol implementations. The
term, weak cache consistency, emphasizes the fact that this
mechanism does not provide the strict server-client consistency
that a cache consistency protocol would provide.
In order to support the weak cache consistency model, the server
will need to be able to get the pre-operation attributes of the
object, perform the intended modify operation, and then get the
post-operation attributes atomically. If there is a window for
the object to get modified between the operation and either of
the get attributes operations, then the client will not be able
to determine whether it was the only entity to modify the
object. Some information will have been lost, thus weakening the
weak cache consistency guarantees.
post_op_fh3
union post_op_fh3 switch (bool handle_follows) {
case TRUE:
nfs_fh3 handle;
case FALSE:
void;
};
One of the principles of this revision of the NFS protocol is to
return the real value from the indicated operation and not an
error from an incidental operation. The post_op_fh3 structure
was designed to allow the server to recover from errors
encountered while constructing a file handle.
This is the structure used to return a file handle from the
CREATE, MKDIR, SYMLINK, MKNOD, and READDIRPLUS requests. In each
case, the client can get the file handle by issuing a LOOKUP
request after a successful return from one of the listed
operations. Returning the file handle is an optimization so that
the client is not forced to immediately issue a LOOKUP request
to get the file handle.
sattr3
enum time_how {
DONT_CHANGE = 0,
SET_TO_SERVER_TIME = 1,
SET_TO_CLIENT_TIME = 2
};
union set_mode3 switch (bool set_it) {
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case TRUE:
mode3 mode;
default:
void;
};
union set_uid3 switch (bool set_it) {
case TRUE:
uid3 uid;
default:
void;
};
union set_gid3 switch (bool set_it) {
case TRUE:
gid3 gid;
default:
void;
};
union set_size3 switch (bool set_it) {
case TRUE:
size3 size;
default:
void;
};
union set_atime switch (time_how set_it) {
case SET_TO_CLIENT_TIME:
nfstime3 atime;
default:
void;
};
union set_mtime switch (time_how set_it) {
case SET_TO_CLIENT_TIME:
nfstime3 mtime;
default:
void;
};
struct sattr3 {
set_mode3 mode;
set_uid3 uid;
set_gid3 gid;
set_size3 size;
set_atime atime;
set_mtime mtime;
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RFC 1813 NFS Version 3 Protocol June 1995
};
The sattr3 structure contains the file attributes that can be
set from the client. The fields are the same as the similarly
named fields in the fattr3 structure. In the NFS version 3
protocol, the settable attributes are described by a structure
containing a set of discriminated unions. Each union indicates
whether the corresponding attribute is to be updated, and if so,
how.
There are two forms of discriminated unions used. In setting the
mode, uid, gid, or size, the discriminated union is switched on
a boolean, set_it; if it is TRUE, a value of the appropriate
type is then encoded.
In setting the atime or mtime, the union is switched on an
enumeration type, set_it. If set_it has the value DONT_CHANGE,
the corresponding attribute is unchanged. If it has the value,
SET_TO_SERVER_TIME, the corresponding attribute is set by the
server to its local time; no data is provided by the client.
Finally, if set_it has the value, SET_TO_CLIENT_TIME, the
attribute is set to the time passed by the client in an nfstime3
structure. (See FSINFO on page 86, which addresses the issue of
time granularity).
diropargs3
struct diropargs3 {
nfs_fh3 dir;
filename3 name;
};
The diropargs3 structure is used in directory operations. The
file handle, dir, identifies the directory in which to
manipulate or access the file, name. See additional comments in
File name component handling on page 101.
3. Server Procedures
The following sections define the RPC procedures that are
supplied by an NFS version 3 protocol server. The RPC
procedure number is given at the top of the page with the
name. The SYNOPSIS provides the name of the procedure, the
list of the names of the arguments, the list of the names of
the results, followed by the XDR argument declarations and
results declarations. The information in the SYNOPSIS is
specified in RPC Data Description Language as defined in
[RFC1014]. The DESCRIPTION section tells what the procedure
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is expected to do and how its arguments and results are used.
The ERRORS section lists the errors returned for specific
types of failures. These lists are not intended to be the
definitive statement of all of the errors which can be
returned by any specific procedure, but as a guide for the
more common errors which may be returned. Client
implementations should be prepared to deal with unexpected
errors coming from a server. The IMPLEMENTATION field gives
information about how the procedure is expected to work and
how it should be used by clients.
program NFS_PROGRAM {
version NFS_V3 {
void
NFSPROC3_NULL(void) = 0;
GETATTR3res
NFSPROC3_GETATTR(GETATTR3args) = 1;
SETATTR3res
NFSPROC3_SETATTR(SETATTR3args) = 2;
LOOKUP3res
NFSPROC3_LOOKUP(LOOKUP3args) = 3;
ACCESS3res
NFSPROC3_ACCESS(ACCESS3args) = 4;
READLINK3res
NFSPROC3_READLINK(READLINK3args) = 5;
READ3res
NFSPROC3_READ(READ3args) = 6;
WRITE3res
NFSPROC3_WRITE(WRITE3args) = 7;
CREATE3res
NFSPROC3_CREATE(CREATE3args) = 8;
MKDIR3res
NFSPROC3_MKDIR(MKDIR3args) = 9;
SYMLINK3res
NFSPROC3_SYMLINK(SYMLINK3args) = 10;
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MKNOD3res
NFSPROC3_MKNOD(MKNOD3args) = 11;
REMOVE3res
NFSPROC3_REMOVE(REMOVE3args) = 12;
RMDIR3res
NFSPROC3_RMDIR(RMDIR3args) = 13;
RENAME3res
NFSPROC3_RENAME(RENAME3args) = 14;
LINK3res
NFSPROC3_LINK(LINK3args) = 15;
READDIR3res
NFSPROC3_READDIR(READDIR3args) = 16;
READDIRPLUS3res
NFSPROC3_READDIRPLUS(READDIRPLUS3args) = 17;
FSSTAT3res
NFSPROC3_FSSTAT(FSSTAT3args) = 18;
FSINFO3res
NFSPROC3_FSINFO(FSINFO3args) = 19;
PATHCONF3res
NFSPROC3_PATHCONF(PATHCONF3args) = 20;
COMMIT3res
NFSPROC3_COMMIT(COMMIT3args) = 21;
} = 3;
} = 100003;
Out of range (undefined) procedure numbers result in RPC
errors. Refer to [RFC1057] for more detail.
3.1 General comments on attributes and consistency data on failure
For those procedures that return either post_op_attr or wcc_data
structures on failure, the discriminated union may contain the
pre-operation attributes of the object or object parent
directory. This depends on the error encountered and may also
depend on the particular server implementation. Implementors are
strongly encouraged to return as much attribute data as possible
upon failure, but client implementors need to be aware that
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their implementation must correctly handle the variant return
instance where no attributes or consistency data is returned.
3.2 General comments on filenames
The following comments apply to all NFS version 3 protocol
procedures in which the client provides one or more filenames in
the arguments: LOOKUP, CREATE, MKDIR, SYMLINK, MKNOD, REMOVE,
RMDIR, RENAME, and LINK.
1. The filename must not be null nor may it be the null
string. The server should return the error, NFS3ERR_ACCES, if
it receives such a filename. On some clients, the filename, ``''
or a null string, is assumed to be an alias for the current
directory. Clients which require this functionality should
implement it for themselves and not depend upon the server to
support such semantics.
2. A filename having the value of "." is assumed to be an
alias for the current directory. Clients which require this
functionality should implement it for themselves and not depend
upon the server to support such semantics. However, the server
should be able to handle such a filename correctly.
3. A filename having the value of ".." is assumed to be an
alias for the parent of the current directory, i.e. the
directory which contains the current directory. The server
should be prepared to handle this semantic, if it supports
directories, even if those directories do not contain UNIX-style
"." or ".." entries.
4. If the filename is longer than the maximum for the file
system (see PATHCONF on page 90, specifically name_max), the
result depends on the value of the PATHCONF flag, no_trunc. If
no_trunc is FALSE, the filename will be silently truncated to
name_max bytes. If no_trunc is TRUE and the filename exceeds the
server's file system maximum filename length, the operation will
fail with the error, NFS3ERR_NAMETOOLONG.
5. In general, there will be characters that a server will
not be able to handle as part of a filename. This set of
characters will vary from server to server and from
implementation to implementation. In most cases, it is the
server which will control the client's view of the file system.
If the server receives a filename containing characters that it
can not handle, the error, NFS3ERR_EACCES, should be returned.
Client implementations should be prepared to handle this side
affect of heterogeneity.
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See also comments in File name component handling on page 101.
3.3.0 Procedure 0: NULL - Do nothing
SYNOPSIS
void NFSPROC3_NULL(void) = 0;
DESCRIPTION
Procedure NULL does not do any work. It is made available to
allow server response testing and timing.
IMPLEMENTATION
It is important that this procedure do no work at all so
that it can be used to measure the overhead of processing
a service request. By convention, the NULL procedure
should never require any authentication. A server may
choose to ignore this convention, in a more secure
implementation, where responding to the NULL procedure
call acknowledges the existence of a resource to an
unauthenticated client.
ERRORS
Since the NULL procedure takes no NFS version 3 protocol
arguments and returns no NFS version 3 protocol response,
it can not return an NFS version 3 protocol error.
However, it is possible that some server implementations
may return RPC errors based on security and authentication
requirements.
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3.3.1 Procedure 1: GETATTR - Get file attributes
SYNOPSIS
GETATTR3res NFSPROC3_GETATTR(GETATTR3args) = 1;
struct GETATTR3args {
nfs_fh3 object;
};
struct GETATTR3resok {
fattr3 obj_attributes;
};
union GETATTR3res switch (nfsstat3 status) {
case NFS3_OK:
GETATTR3resok resok;
default:
void;
};
DESCRIPTION
Procedure GETATTR retrieves the attributes for a specified
file system object. The object is identified by the file
handle that the server returned as part of the response
from a LOOKUP, CREATE, MKDIR, SYMLINK, MKNOD, or
READDIRPLUS procedure (or from the MOUNT service,
described elsewhere). On entry, the arguments in
GETATTR3args are:
object
The file handle of an object whose attributes are to be
retrieved.
On successful return, GETATTR3res.status is NFS3_OK and
GETATTR3res.resok contains:
obj_attributes
The attributes for the object.
Otherwise, GETATTR3res.status contains the error on failure and
no other results are returned.
IMPLEMENTATION
The attributes of file system objects is a point of major
disagreement between different operating systems. Servers
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should make a best attempt to support all of the
attributes in the fattr3 structure so that clients can
count on this as a common ground. Some mapping may be
required to map local attributes to those in the fattr3
structure.
Today, most client NFS version 3 protocol implementations
implement a time-bounded attribute caching scheme to
reduce over-the-wire attribute checks.
ERRORS
NFS3ERR_IO
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
ACCESS.
3.3.2 Procedure 2: SETATTR - Set file attributes
SYNOPSIS
SETATTR3res NFSPROC3_SETATTR(SETATTR3args) = 2;
union sattrguard3 switch (bool check) {
case TRUE:
nfstime3 obj_ctime;
case FALSE:
void;
};
struct SETATTR3args {
nfs_fh3 object;
sattr3 new_attributes;
sattrguard3 guard;
};
struct SETATTR3resok {
wcc_data obj_wcc;
};
struct SETATTR3resfail {
wcc_data obj_wcc;
};
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union SETATTR3res switch (nfsstat3 status) {
case NFS3_OK:
SETATTR3resok resok;
default:
SETATTR3resfail resfail;
};
DESCRIPTION
Procedure SETATTR changes one or more of the attributes of
a file system object on the server. The new attributes are
specified by a sattr3 structure. On entry, the arguments
in SETATTR3args are:
object
The file handle for the object.
new_attributes
A sattr3 structure containing booleans and
enumerations describing the attributes to be set and the new
values for those attributes.
guard
A sattrguard3 union:
check
TRUE if the server is to verify that guard.obj_ctime
matches the ctime for the object; FALSE otherwise.
A client may request that the server check that the object
is in an expected state before performing the SETATTR
operation. To do this, it sets the argument guard.check to
TRUE and the client passes a time value in guard.obj_ctime.
If guard.check is TRUE, the server must compare the value of
guard.obj_ctime to the current ctime of the object. If the
values are different, the server must preserve the object
attributes and must return a status of NFS3ERR_NOT_SYNC.
If guard.check is FALSE, the server will not perform this
check.
On successful return, SETATTR3res.status is NFS3_OK and
SETATTR3res.resok contains:
obj_wcc
A wcc_data structure containing the old and new
attributes for the object.
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Otherwise, SETATTR3res.status contains the error on
failure and SETATTR3res.resfail contains the following:
obj_wcc
A wcc_data structure containing the old and new
attributes for the object.
IMPLEMENTATION
The guard.check mechanism allows the client to avoid
changing the attributes of an object on the basis of stale
attributes. It does not guarantee exactly-once semantics.
In particular, if a reply is lost and the server does not
detect the retransmission of the request, the procedure
can fail with the error, NFS3ERR_NOT_SYNC, even though the
attribute setting was previously performed successfully.
The client can attempt to recover from this error by
getting fresh attributes from the server and sending a new
SETATTR request using the new ctime. The client can
optionally check the attributes to avoid the second
SETATTR request if the new attributes show that the
attributes have already been set as desired (though it may
not have been the issuing client that set the
attributes).
The new_attributes.size field is used to request changes
to the size of a file. A value of 0 causes the file to be
truncated, a value less than the current size of the file
causes data from new size to the end of the file to be
discarded, and a size greater than the current size of the
file causes logically zeroed data bytes to be added to the
end of the file. Servers are free to implement this using
holes or actual zero data bytes. Clients should not make
any assumptions regarding a server's implementation of
this feature, beyond that the bytes returned will be
zeroed. Servers must support extending the file size via
SETATTR.
SETATTR is not guaranteed atomic. A failed SETATTR may
partially change a file's attributes.
Changing the size of a file with SETATTR indirectly
changes the mtime. A client must account for this as size
changes can result in data deletion.
If server and client times differ, programs that compare
client time to file times can break. A time maintenance
protocol should be used to limit client/server time skew.
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In a heterogeneous environment, it is quite possible that
the server will not be able to support the full range of
SETATTR requests. The error, NFS3ERR_INVAL, may be
returned if the server can not store a uid or gid in its
own representation of uids or gids, respectively. If the
server can only support 32 bit offsets and sizes, a
SETATTR request to set the size of a file to larger than
can be represented in 32 bits will be rejected with this
same error.
ERRORS
NFS3ERR_PERM
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_INVAL
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_DQUOT
NFS3ERR_NOT_SYNC
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
CREATE, MKDIR, SYMLINK, and MKNOD.
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3.3.3 Procedure 3: LOOKUP - Lookup filename
SYNOPSIS
LOOKUP3res NFSPROC3_LOOKUP(LOOKUP3args) = 3;
struct LOOKUP3args {
diropargs3 what;
};
struct LOOKUP3resok {
nfs_fh3 object;
post_op_attr obj_attributes;
post_op_attr dir_attributes;
};
struct LOOKUP3resfail {
post_op_attr dir_attributes;
};
union LOOKUP3res switch (nfsstat3 status) {
case NFS3_OK:
LOOKUP3resok resok;
default:
LOOKUP3resfail resfail;
};
DESCRIPTION
Procedure LOOKUP searches a directory for a specific name
and returns the file handle for the corresponding file
system object. On entry, the arguments in LOOKUP3args
are:
what
Object to look up:
dir
The file handle for the directory to search.
name
The filename to be searched for. Refer to General
comments on filenames on page 30.
On successful return, LOOKUP3res.status is NFS3_OK and
LOOKUP3res.resok contains:
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object
The file handle of the object corresponding to
what.name.
obj_attributes
The attributes of the object corresponding to
what.name.
dir_attributes
The post-operation attributes of the directory,
what.dir.
Otherwise, LOOKUP3res.status contains the error on failure and
LOOKUP3res.resfail contains the following:
dir_attributes
The post-operation attributes for the directory,
what.dir.
IMPLEMENTATION
At first glance, in the case where what.name refers to a
mount point on the server, two different replies seem
possible. The server can return either the file handle for
the underlying directory that is mounted on or the file
handle of the root of the mounted directory. This
ambiguity is simply resolved. A server will not allow a
LOOKUP operation to cross a mountpoint to the root of a
different filesystem, even if the filesystem is exported.
This does not prevent a client from accessing a hierarchy
of filesystems exported by a server, but the client must
mount each of the filesystems individually so that the
mountpoint crossing takes place on the client. A given
server implementation may refine these rules given
capabilities or limitations particular to that
implementation. Refer to [X/OpenNFS] for a discussion on
exporting file systems.
Two filenames are distinguished, as in the NFS version 2
protocol. The name, ".", is an alias for the current
directory and the name, "..", is an alias for the parent
directory; that is, the directory that includes the
specified directory as a member. There is no facility for
dealing with a multiparented directory and the NFS
protocol assumes a hierarchical organization, organized as
a single-rooted tree.
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Note that this procedure does not follow symbolic links.
The client is responsible for all parsing of filenames
including filenames that are modified by symbolic links
encountered during the lookup process.
ERRORS
NFS3ERR_IO
NFS3ERR_NOENT
NFS3ERR_ACCES
NFS3ERR_NOTDIR
NFS3ERR_NAMETOOLONG
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
CREATE, MKDIR, SYMLINK, MKNOD, READDIRPLUS, and PATHCONF.
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3.3.4 Procedure 4: ACCESS - Check Access Permission
SYNOPSIS
ACCESS3res NFSPROC3_ACCESS(ACCESS3args) = 4;
const ACCESS3_READ = 0x0001;
const ACCESS3_LOOKUP = 0x0002;
const ACCESS3_MODIFY = 0x0004;
const ACCESS3_EXTEND = 0x0008;
const ACCESS3_DELETE = 0x0010;
const ACCESS3_EXECUTE = 0x0020;
struct ACCESS3args {
nfs_fh3 object;
uint32 access;
};
struct ACCESS3resok {
post_op_attr obj_attributes;
uint32 access;
};
struct ACCESS3resfail {
post_op_attr obj_attributes;
};
union ACCESS3res switch (nfsstat3 status) {
case NFS3_OK:
ACCESS3resok resok;
default:
ACCESS3resfail resfail;
};
DESCRIPTION
Procedure ACCESS determines the access rights that a user,
as identified by the credentials in the request, has with
respect to a file system object. The client encodes the
set of permissions that are to be checked in a bit mask.
The server checks the permissions encoded in the bit mask.
A status of NFS3_OK is returned along with a bit mask
encoded with the permissions that the client is allowed.
The results of this procedure are necessarily advisory in
nature. That is, a return status of NFS3_OK and the
appropriate bit set in the bit mask does not imply that
such access will be allowed to the file system object in
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the future, as access rights can be revoked by the server
at any time.
On entry, the arguments in ACCESS3args are:
object
The file handle for the file system object to which
access is to be checked.
access
A bit mask of access permissions to check.
The following access permissions may be requested:
ACCESS3_READ
Read data from file or read a directory.
ACCESS3_LOOKUP
Look up a name in a directory (no meaning for
non-directory objects).
ACCESS3_MODIFY
Rewrite existing file data or modify existing
directory entries.
ACCESS3_EXTEND
Write new data or add directory entries.
ACCESS3_DELETE
Delete an existing directory entry.
ACCESS3_EXECUTE
Execute file (no meaning for a directory).
On successful return, ACCESS3res.status is NFS3_OK. The
server should return a status of NFS3_OK if no errors
occurred that prevented the server from making the
required access checks. The results in ACCESS3res.resok
are:
obj_attributes
The post-operation attributes of object.
access
A bit mask of access permissions indicating access
rights for the authentication credentials provided with
the request.
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Otherwise, ACCESS3res.status contains the error on failure
and ACCESS3res.resfail contains the following:
obj_attributes
The attributes of object - if access to attributes is
permitted.
IMPLEMENTATION
In general, it is not sufficient for the client to attempt
to deduce access permissions by inspecting the uid, gid,
and mode fields in the file attributes, since the server
may perform uid or gid mapping or enforce additional
access control restrictions. It is also possible that the
NFS version 3 protocol server may not be in the same ID
space as the NFS version 3 protocol client. In these cases
(and perhaps others), the NFS version 3 protocol client
can not reliably perform an access check with only current
file attributes.
In the NFS version 2 protocol, the only reliable way to
determine whether an operation was allowed was to try it
and see if it succeeded or failed. Using the ACCESS
procedure in the NFS version 3 protocol, the client can
ask the server to indicate whether or not one or more
classes of operations are permitted. The ACCESS operation
is provided to allow clients to check before doing a
series of operations. This is useful in operating systems
(such as UNIX) where permission checking is done only when
a file or directory is opened. This procedure is also
invoked by NFS client access procedure (called possibly
through access(2)). The intent is to make the behavior of
opening a remote file more consistent with the behavior of
opening a local file.
The information returned by the server in response to an
ACCESS call is not permanent. It was correct at the exact
time that the server performed the checks, but not
necessarily afterwards. The server can revoke access
permission at any time.
The NFS version 3 protocol client should use the effective
credentials of the user to build the authentication
information in the ACCESS request used to determine access
rights. It is the effective user and group credentials
that are used in subsequent read and write operations. See
the comments in Permission issues on page 98 for more
information on this topic.
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Many implementations do not directly support the
ACCESS3_DELETE permission. Operating systems like UNIX
will ignore the ACCESS3_DELETE bit if set on an access
request on a non-directory object. In these systems,
delete permission on a file is determined by the access
permissions on the directory in which the file resides,
instead of being determined by the permissions of the file
itself. Thus, the bit mask returned for such a request
will have the ACCESS3_DELETE bit set to 0, indicating that
the client does not have this permission.
ERRORS
NFS3ERR_IO
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
GETATTR.
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3.3.5 Procedure 5: READLINK - Read from symbolic link
SYNOPSIS
READLINK3res NFSPROC3_READLINK(READLINK3args) = 5;
struct READLINK3args {
nfs_fh3 symlink;
};
struct READLINK3resok {
post_op_attr symlink_attributes;
nfspath3 data;
};
struct READLINK3resfail {
post_op_attr symlink_attributes;
};
union READLINK3res switch (nfsstat3 status) {
case NFS3_OK:
READLINK3resok resok;
default:
READLINK3resfail resfail;
};
DESCRIPTION
Procedure READLINK reads the data associated with a
symbolic link. The data is an ASCII string that is opaque
to the server. That is, whether created by the NFS
version 3 protocol software from a client or created
locally on the server, the data in a symbolic link is not
interpreted when created, but is simply stored. On entry,
the arguments in READLINK3args are:
symlink
The file handle for a symbolic link (file system object
of type NF3LNK).
On successful return, READLINK3res.status is NFS3_OK and
READLINK3res.resok contains:
data
The data associated with the symbolic link.
symlink_attributes
The post-operation attributes for the symbolic link.
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Otherwise, READLINK3res.status contains the error on
failure and READLINK3res.resfail contains the following:
symlink_attributes
The post-operation attributes for the symbolic link.
IMPLEMENTATION
A symbolic link is nominally a pointer to another file.
The data is not necessarily interpreted by the server,
just stored in the file. It is possible for a client
implementation to store a path name that is not meaningful
to the server operating system in a symbolic link. A
READLINK operation returns the data to the client for
interpretation. If different implementations want to share
access to symbolic links, then they must agree on the
interpretation of the data in the symbolic link.
The READLINK operation is only allowed on objects of type,
NF3LNK. The server should return the error,
NFS3ERR_INVAL, if the object is not of type, NF3LNK.
(Note: The X/Open XNFS Specification for the NFS version 2
protocol defined the error status in this case as
NFSERR_NXIO. This is inconsistent with existing server
practice.)
ERRORS
NFS3ERR_IO
NFS3ERR_INVAL
NFS3ERR_ACCES
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
READLINK, SYMLINK.
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3.3.6 Procedure 6: READ - Read From file
SYNOPSIS
READ3res NFSPROC3_READ(READ3args) = 6;
struct READ3args {
nfs_fh3 file;
offset3 offset;
count3 count;
};
struct READ3resok {
post_op_attr file_attributes;
count3 count;
bool eof;
opaque data<>;
};
struct READ3resfail {
post_op_attr file_attributes;
};
union READ3res switch (nfsstat3 status) {
case NFS3_OK:
READ3resok resok;
default:
READ3resfail resfail;
};
DESCRIPTION
Procedure READ reads data from a file. On entry, the
arguments in READ3args are:
file
The file handle of the file from which data is to be
read. This must identify a file system object of type,
NF3REG.
offset
The position within the file at which the read is to
begin. An offset of 0 means to read data starting at
the beginning of the file. If offset is greater than or
equal to the size of the file, the status, NFS3_OK, is
returned with count set to 0 and eof set to TRUE,
subject to access permissions checking.
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count
The number of bytes of data that are to be read. If
count is 0, the READ will succeed and return 0 bytes of
data, subject to access permissions checking. count
must be less than or equal to the value of the rtmax
field in the FSINFO reply structure for the file system
that contains file. If greater, the server may return
only rtmax bytes, resulting in a short read.
On successful return, READ3res.status is NFS3_OK and
READ3res.resok contains:
file_attributes
The attributes of the file on completion of the read.
count
The number of bytes of data returned by the read.
eof
If the read ended at the end-of-file (formally, in a
correctly formed READ request, if READ3args.offset plus
READ3resok.count is equal to the size of the file), eof
is returned as TRUE; otherwise it is FALSE. A
successful READ of an empty file will always return eof
as TRUE.
data
The counted data read from the file.
Otherwise, READ3res.status contains the error on failure
and READ3res.resfail contains the following:
file_attributes
The post-operation attributes of the file.
IMPLEMENTATION
The nfsdata type used for the READ and WRITE operations in
the NFS version 2 protocol defining the data portion of a
request or reply has been changed to a variable-length
opaque byte array. The maximum size allowed by the
protocol is now limited by what XDR and underlying
transports will allow. There are no artificial limits
imposed by the NFS version 3 protocol. Consult the FSINFO
procedure description for details.
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It is possible for the server to return fewer than count
bytes of data. If the server returns less than the count
requested and eof set to FALSE, the client should issue
another READ to get the remaining data. A server may
return less data than requested under several
circumstances. The file may have been truncated by another
client or perhaps on the server itself, changing the file
size from what the requesting client believes to be the
case. This would reduce the actual amount of data
available to the client. It is possible that the server
may back off the transfer size and reduce the read request
return. Server resource exhaustion may also occur
necessitating a smaller read return.
Some NFS version 2 protocol client implementations chose
to interpret a short read response as indicating EOF. The
addition of the eof flag in the NFS version 3 protocol
provides a correct way of handling EOF.
Some NFS version 2 protocol server implementations
incorrectly returned NFSERR_ISDIR if the file system
object type was not a regular file. The correct return
value for the NFS version 3 protocol is NFS3ERR_INVAL.
ERRORS
NFS3ERR_IO
NFS3ERR_NXIO
NFS3ERR_ACCES
NFS3ERR_INVAL
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
READLINK.
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3.3.7 Procedure 7: WRITE - Write to file
SYNOPSIS
WRITE3res NFSPROC3_WRITE(WRITE3args) = 7;
enum stable_how {
UNSTABLE = 0,
DATA_SYNC = 1,
FILE_SYNC = 2
};
struct WRITE3args {
nfs_fh3 file;
offset3 offset;
count3 count;
stable_how stable;
opaque data<>;
};
struct WRITE3resok {
wcc_data file_wcc;
count3 count;
stable_how committed;
writeverf3 verf;
};
struct WRITE3resfail {
wcc_data file_wcc;
};
union WRITE3res switch (nfsstat3 status) {
case NFS3_OK:
WRITE3resok resok;
default:
WRITE3resfail resfail;
};
DESCRIPTION
Procedure WRITE writes data to a file. On entry, the
arguments in WRITE3args are:
file
The file handle for the file to which data is to be
written. This must identify a file system object of
type, NF3REG.
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offset
The position within the file at which the write is to
begin. An offset of 0 means to write data starting at
the beginning of the file.
count
The number of bytes of data to be written. If count is
0, the WRITE will succeed and return a count of 0,
barring errors due to permissions checking. The size of
data must be less than or equal to the value of the
wtmax field in the FSINFO reply structure for the file
system that contains file. If greater, the server may
write only wtmax bytes, resulting in a short write.
stable
If stable is FILE_SYNC, the server must commit the data
written plus all file system metadata to stable storage
before returning results. This corresponds to the NFS
version 2 protocol semantics. Any other behavior
constitutes a protocol violation. If stable is
DATA_SYNC, then the server must commit all of the data
to stable storage and enough of the metadata to
retrieve the data before returning. The server
implementor is free to implement DATA_SYNC in the same
fashion as FILE_SYNC, but with a possible performance
drop. If stable is UNSTABLE, the server is free to
commit any part of the data and the metadata to stable
storage, including all or none, before returning a
reply to the client. There is no guarantee whether or
when any uncommitted data will subsequently be
committed to stable storage. The only guarantees made
by the server are that it will not destroy any data
without changing the value of verf and that it will not
commit the data and metadata at a level less than that
requested by the client. See the discussion on COMMIT
on page 92 for more information on if and when
data is committed to stable storage.
data
The data to be written to the file.
On successful return, WRITE3res.status is NFS3_OK and
WRITE3res.resok contains:
file_wcc
Weak cache consistency data for the file. For a client
that requires only the post-write file attributes,
these can be found in file_wcc.after.
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count
The number of bytes of data written to the file. The
server may write fewer bytes than requested. If so, the
actual number of bytes written starting at location,
offset, is returned.
committed
The server should return an indication of the level of
commitment of the data and metadata via committed. If
the server committed all data and metadata to stable
storage, committed should be set to FILE_SYNC. If the
level of commitment was at least as strong as
DATA_SYNC, then committed should be set to DATA_SYNC.
Otherwise, committed must be returned as UNSTABLE. If
stable was FILE_SYNC, then committed must also be
FILE_SYNC: anything else constitutes a protocol
violation. If stable was DATA_SYNC, then committed may
be FILE_SYNC or DATA_SYNC: anything else constitutes a
protocol violation. If stable was UNSTABLE, then
committed may be either FILE_SYNC, DATA_SYNC, or
UNSTABLE.
verf
This is a cookie that the client can use to determine
whether the server has changed state between a call to
WRITE and a subsequent call to either WRITE or COMMIT.
This cookie must be consistent during a single instance
of the NFS version 3 protocol service and must be
unique between instances of the NFS version 3 protocol
server, where uncommitted data may be lost.
Otherwise, WRITE3res.status contains the error on failure
and WRITE3res.resfail contains the following:
file_wcc
Weak cache consistency data for the file. For a client
that requires only the post-write file attributes,
these can be found in file_wcc.after. Even though the
write failed, full wcc_data is returned to allow the
client to determine whether the failed write resulted
in any change to the file.
If a client writes data to the server with the stable
argument set to UNSTABLE and the reply yields a committed
response of DATA_SYNC or UNSTABLE, the client will follow
up some time in the future with a COMMIT operation to
synchronize outstanding asynchronous data and metadata
with the server's stable storage, barring client error. It
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is possible that due to client crash or other error that a
subsequent COMMIT will not be received by the server.
IMPLEMENTATION
The nfsdata type used for the READ and WRITE operations in
the NFS version 2 protocol defining the data portion of a
request or reply has been changed to a variable-length
opaque byte array. The maximum size allowed by the
protocol is now limited by what XDR and underlying
transports will allow. There are no artificial limits
imposed by the NFS version 3 protocol. Consult the FSINFO
procedure description for details.
It is possible for the server to write fewer than count
bytes of data. In this case, the server should not return
an error unless no data was written at all. If the server
writes less than count bytes, the client should issue
another WRITE to write the remaining data.
It is assumed that the act of writing data to a file will
cause the mtime of the file to be updated. However, the
mtime of the file should not be changed unless the
contents of the file are changed. Thus, a WRITE request
with count set to 0 should not cause the mtime of the file
to be updated.
The NFS version 3 protocol introduces safe asynchronous
writes. The combination of WRITE with stable set to
UNSTABLE followed by a COMMIT addresses the performance
bottleneck found in the NFS version 2 protocol, the need
to synchronously commit all writes to stable storage.
The definition of stable storage has been historically a
point of contention. The following expected properties of
stable storage may help in resolving design issues in the
implementation. Stable storage is persistent storage that
survives:
1. Repeated power failures.
2. Hardware failures (of any board, power supply, and so on.).
3. Repeated software crashes, including reboot cycle.
This definition does not address failure of the stable
storage module itself.
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A cookie, verf, is defined to allow a client to detect
different instances of an NFS version 3 protocol server
over which cached, uncommitted data may be lost. In the
most likely case, the verf allows the client to detect
server reboots. This information is required so that the
client can safely determine whether the server could have
lost cached data. If the server fails unexpectedly and the
client has uncommitted data from previous WRITE requests
(done with the stable argument set to UNSTABLE and in
which the result committed was returned as UNSTABLE as
well) it may not have flushed cached data to stable
storage. The burden of recovery is on the client and the
client will need to retransmit the data to the server.
A suggested verf cookie would be to use the time that the
server was booted or the time the server was last started
(if restarting the server without a reboot results in lost
buffers).
The committed field in the results allows the client to do
more effective caching. If the server is committing all
WRITE requests to stable storage, then it should return
with committed set to FILE_SYNC, regardless of the value
of the stable field in the arguments. A server that uses
an NVRAM accelerator may choose to implement this policy.
The client can use this to increase the effectiveness of
the cache by discarding cached data that has already been
committed on the server.
Some implementations may return NFS3ERR_NOSPC instead of
NFS3ERR_DQUOT when a user's quota is exceeded.
Some NFS version 2 protocol server implementations
incorrectly returned NFSERR_ISDIR if the file system
object type was not a regular file. The correct return
value for the NFS version 3 protocol is NFS3ERR_INVAL.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_FBIG
NFS3ERR_DQUOT
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_INVAL
NFS3ERR_STALE
NFS3ERR_BADHANDLE
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NFS3ERR_SERVERFAULT
SEE ALSO
COMMIT.
3.3.8 Procedure 8: CREATE - Create a file
SYNOPSIS
CREATE3res NFSPROC3_CREATE(CREATE3args) = 8;
enum createmode3 {
UNCHECKED = 0,
GUARDED = 1,
EXCLUSIVE = 2
};
union createhow3 switch (createmode3 mode) {
case UNCHECKED:
case GUARDED:
sattr3 obj_attributes;
case EXCLUSIVE:
createverf3 verf;
};
struct CREATE3args {
diropargs3 where;
createhow3 how;
};
struct CREATE3resok {
post_op_fh3 obj;
post_op_attr obj_attributes;
wcc_data dir_wcc;
};
struct CREATE3resfail {
wcc_data dir_wcc;
};
union CREATE3res switch (nfsstat3 status) {
case NFS3_OK:
CREATE3resok resok;
default:
CREATE3resfail resfail;
};
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DESCRIPTION
Procedure CREATE creates a regular file. On entry, the
arguments in CREATE3args are:
where
The location of the file to be created:
dir
The file handle for the directory in which the file
is to be created.
name
The name that is to be associated with the created
file. Refer to General comments on filenames on
page 30.
When creating a regular file, there are three ways to
create the file as defined by:
how
A discriminated union describing how the server is to
handle the file creation along with the appropriate
attributes:
mode
One of UNCHECKED, GUARDED, and EXCLUSIVE. UNCHECKED
means that the file should be created without checking
for the existence of a duplicate file in the same
directory. In this case, how.obj_attributes is a sattr3
describing the initial attributes for the file. GUARDED
specifies that the server should check for the presence
of a duplicate file before performing the create and
should fail the request with NFS3ERR_EXIST if a
duplicate file exists. If the file does not exist, the
request is performed as described for UNCHECKED.
EXCLUSIVE specifies that the server is to follow
exclusive creation semantics, using the verifier to
ensure exclusive creation of the target. No attributes
may be provided in this case, since the server may use
the target file metadata to store the createverf3
verifier.
On successful return, CREATE3res.status is NFS3_OK and the
results in CREATE3res.resok are:
obj
The file handle of the newly created regular file.
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obj_attributes
The attributes of the regular file just created.
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires on the
post-CREATE directory attributes, these can be found in
dir_wcc.after.
Otherwise, CREATE3res.status contains the error on failure
and CREATE3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-CREATE directory attributes, these can be found in
dir_wcc.after. Even though the CREATE failed, full
wcc_data is returned to allow the client to determine
whether the failing CREATE resulted in any change to
the directory.
IMPLEMENTATION
Unlike the NFS version 2 protocol, in which certain fields
in the initial attributes structure were overloaded to
indicate creation of devices and FIFOs in addition to
regular files, this procedure only supports the creation
of regular files. The MKNOD procedure was introduced in
the NFS version 3 protocol to handle creation of devices
and FIFOs. Implementations should have no reason in the
NFS version 3 protocol to overload CREATE semantics.
One aspect of the NFS version 3 protocol CREATE procedure
warrants particularly careful consideration: the mechanism
introduced to support the reliable exclusive creation of
regular files. The mechanism comes into play when how.mode
is EXCLUSIVE. In this case, how.verf contains a verifier
that can reasonably be expected to be unique. A
combination of a client identifier, perhaps the client
network address, and a unique number generated by the
client, perhaps the RPC transaction identifier, may be
appropriate.
If the file does not exist, the server creates the file
and stores the verifier in stable storage. For file
systems that do not provide a mechanism for the storage of
arbitrary file attributes, the server may use one or more
elements of the file metadata to store the verifier. The
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verifier must be stored in stable storage to prevent
erroneous failure on retransmission of the request. It is
assumed that an exclusive create is being performed
because exclusive semantics are critical to the
application. Because of the expected usage, exclusive
CREATE does not rely solely on the normally volatile
duplicate request cache for storage of the verifier. The
duplicate request cache in volatile storage does not
survive a crash and may actually flush on a long network
partition, opening failure windows. In the UNIX local
file system environment, the expected storage location for
the verifier on creation is the metadata (time stamps) of
the file. For this reason, an exclusive file create may
not include initial attributes because the server would
have nowhere to store the verifier.
If the server can not support these exclusive create
semantics, possibly because of the requirement to commit
the verifier to stable storage, it should fail the CREATE
request with the error, NFS3ERR_NOTSUPP.
During an exclusive CREATE request, if the file already
exists, the server reconstructs the file's verifier and
compares it with the verifier in the request. If they
match, the server treats the request as a success. The
request is presumed to be a duplicate of an earlier,
successful request for which the reply was lost and that
the server duplicate request cache mechanism did not
detect. If the verifiers do not match, the request is
rejected with the status, NFS3ERR_EXIST.
Once the client has performed a successful exclusive
create, it must issue a SETATTR to set the correct file
attributes. Until it does so, it should not rely upon any
of the file attributes, since the server implementation
may need to overload file metadata to store the verifier.
Use of the GUARDED attribute does not provide exactly-once
semantics. In particular, if a reply is lost and the
server does not detect the retransmission of the request,
the procedure can fail with NFS3ERR_EXIST, even though the
create was performed successfully.
Refer to General comments on filenames on page 30.
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ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_NOTDIR
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_NAMETOOLONG
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
MKDIR, SYMLINK, MKNOD, and PATHCONF.
3.3.9 Procedure 9: MKDIR - Create a directory
SYNOPSIS
MKDIR3res NFSPROC3_MKDIR(MKDIR3args) = 9;
struct MKDIR3args {
diropargs3 where;
sattr3 attributes;
};
struct MKDIR3resok {
post_op_fh3 obj;
post_op_attr obj_attributes;
wcc_data dir_wcc;
};
struct MKDIR3resfail {
wcc_data dir_wcc;
};
union MKDIR3res switch (nfsstat3 status) {
case NFS3_OK:
MKDIR3resok resok;
default:
MKDIR3resfail resfail;
};
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DESCRIPTION
Procedure MKDIR creates a new subdirectory. On entry, the
arguments in MKDIR3args are:
where
The location of the subdirectory to be created:
dir
The file handle for the directory in which the
subdirectory is to be created.
name
The name that is to be associated with the created
subdirectory. Refer to General comments on filenames
on page 30.
attributes
The initial attributes for the subdirectory.
On successful return, MKDIR3res.status is NFS3_OK and the
results in MKDIR3res.resok are:
obj
The file handle for the newly created directory.
obj_attributes
The attributes for the newly created subdirectory.
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-MKDIR directory attributes, these can be found in
dir_wcc.after.
Otherwise, MKDIR3res.status contains the error on failure
and MKDIR3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-MKDIR directory attributes, these can be found in
dir_wcc.after. Even though the MKDIR failed, full
wcc_data is returned to allow the client to determine
whether the failing MKDIR resulted in any change to the
directory.
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IMPLEMENTATION
Many server implementations will not allow the filenames,
"." or "..", to be used as targets in a MKDIR operation.
In this case, the server should return NFS3ERR_EXIST.
Refer to General comments on filenames on page 30.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_NOTDIR
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_NAMETOOLONG
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
CREATE, SYMLINK, MKNOD, and PATHCONF.
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3.3.10 Procedure 10: SYMLINK - Create a symbolic link
SYNOPSIS
SYMLINK3res NFSPROC3_SYMLINK(SYMLINK3args) = 10;
struct symlinkdata3 {
sattr3 symlink_attributes;
nfspath3 symlink_data;
};
struct SYMLINK3args {
diropargs3 where;
symlinkdata3 symlink;
};
struct SYMLINK3resok {
post_op_fh3 obj;
post_op_attr obj_attributes;
wcc_data dir_wcc;
};
struct SYMLINK3resfail {
wcc_data dir_wcc;
};
union SYMLINK3res switch (nfsstat3 status) {
case NFS3_OK:
SYMLINK3resok resok;
default:
SYMLINK3resfail resfail;
};
DESCRIPTION
Procedure SYMLINK creates a new symbolic link. On entry,
the arguments in SYMLINK3args are:
where
The location of the symbolic link to be created:
dir
The file handle for the directory in which the
symbolic link is to be created.
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name
The name that is to be associated with the created
symbolic link. Refer to General comments on
filenames on page 30.
symlink
The symbolic link to create:
symlink_attributes
The initial attributes for the symbolic link.
symlink_data
The string containing the symbolic link data.
On successful return, SYMLINK3res.status is NFS3_OK and
SYMLINK3res.resok contains:
obj
The file handle for the newly created symbolic link.
obj_attributes
The attributes for the newly created symbolic link.
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-SYMLINK directory attributes, these can be found
in dir_wcc.after.
Otherwise, SYMLINK3res.status contains the error on
failure and SYMLINK3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-SYMLINK directory attributes, these can be found
in dir_wcc.after. Even though the SYMLINK failed, full
wcc_data is returned to allow the client to determine
whether the failing SYMLINK changed the directory.
IMPLEMENTATION
Refer to General comments on filenames on page 30.
For symbolic links, the actual file system node and its
contents are expected to be created in a single atomic
operation. That is, once the symbolic link is visible,
there must not be a window where a READLINK would fail or
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return incorrect data.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_NOTDIR
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_NAMETOOLONG
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
READLINK, CREATE, MKDIR, MKNOD, FSINFO, and PATHCONF.
3.3.11 Procedure 11: MKNOD - Create a special device
SYNOPSIS
MKNOD3res NFSPROC3_MKNOD(MKNOD3args) = 11;
struct devicedata3 {
sattr3 dev_attributes;
specdata3 spec;
};
union mknoddata3 switch (ftype3 type) {
case NF3CHR:
case NF3BLK:
devicedata3 device;
case NF3SOCK:
case NF3FIFO:
sattr3 pipe_attributes;
default:
void;
};
struct MKNOD3args {
diropargs3 where;
mknoddata3 what;
};
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struct MKNOD3resok {
post_op_fh3 obj;
post_op_attr obj_attributes;
wcc_data dir_wcc;
};
struct MKNOD3resfail {
wcc_data dir_wcc;
};
union MKNOD3res switch (nfsstat3 status) {
case NFS3_OK:
MKNOD3resok resok;
default:
MKNOD3resfail resfail;
};
DESCRIPTION
Procedure MKNOD creates a new special file of the type,
what.type. Special files can be device files or named
pipes. On entry, the arguments in MKNOD3args are:
where
The location of the special file to be created:
dir
The file handle for the directory in which the
special file is to be created.
name
The name that is to be associated with the created
special file. Refer to General comments on filenames
on page 30.
what
A discriminated union identifying the type of the
special file to be created along with the data and
attributes appropriate to the type of the special
file:
type
The type of the object to be created.
When creating a character special file (what.type is
NF3CHR) or a block special file (what.type is NF3BLK),
what includes:
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device
A structure devicedata3 with the following components:
dev_attributes
The initial attributes for the special file.
spec
The major number stored in device.spec.specdata1 and
the minor number stored in device.spec.specdata2.
When creating a socket (what.type is NF3SOCK) or a FIFO
(what.type is NF3FIFO), what includes:
pipe_attributes
The initial attributes for the special file.
On successful return, MKNOD3res.status is NFS3_OK and
MKNOD3res.resok contains:
obj
The file handle for the newly created special file.
obj_attributes
The attributes for the newly created special file.
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-MKNOD directory attributes, these can be found in
dir_wcc.after.
Otherwise, MKNOD3res.status contains the error on failure
and MKNOD3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
where.dir. For a client that requires only the
post-MKNOD directory attributes, these can be found in
dir_wcc.after. Even though the MKNOD failed, full
wcc_data is returned to allow the client to determine
whether the failing MKNOD changed the directory.
IMPLEMENTATION
Refer to General comments on filenames on page 30.
Without explicit support for special file type creation in
the NFS version 2 protocol, fields in the CREATE arguments
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were overloaded to indicate creation of certain types of
objects. This overloading is not necessary in the NFS
version 3 protocol.
If the server does not support any of the defined types,
the error, NFS3ERR_NOTSUPP, should be returned. Otherwise,
if the server does not support the target type or the
target type is illegal, the error, NFS3ERR_BADTYPE, should
be returned. Note that NF3REG, NF3DIR, and NF3LNK are
illegal types for MKNOD. The procedures, CREATE, MKDIR,
and SYMLINK should be used to create these file types,
respectively, instead of MKNOD.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_NOTDIR
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_NAMETOOLONG
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
NFS3ERR_BADTYPE
SEE ALSO
CREATE, MKDIR, SYMLINK, and PATHCONF.
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3.3.12 Procedure 12: REMOVE - Remove a File
SYNOPSIS
REMOVE3res NFSPROC3_REMOVE(REMOVE3args) = 12;
struct REMOVE3args {
diropargs3 object;
};
struct REMOVE3resok {
wcc_data dir_wcc;
};
struct REMOVE3resfail {
wcc_data dir_wcc;
};
union REMOVE3res switch (nfsstat3 status) {
case NFS3_OK:
REMOVE3resok resok;
default:
REMOVE3resfail resfail;
};
DESCRIPTION
Procedure REMOVE removes (deletes) an entry from a
directory. If the entry in the directory was the last
reference to the corresponding file system object, the
object may be destroyed. On entry, the arguments in
REMOVE3args are:
object
A diropargs3 structure identifying the entry to be
removed:
dir
The file handle for the directory from which the entry
is to be removed.
name
The name of the entry to be removed. Refer to General
comments on filenames on page 30.
On successful return, REMOVE3res.status is NFS3_OK and
REMOVE3res.resok contains:
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dir_wcc
Weak cache consistency data for the directory,
object.dir. For a client that requires only the
post-REMOVE directory attributes, these can be found in
dir_wcc.after.
Otherwise, REMOVE3res.status contains the error on failure
and REMOVE3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
object.dir. For a client that requires only the
post-REMOVE directory attributes, these can be found in
dir_wcc.after. Even though the REMOVE failed, full
wcc_data is returned to allow the client to determine
whether the failing REMOVE changed the directory.
IMPLEMENTATION
In general, REMOVE is intended to remove non-directory
file objects and RMDIR is to be used to remove
directories. However, REMOVE can be used to remove
directories, subject to restrictions imposed by either the
client or server interfaces. This had been a source of
confusion in the NFS version 2 protocol.
The concept of last reference is server specific. However,
if the nlink field in the previous attributes of the
object had the value 1, the client should not rely on
referring to the object via a file handle. Likewise, the
client should not rely on the resources (disk space,
directory entry, and so on.) formerly associated with the
object becoming immediately available. Thus, if a client
needs to be able to continue to access a file after using
REMOVE to remove it, the client should take steps to make
sure that the file will still be accessible. The usual
mechanism used is to use RENAME to rename the file from
its old name to a new hidden name.
Refer to General comments on filenames on page 30.
ERRORS
NFS3ERR_NOENT
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_NOTDIR
NFS3ERR_NAMETOOLONG
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NFS3ERR_ROFS
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
RMDIR and RENAME.
3.3.13 Procedure 13: RMDIR - Remove a Directory
SYNOPSIS
RMDIR3res NFSPROC3_RMDIR(RMDIR3args) = 13;
struct RMDIR3args {
diropargs3 object;
};
struct RMDIR3resok {
wcc_data dir_wcc;
};
struct RMDIR3resfail {
wcc_data dir_wcc;
};
union RMDIR3res switch (nfsstat3 status) {
case NFS3_OK:
RMDIR3resok resok;
default:
RMDIR3resfail resfail;
};
DESCRIPTION
Procedure RMDIR removes (deletes) a subdirectory from a
directory. If the directory entry of the subdirectory is
the last reference to the subdirectory, the subdirectory
may be destroyed. On entry, the arguments in RMDIR3args
are:
object
A diropargs3 structure identifying the directory entry
to be removed:
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dir
The file handle for the directory from which the
subdirectory is to be removed.
name
The name of the subdirectory to be removed. Refer to
General comments on filenames on page 30.
On successful return, RMDIR3res.status is NFS3_OK and
RMDIR3res.resok contains:
dir_wcc
Weak cache consistency data for the directory,
object.dir. For a client that requires only the
post-RMDIR directory attributes, these can be found in
dir_wcc.after.
Otherwise, RMDIR3res.status contains the error on failure
and RMDIR3res.resfail contains the following:
dir_wcc
Weak cache consistency data for the directory,
object.dir. For a client that requires only the
post-RMDIR directory attributes, these can be found in
dir_wcc.after. Note that even though the RMDIR failed,
full wcc_data is returned to allow the client to
determine whether the failing RMDIR changed the
directory.
IMPLEMENTATION
Note that on some servers, removal of a non-empty
directory is disallowed.
On some servers, the filename, ".", is illegal. These
servers will return the error, NFS3ERR_INVAL. On some
servers, the filename, "..", is illegal. These servers
will return the error, NFS3ERR_EXIST. This would seem
inconsistent, but allows these servers to comply with
their own specific interface definitions. Clients should
be prepared to handle both cases.
The client should not rely on the resources (disk space,
directory entry, and so on.) formerly associated with the
directory becoming immediately available.
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ERRORS
NFS3ERR_NOENT
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_INVAL
NFS3ERR_EXIST
NFS3ERR_NOTDIR
NFS3ERR_NAMETOOLONG
NFS3ERR_ROFS
NFS3ERR_NOTEMPTY
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
REMOVE.
3.3.14 Procedure 14: RENAME - Rename a File or Directory
SYNOPSIS
RENAME3res NFSPROC3_RENAME(RENAME3args) = 14;
struct RENAME3args {
diropargs3 from;
diropargs3 to;
};
struct RENAME3resok {
wcc_data fromdir_wcc;
wcc_data todir_wcc;
};
struct RENAME3resfail {
wcc_data fromdir_wcc;
wcc_data todir_wcc;
};
union RENAME3res switch (nfsstat3 status) {
case NFS3_OK:
RENAME3resok resok;
default:
RENAME3resfail resfail;
};
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DESCRIPTION
Procedure RENAME renames the file identified by from.name
in the directory, from.dir, to to.name in the di- rectory,
to.dir. The operation is required to be atomic to the
client. To.dir and from.dir must reside on the same file
system and server. On entry, the arguments in RENAME3args
are:
from
A diropargs3 structure identifying the source (the file
system object to be re-named):
from.dir
The file handle for the directory from which the
entry is to be renamed.
from.name
The name of the entry that identifies the object to
be renamed. Refer to General comments on filenames
on page 30.
to
A diropargs3 structure identifying the target (the new
name of the object):
to.dir
The file handle for the directory to which the
object is to be renamed.
to.name
The new name for the object. Refer to General
comments on filenames on page 30.
If the directory, to.dir, already contains an entry with
the name, to.name, the source object must be compatible
with the target: either both are non-directories or both
are directories and the target must be empty. If
compatible, the existing target is removed before the
rename occurs. If they are not compatible or if the target
is a directory but not empty, the server should return the
error, NFS3ERR_EXIST.
On successful return, RENAME3res.status is NFS3_OK and
RENAME3res.resok contains:
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fromdir_wcc
Weak cache consistency data for the directory,
from.dir.
todir_wcc
Weak cache consistency data for the directory, to.dir.
Otherwise, RENAME3res.status contains the error on failure
and RENAME3res.resfail contains the following:
fromdir_wcc
Weak cache consistency data for the directory,
from.dir.
todir_wcc
Weak cache consistency data for the directory, to.dir.
IMPLEMENTATION
The RENAME operation must be atomic to the client. The
message "to.dir and from.dir must reside on the same file
system on the server, [or the operation will fail]" means
that the fsid fields in the attributes for the directories
are the same. If they reside on different file systems,
the error, NFS3ERR_XDEV, is returned. Even though the
operation is atomic, the status, NFS3ERR_MLINK, may be
returned if the server used a "unlink/link/unlink"
sequence internally.
A file handle may or may not become stale on a rename.
However, server implementors are strongly encouraged to
attempt to keep file handles from becoming stale in this
fashion.
On some servers, the filenames, "." and "..", are illegal
as either from.name or to.name. In addition, neither
from.name nor to.name can be an alias for from.dir. These
servers will return the error, NFS3ERR_INVAL, in these
cases.
If from and to both refer to the same file (they might
be hard links of each other), then RENAME should perform
no action and return NFS3_OK.
Refer to General comments on filenames on page 30.
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ERRORS
NFS3ERR_NOENT
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_XDEV
NFS3ERR_NOTDIR
NFS3ERR_ISDIR
NFS3ERR_INVAL
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_MLINK
NFS3ERR_NAMETOOLONG
NFS3ERR_NOTEMPTY
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
REMOVE and LINK.
3.3.15 Procedure 15: LINK - Create Link to an object
SYNOPSIS
LINK3res NFSPROC3_LINK(LINK3args) = 15;
struct LINK3args {
nfs_fh3 file;
diropargs3 link;
};
struct LINK3resok {
post_op_attr file_attributes;
wcc_data linkdir_wcc;
};
struct LINK3resfail {
post_op_attr file_attributes;
wcc_data linkdir_wcc;
};
union LINK3res switch (nfsstat3 status) {
case NFS3_OK:
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LINK3resok resok;
default:
LINK3resfail resfail;
};
DESCRIPTION
Procedure LINK creates a hard link from file to link.name,
in the directory, link.dir. file and link.dir must reside
on the same file system and server. On entry, the
arguments in LINK3args are:
file
The file handle for the existing file system object.
link
The location of the link to be created:
link.dir
The file handle for the directory in which the link
is to be created.
link.name
The name that is to be associated with the created
link. Refer to General comments on filenames on page
17.
On successful return, LINK3res.status is NFS3_OK and
LINK3res.resok contains:
file_attributes
The post-operation attributes of the file system object
identified by file.
linkdir_wcc
Weak cache consistency data for the directory,
link.dir.
Otherwise, LINK3res.status contains the error on failure
and LINK3res.resfail contains the following:
file_attributes
The post-operation attributes of the file system object
identified by file.
linkdir_wcc
Weak cache consistency data for the directory,
link.dir.
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IMPLEMENTATION
Changes to any property of the hard-linked files are
reflected in all of the linked files. When a hard link is
made to a file, the attributes for the file should have a
value for nlink that is one greater than the value before
the LINK.
The comments under RENAME regarding object and target
residing on the same file system apply here as well. The
comments regarding the target name applies as well. Refer
to General comments on filenames on page 30.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_EXIST
NFS3ERR_XDEV
NFS3ERR_NOTDIR
NFS3ERR_INVAL
NFS3ERR_NOSPC
NFS3ERR_ROFS
NFS3ERR_MLINK
NFS3ERR_NAMETOOLONG
NFS3ERR_DQUOT
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
SYMLINK, RENAME and FSINFO.
3.3.16 Procedure 16: READDIR - Read From Directory
SYNOPSIS
READDIR3res NFSPROC3_READDIR(READDIR3args) = 16;
struct READDIR3args {
nfs_fh3 dir;
cookie3 cookie;
cookieverf3 cookieverf;
count3 count;
};
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struct entry3 {
fileid3 fileid;
filename3 name;
cookie3 cookie;
entry3 *nextentry;
};
struct dirlist3 {
entry3 *entries;
bool eof;
};
struct READDIR3resok {
post_op_attr dir_attributes;
cookieverf3 cookieverf;
dirlist3 reply;
};
struct READDIR3resfail {
post_op_attr dir_attributes;
};
union READDIR3res switch (nfsstat3 status) {
case NFS3_OK:
READDIR3resok resok;
default:
READDIR3resfail resfail;
};
DESCRIPTION
Procedure READDIR retrieves a variable number of entries,
in sequence, from a directory and returns the name and
file identifier for each, with information to allow the
client to request additional directory entries in a
subsequent READDIR request. On entry, the arguments in
READDIR3args are:
dir
The file handle for the directory to be read.
cookie
This should be set to 0 in the first request to read
the directory. On subsequent requests, it should be a
cookie as returned by the server.
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cookieverf
This should be set to 0 in the first request to read
the directory. On subsequent requests, it should be a
cookieverf as returned by the server. The cookieverf
must match that returned by the READDIR in which the
cookie was acquired.
count
The maximum size of the READDIR3resok structure, in
bytes. The size must include all XDR overhead. The
server is free to return less than count bytes of
data.
On successful return, READDIR3res.status is NFS3_OK and
READDIR3res.resok contains:
dir_attributes
The attributes of the directory, dir.
cookieverf
The cookie verifier.
reply
The directory list:
entries
Zero or more directory (entry3) entries.
eof
TRUE if the last member of reply.entries is the last
entry in the directory or the list reply.entries is
empty and the cookie corresponded to the end of the
directory. If FALSE, there may be more entries to
read.
Otherwise, READDIR3res.status contains the error on
failure and READDIR3res.resfail contains the following:
dir_attributes
The attributes of the directory, dir.
IMPLEMENTATION
In the NFS version 2 protocol, each directory entry
returned included a cookie identifying a point in the
directory. By including this cookie in a subsequent
READDIR, the client could resume the directory read at any
point in the directory. One problem with this scheme was
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that there was no easy way for a server to verify that a
cookie was valid. If two READDIRs were separated by one or
more operations that changed the directory in some way
(for example, reordering or compressing it), it was
possible that the second READDIR could miss entries, or
process entries more than once. If the cookie was no
longer usable, for example, pointing into the middle of a
directory entry, the server would have to either round the
cookie down to the cookie of the previous entry or round
it up to the cookie of the next entry in the directory.
Either way would possibly lead to incorrect results and
the client would be unaware that any problem existed.
In the NFS version 3 protocol, each READDIR request
includes both a cookie and a cookie verifier. For the
first call, both are set to 0. The response includes a
new cookie verifier, with a cookie per entry. For
subsequent READDIRs, the client must present both the
cookie and the corresponding cookie verifier. If the
server detects that the cookie is no longer valid, the
server will reject the READDIR request with the status,
NFS3ERR_BAD_COOKIE. The client should be careful to
avoid holding directory entry cookies across operations
that modify the directory contents, such as REMOVE and
CREATE.
One implementation of the cookie-verifier mechanism might
be for the server to use the modification time of the
directory. This might be overly restrictive, however. A
better approach would be to record the time of the last
directory modification that changed the directory
organization in a way that would make it impossible to
reliably interpret a cookie. Servers in which directory
cookies are always valid are free to use zero as the
verifier always.
The server may return fewer than count bytes of
XDR-encoded entries. The count specified by the client in
the request should be greater than or equal to FSINFO
dtpref.
Since UNIX clients give a special meaning to the fileid
value zero, UNIX clients should be careful to map zero
fileid values to some other value and servers should try
to avoid sending a zero fileid.
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ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_NOTDIR
NFS3ERR_BAD_COOKIE
NFS3ERR_TOOSMALL
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
READDIRPLUS and FSINFO.
3.3.17 Procedure 17: READDIRPLUS - Extended read from directory
SYNOPSIS
READDIRPLUS3res NFSPROC3_READDIRPLUS(READDIRPLUS3args) = 17;
struct READDIRPLUS3args {
nfs_fh3 dir;
cookie3 cookie;
cookieverf3 cookieverf;
count3 dircount;
count3 maxcount;
};
struct entryplus3 {
fileid3 fileid;
filename3 name;
cookie3 cookie;
post_op_attr name_attributes;
post_op_fh3 name_handle;
entryplus3 *nextentry;
};
struct dirlistplus3 {
entryplus3 *entries;
bool eof;
};
struct READDIRPLUS3resok {
post_op_attr dir_attributes;
cookieverf3 cookieverf;
dirlistplus3 reply;
};
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struct READDIRPLUS3resfail {
post_op_attr dir_attributes;
};
union READDIRPLUS3res switch (nfsstat3 status) {
case NFS3_OK:
READDIRPLUS3resok resok;
default:
READDIRPLUS3resfail resfail;
};
DESCRIPTION
Procedure READDIRPLUS retrieves a variable number of
entries from a file system directory and returns complete
information about each along with information to allow the
client to request additional directory entries in a
subsequent READDIRPLUS. READDIRPLUS differs from READDIR
only in the amount of information returned for each
entry. In READDIR, each entry returns the filename and
the fileid. In READDIRPLUS, each entry returns the name,
the fileid, attributes (including the fileid), and file
handle. On entry, the arguments in READDIRPLUS3args are:
dir
The file handle for the directory to be read.
cookie
This should be set to 0 on the first request to read a
directory. On subsequent requests, it should be a
cookie as returned by the server.
cookieverf
This should be set to 0 on the first request to read a
directory. On subsequent requests, it should be a
cookieverf as returned by the server. The cookieverf
must match that returned by the READDIRPLUS call in
which the cookie was acquired.
dircount
The maximum number of bytes of directory information
returned. This number should not include the size of
the attributes and file handle portions of the result.
maxcount
The maximum size of the READDIRPLUS3resok structure, in
bytes. The size must include all XDR overhead. The
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server is free to return fewer than maxcount bytes of
data.
On successful return, READDIRPLUS3res.status is NFS3_OK
and READDIRPLUS3res.resok contains:
dir_attributes
The attributes of the directory, dir.
cookieverf
The cookie verifier.
reply
The directory list:
entries
Zero or more directory (entryplus3) entries.
eof
TRUE if the last member of reply.entries is the last
entry in the directory or the list reply.entries is
empty and the cookie corresponded to the end of the
directory. If FALSE, there may be more entries to
read.
Otherwise, READDIRPLUS3res.status contains the error on
failure and READDIRPLUS3res.resfail contains the following:
dir_attributes
The attributes of the directory, dir.
IMPLEMENTATION
Issues that need to be understood for this procedure
include increased cache flushing activity on the client
(as new file handles are returned with names which are
entered into caches) and over-the-wire overhead versus
expected subsequent LOOKUP elimination. It is thought that
this procedure may improve performance for directory
browsing where attributes are always required as on the
Apple Macintosh operating system and for MS-DOS.
The dircount and maxcount fields are included as an
optimization. Consider a READDIRPLUS call on a UNIX
operating system implementation for 1048 bytes; the reply
does not contain many entries because of the overhead due
to attributes and file handles. An alternative is to issue
a READDIRPLUS call for 8192 bytes and then only use the
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first 1048 bytes of directory information. However, the
server doesn't know that all that is needed is 1048 bytes
of directory information (as would be returned by
READDIR). It sees the 8192 byte request and issues a
VOP_READDIR for 8192 bytes. It then steps through all of
those directory entries, obtaining attributes and file
handles for each entry. When it encodes the result, the
server only encodes until it gets 8192 bytes of results
which include the attributes and file handles. Thus, it
has done a larger VOP_READDIR and many more attribute
fetches than it needed to. The ratio of the directory
entry size to the size of the attributes plus the size of
the file handle is usually at least 8 to 1. The server has
done much more work than it needed to.
The solution to this problem is for the client to provide
two counts to the server. The first is the number of bytes
of directory information that the client really wants,
dircount. The second is the maximum number of bytes in
the result, including the attributes and file handles,
maxcount. Thus, the server will issue a VOP_READDIR for
only the number of bytes that the client really wants to
get, not an inflated number. This should help to reduce
the size of VOP_READDIR requests on the server, thus
reducing the amount of work done there, and to reduce the
number of VOP_LOOKUP, VOP_GETATTR, and other calls done by
the server to construct attributes and file handles.
ERRORS
NFS3ERR_IO
NFS3ERR_ACCES
NFS3ERR_NOTDIR
NFS3ERR_BAD_COOKIE
NFS3ERR_TOOSMALL
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_NOTSUPP
NFS3ERR_SERVERFAULT
SEE ALSO
READDIR.
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3.3.18 Procedure 18: FSSTAT - Get dynamic file system information
SYNOPSIS
FSSTAT3res NFSPROC3_FSSTAT(FSSTAT3args) = 18;
struct FSSTAT3args {
nfs_fh3 fsroot;
};
struct FSSTAT3resok {
post_op_attr obj_attributes;
size3 tbytes;
size3 fbytes;
size3 abytes;
size3 tfiles;
size3 ffiles;
size3 afiles;
uint32 invarsec;
};
struct FSSTAT3resfail {
post_op_attr obj_attributes;
};
union FSSTAT3res switch (nfsstat3 status) {
case NFS3_OK:
FSSTAT3resok resok;
default:
FSSTAT3resfail resfail;
};
DESCRIPTION
Procedure FSSTAT retrieves volatile file system state
information. On entry, the arguments in FSSTAT3args are:
fsroot
A file handle identifying a object in the file system.
This is normally a file handle for a mount point for a
file system, as originally obtained from the MOUNT
service on the server.
On successful return, FSSTAT3res.status is NFS3_OK and
FSSTAT3res.resok contains:
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obj_attributes
The attributes of the file system object specified in
fsroot.
tbytes
The total size, in bytes, of the file system.
fbytes
The amount of free space, in bytes, in the file
system.
abytes
The amount of free space, in bytes, available to the
user identified by the authentication information in
the RPC. (This reflects space that is reserved by the
file system; it does not reflect any quota system
implemented by the server.)
tfiles
The total number of file slots in the file system. (On
a UNIX server, this often corresponds to the number of
inodes configured.)
ffiles
The number of free file slots in the file system.
afiles
The number of free file slots that are available to the
user corresponding to the authentication information in
the RPC. (This reflects slots that are reserved by the
file system; it does not reflect any quota system
implemented by the server.)
invarsec
A measure of file system volatility: this is the number
of seconds for which the file system is not expected to
change. For a volatile, frequently updated file system,
this will be 0. For an immutable file system, such as a
CD-ROM, this would be the largest unsigned integer. For
file systems that are infrequently modified, for
example, one containing local executable programs and
on-line documentation, a value corresponding to a few
hours or days might be used. The client may use this as
a hint in tuning its cache management. Note however,
this measure is assumed to be dynamic and may change at
any time.
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Otherwise, FSSTAT3res.status contains the error on failure
and FSSTAT3res.resfail contains the following:
obj_attributes
The attributes of the file system object specified in
fsroot.
IMPLEMENTATION
Not all implementations can support the entire list of
attributes. It is expected that servers will make a best
effort at supporting all the attributes.
ERRORS
NFS3ERR_IO
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
FSINFO.
3.3.19 Procedure 19: FSINFO - Get static file system Information
SYNOPSIS
FSINFO3res NFSPROC3_FSINFO(FSINFO3args) = 19;
const FSF3_LINK = 0x0001;
const FSF3_SYMLINK = 0x0002;
const FSF3_HOMOGENEOUS = 0x0008;
const FSF3_CANSETTIME = 0x0010;
struct FSINFOargs {
nfs_fh3 fsroot;
};
struct FSINFO3resok {
post_op_attr obj_attributes;
uint32 rtmax;
uint32 rtpref;
uint32 rtmult;
uint32 wtmax;
uint32 wtpref;
uint32 wtmult;
uint32 dtpref;
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size3 maxfilesize;
nfstime3 time_delta;
uint32 properties;
};
struct FSINFO3resfail {
post_op_attr obj_attributes;
};
union FSINFO3res switch (nfsstat3 status) {
case NFS3_OK:
FSINFO3resok resok;
default:
FSINFO3resfail resfail;
};
DESCRIPTION
Procedure FSINFO retrieves nonvolatile file system state
information and general information about the NFS version
3 protocol server implementation. On entry, the arguments
in FSINFO3args are:
fsroot
A file handle identifying a file object. Normal usage
is to provide a file handle for a mount point for a
file system, as originally obtained from the MOUNT
service on the server.
On successful return, FSINFO3res.status is NFS3_OK and
FSINFO3res.resok contains:
obj_attributes
The attributes of the file system object specified in
fsroot.
rtmax
The maximum size in bytes of a READ request supported
by the server. Any READ with a number greater than
rtmax will result in a short read of rtmax bytes or
less.
rtpref
The preferred size of a READ request. This should be
the same as rtmax unless there is a clear benefit in
performance or efficiency.
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rtmult
The suggested multiple for the size of a READ request.
wtmax
The maximum size of a WRITE request supported by the
server. In general, the client is limited by wtmax
since there is no guarantee that a server can handle a
larger write. Any WRITE with a count greater than wtmax
will result in a short write of at most wtmax bytes.
wtpref
The preferred size of a WRITE request. This should be
the same as wtmax unless there is a clear benefit in
performance or efficiency.
wtmult
The suggested multiple for the size of a WRITE
request.
dtpref
The preferred size of a READDIR request.
maxfilesize
The maximum size of a file on the file system.
time_delta
The server time granularity. When setting a file time
using SETATTR, the server guarantees only to preserve
times to this accuracy. If this is {0, 1}, the server
can support nanosecond times, {0, 1000000} denotes
millisecond precision, and {1, 0} indicates that times
are accurate only to the nearest second.
properties
A bit mask of file system properties. The following
values are defined:
FSF_LINK
If this bit is 1 (TRUE), the file system supports
hard links.
FSF_SYMLINK
If this bit is 1 (TRUE), the file system supports
symbolic links.
FSF_HOMOGENEOUS
If this bit is 1 (TRUE), the information returned by
PATHCONF is identical for every file and directory
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in the file system. If it is 0 (FALSE), the client
should retrieve PATHCONF information for each file
and directory as required.
FSF_CANSETTIME
If this bit is 1 (TRUE), the server will set the
times for a file via SETATTR if requested (to the
accuracy indicated by time_delta). If it is 0
(FALSE), the server cannot set times as requested.
Otherwise, FSINFO3res.status contains the error on failure
and FSINFO3res.resfail contains the following:
attributes
The attributes of the file system object specified in
fsroot.
IMPLEMENTATION
Not all implementations can support the entire list of
attributes. It is expected that a server will make a best
effort at supporting all the attributes.
The file handle provided is expected to be the file handle
of the file system root, as returned to the MOUNT
operation. Since mounts may occur anywhere within an
exported tree, the server should expect FSINFO requests
specifying file handles within the exported file system.
A server may export different types of file systems with
different attributes returned to the FSINFO call. The
client should retrieve FSINFO information for each mount
completed. Though a server may return different FSINFO
information for different files within a file system,
there is no requirement that a client obtain FSINFO
information for other than the file handle returned at
mount.
The maxfilesize field determines whether a server's
particular file system uses 32 bit sizes and offsets or 64
bit file sizes and offsets. This may affect a client's
processing.
The preferred sizes for requests are nominally tied to an
exported file system mounted by a client. A surmountable
issue arises in that the transfer size for an NFS version
3 protocol request is not only dependent on
characteristics of the file system but also on
characteristics of the network interface, particularly the
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maximum transfer unit (MTU). A server implementation can
advertise different transfer sizes (for the fields, rtmax,
rtpref, wtmax, wtpref, and dtpref) depending on the
interface on which the FSINFO request is received. This is
an implementation issue.
ERRORS
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
READLINK, WRITE, READDIR, FSSTAT and PATHCONF.
3.3.20 Procedure 20: PATHCONF - Retrieve POSIX information
SYNOPSIS
PATHCONF3res NFSPROC3_PATHCONF(PATHCONF3args) = 20;
struct PATHCONF3args {
nfs_fh3 object;
};
struct PATHCONF3resok {
post_op_attr obj_attributes;
uint32 linkmax;
uint32 name_max;
bool no_trunc;
bool chown_restricted;
bool case_insensitive;
bool case_preserving;
};
struct PATHCONF3resfail {
post_op_attr obj_attributes;
};
union PATHCONF3res switch (nfsstat3 status) {
case NFS3_OK:
PATHCONF3resok resok;
default:
PATHCONF3resfail resfail;
};
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DESCRIPTION
Procedure PATHCONF retrieves the pathconf information for
a file or directory. If the FSF_HOMOGENEOUS bit is set in
FSFINFO3resok.properties, the pathconf information will be
the same for all files and directories in the exported
file system in which this file or directory resides. On
entry, the arguments in PATHCONF3args are:
object
The file handle for the file system object.
On successful return, PATHCONF3res.status is NFS3_OK and
PATHCONF3res.resok contains:
obj_attributes
The attributes of the object specified by object.
linkmax
The maximum number of hard links to an object.
name_max
The maximum length of a component of a filename.
no_trunc
If TRUE, the server will reject any request that
includes a name longer than name_max with the error,
NFS3ERR_NAMETOOLONG. If FALSE, any length name over
name_max bytes will be silently truncated to name_max
bytes.
chown_restricted
If TRUE, the server will reject any request to change
either the owner or the group associated with a file if
the caller is not the privileged user. (Uid 0.)
case_insensitive
If TRUE, the server file system does not distinguish
case when interpreting filenames.
case_preserving
If TRUE, the server file system will preserve the case
of a name during a CREATE, MKDIR, MKNOD, SYMLINK,
RENAME, or LINK operation.
Otherwise, PATHCONF3res.status contains the error on
failure and PATHCONF3res.resfail contains the following:
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obj_attributes
The attributes of the object specified by object.
IMPLEMENTATION
In some implementations of the NFS version 2 protocol,
pathconf information was obtained at mount time through
the MOUNT protocol. The proper place to obtain it, is as
here, in the NFS version 3 protocol itself.
ERRORS
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
LOOKUP, CREATE, MKDIR, SYMLINK, MKNOD, RENAME, LINK and FSINFO.
3.3.21 Procedure 21: COMMIT - Commit cached data on a server to stable
storage
SYNOPSIS
COMMIT3res NFSPROC3_COMMIT(COMMIT3args) = 21;
struct COMMIT3args {
nfs_fh3 file;
offset3 offset;
count3 count;
};
struct COMMIT3resok {
wcc_data file_wcc;
writeverf3 verf;
};
struct COMMIT3resfail {
wcc_data file_wcc;
};
union COMMIT3res switch (nfsstat3 status) {
case NFS3_OK:
COMMIT3resok resok;
default:
COMMIT3resfail resfail;
};
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DESCRIPTION
Procedure COMMIT forces or flushes data to stable storage
that was previously written with a WRITE procedure call
with the stable field set to UNSTABLE. On entry, the
arguments in COMMIT3args are:
file
The file handle for the file to which data is to be
flushed (committed). This must identify a file system
object of type, NF3REG.
offset
The position within the file at which the flush is to
begin. An offset of 0 means to flush data starting at
the beginning of the file.
count
The number of bytes of data to flush. If count is 0, a
flush from offset to the end of file is done.
On successful return, COMMIT3res.status is NFS3_OK and
COMMIT3res.resok contains:
file_wcc
Weak cache consistency data for the file. For a client
that requires only the post-operation file attributes,
these can be found in file_wcc.after.
verf
This is a cookie that the client can use to determine
whether the server has rebooted between a call to WRITE
and a subsequent call to COMMIT. This cookie must be
consistent during a single boot session and must be
unique between instances of the NFS version 3 protocol
server where uncommitted data may be lost.
Otherwise, COMMIT3res.status contains the error on failure
and COMMIT3res.resfail contains the following:
file_wcc
Weak cache consistency data for the file. For a client
that requires only the post-write file attributes,
these can be found in file_wcc.after. Even though the
COMMIT failed, full wcc_data is returned to allow the
client to determine whether the file changed on the
server between calls to WRITE and COMMIT.
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IMPLEMENTATION
Procedure COMMIT is similar in operation and semantics to
the POSIX fsync(2) system call that synchronizes a file's
state with the disk, that is it flushes the file's data
and metadata to disk. COMMIT performs the same operation
for a client, flushing any unsynchronized data and
metadata on the server to the server's disk for the
specified file. Like fsync(2), it may be that there is
some modified data or no modified data to synchronize. The
data may have been synchronized by the server's normal
periodic buffer synchronization activity. COMMIT will
always return NFS3_OK, unless there has been an unexpected
error.
COMMIT differs from fsync(2) in that it is possible for
the client to flush a range of the file (most likely
triggered by a buffer-reclamation scheme on the client
before file has been completely written).
The server implementation of COMMIT is reasonably simple.
If the server receives a full file COMMIT request, that is
starting at offset 0 and count 0, it should do the
equivalent of fsync()'ing the file. Otherwise, it should
arrange to have the cached data in the range specified by
offset and count to be flushed to stable storage. In both
cases, any metadata associated with the file must be
flushed to stable storage before returning. It is not an
error for there to be nothing to flush on the server.
This means that the data and metadata that needed to be
flushed have already been flushed or lost during the last
server failure.
The client implementation of COMMIT is a little more
complex. There are two reasons for wanting to commit a
client buffer to stable storage. The first is that the
client wants to reuse a buffer. In this case, the offset
and count of the buffer are sent to the server in the
COMMIT request. The server then flushes any cached data
based on the offset and count, and flushes any metadata
associated with the file. It then returns the status of
the flush and the verf verifier. The other reason for the
client to generate a COMMIT is for a full file flush, such
as may be done at close. In this case, the client would
gather all of the buffers for this file that contain
uncommitted data, do the COMMIT operation with an offset
of 0 and count of 0, and then free all of those buffers.
Any other dirty buffers would be sent to the server in the
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normal fashion.
This implementation will require some modifications to the
buffer cache on the client. After a buffer is written with
stable UNSTABLE, it must be considered as dirty by the
client system until it is either flushed via a COMMIT
operation or written via a WRITE operation with stable set
to FILE_SYNC or DATA_SYNC. This is done to prevent the
buffer from being freed and reused before the data can be
flushed to stable storage on the server.
When a response comes back from either a WRITE or a COMMIT
operation that contains an unexpected verf, the client
will need to retransmit all of the buffers containing
uncommitted cached data to the server. How this is to be
done is up to the implementor. If there is only one buffer
of interest, then it should probably be sent back over in
a WRITE request with the appropriate stable flag. If there
more than one, it might be worthwhile retransmitting all
of the buffers in WRITE requests with stable set to
UNSTABLE and then retransmitting the COMMIT operation to
flush all of the data on the server to stable storage. The
timing of these retransmissions is left to the
implementor.
The above description applies to page-cache-based systems
as well as buffer-cache-based systems. In those systems,
the virtual memory system will need to be modified instead
of the buffer cache.
See additional comments on WRITE on page 49.
ERRORS
NFS3ERR_IO
NFS3ERR_STALE
NFS3ERR_BADHANDLE
NFS3ERR_SERVERFAULT
SEE ALSO
WRITE.
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4. Implementation issues
The NFS version 3 protocol was designed to allow different
operating systems to share files. However, since it was
designed in a UNIX environment, many operations have
semantics similar to the operations of the UNIX file system.
This section discusses some of the general
implementation-specific details and semantic issues.
Procedure descriptions have implementation comments specific
to that procedure.
A number of papers have been written describing issues
encountered when constructing an NFS version 2 protocol
implementation. The best overview paper is still [Sandberg].
[Israel], [Macklem], and [Pawlowski] describe other
implementations. [X/OpenNFS] provides a complete description
of the NFS version 2 protocol and supporting protocols, as
well as a discussion on implementation issues and procedure
and error semantics. Many of the issues encountered when
constructing an NFS version 2 protocol implementation will be
encountered when constructing an NFS version 3 protocol
implementation.
4.1 Multiple version support
The RPC protocol provides explicit support for versioning of
a service. Client and server implementations of NFS version 3
protocol should support both versions, for full backwards
compatibility, when possible. Default behavior of the RPC
binding protocol is the client and server bind using the
highest version number they both support. Client or server
implementations that cannot easily support both versions (for
example, because of memory restrictions) will have to choose
what version to support. The NFS version 2 protocol would be
a safe choice since fully capable clients and servers should
support both versions. However, this choice would need to be
made keeping all requirements in mind.
4.2 Server/client relationship
The NFS version 3 protocol is designed to allow servers to be
as simple and general as possible. Sometimes the simplicity
of the server can be a problem, if the client implements
complicated file system semantics.
For example, some operating systems allow removal of open
files. A process can open a file and, while it is open,
remove it from the directory. The file can be read and
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written as long as the process keeps it open, even though the
file has no name in the file system. It is impossible for a
stateless server to implement these semantics. The client
can do some tricks such as renaming the file on remove (to a
hidden name), and only physically deleting it on close. The
NFS version 3 protocol provides sufficient functionality to
implement most file system semantics on a client.
Every NFS version 3 protocol client can also potentially be a
server, and remote and local mounted file systems can be
freely mixed. This leads to some problems when a client
travels down the directory tree of a remote file system and
reaches the mount point on the server for another remote file
system. Allowing the server to follow the second remote mount
would require loop detection, server lookup, and user
revalidation. Instead, both NFS version 2 protocol and NFS
version 3 protocol implementations do not typically let
clients cross a server's mount point. When a client does a
LOOKUP on a directory on which the server has mounted a file
system, the client sees the underlying directory instead of
the mounted directory.
For example, if a server has a file system called /usr and
mounts another file system on /usr/src, if a client mounts
/usr, it does not see the mounted version of /usr/src. A
client could do remote mounts that match the server's mount
points to maintain the server's view. In this example, the
client would also have to mount /usr/src in addition to /usr,
even if they are from the same server.
4.3 Path name interpretation
There are a few complications to the rule that path names are
always parsed on the client. For example, symbolic links
could have different interpretations on different clients.
There is no answer to this problem in this specification.
Another common problem for non-UNIX implementations is the
special interpretation of the pathname, "..", to mean the
parent of a given directory. A future revision of the
protocol may use an explicit flag to indicate the parent
instead - however it is not a problem as many working
non-UNIX implementations exist.
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4.4 Permission issues
The NFS version 3 protocol, strictly speaking, does not
define the permission checking used by servers. However, it
is expected that a server will do normal operating system
permission checking using AUTH_UNIX style authentication as
the basis of its protection mechanism, or another stronger
form of authentication such as AUTH_DES or AUTH_KERB. With
AUTH_UNIX authentication, the server gets the client's
effective uid, effective gid, and groups on each call and
uses them to check permission. These are the so-called UNIX
credentials. AUTH_DES and AUTH_KERB use a network name, or
netname, as the basis for identification (from which a UNIX
server derives the necessary standard UNIX credentials).
There are problems with this method that have been solved.
Using uid and gid implies that the client and server share
the same uid list. Every server and client pair must have the
same mapping from user to uid and from group to gid. Since
every client can also be a server, this tends to imply that
the whole network shares the same uid/gid space. If this is
not the case, then it usually falls upon the server to
perform some custom mapping of credentials from one
authentication domain into another. A discussion of
techniques for managing a shared user space or for providing
mechanisms for user ID mapping is beyond the scope of this
specification.
Another problem arises due to the usually stateful open
operation. Most operating systems check permission at open
time, and then check that the file is open on each read and
write request. With stateless servers, the server cannot
detect that the file is open and must do permission checking
on each read and write call. UNIX client semantics of access
permission checking on open can be provided with the ACCESS
procedure call in this revision, which allows a client to
explicitly check access permissions without resorting to
trying the operation. On a local file system, a user can open
a file and then change the permissions so that no one is
allowed to touch it, but will still be able to write to the
file because it is open. On a remote file system, by
contrast, the write would fail. To get around this problem,
the server's permission checking algorithm should allow the
owner of a file to access it regardless of the permission
setting. This is needed in a practical NFS version 3 protocol
server implementation, but it does depart from correct local
file system semantics. This should not affect the return
result of access permissions as returned by the ACCESS
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procedure, however.
A similar problem has to do with paging in an executable
program over the network. The operating system usually checks
for execute permission before opening a file for demand
paging, and then reads blocks from the open file. In a local
UNIX file system, an executable file does not need read
permission to execute (pagein). An NFS version 3 protocol
server can not tell the difference between a normal file read
(where the read permission bit is meaningful) and a demand
pagein read (where the server should allow access to the
executable file if the execute bit is set for that user or
group or public). To make this work, the server allows
reading of files if the uid given in the call has either
execute or read permission on the file, through ownership,
group membership or public access. Again, this departs from
correct local file system semantics.
In most operating systems, a particular user (on UNIX, the
uid 0) has access to all files, no matter what permission and
ownership they have. This superuser permission may not be
allowed on the server, since anyone who can become superuser
on their client could gain access to all remote files. A UNIX
server by default maps uid 0 to a distinguished value
(UID_NOBODY), as well as mapping the groups list, before
doing its access checking. A server implementation may
provide a mechanism to change this mapping. This works except
for NFS version 3 protocol root file systems (required for
diskless NFS version 3 protocol client support), where
superuser access cannot be avoided. Export options are used,
on the server, to restrict the set of clients allowed
superuser access.
4.5 Duplicate request cache
The typical NFS version 3 protocol failure recovery model
uses client time-out and retry to handle server crashes,
network partitions, and lost server replies. A retried
request is called a duplicate of the original.
When used in a file server context, the term idempotent can
be used to distinguish between operation types. An idempotent
request is one that a server can perform more than once with
equivalent results (though it may in fact change, as a side
effect, the access time on a file, say for READ). Some NFS
operations are obviously non-idempotent. They cannot be
reprocessed without special attention simply because they may
fail if tried a second time. The CREATE request, for example,
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can be used to create a file for which the owner does not
have write permission. A duplicate of this request cannot
succeed if the original succeeded. Likewise, a file can be
removed only once.
The side effects caused by performing a duplicate
non-idempotent request can be destructive (for example, a
truncate operation causing lost writes). The combination of a
stateless design with the common choice of an unreliable
network transport (UDP) implies the possibility of
destructive replays of non-idempotent requests. Though to be
more accurate, it is the inherent stateless design of the NFS
version 3 protocol on top of an unreliable RPC mechanism that
yields the possibility of destructive replays of
non-idempotent requests, since even in an implementation of
the NFS version 3 protocol over a reliable
connection-oriented transport, a connection break with
automatic reestablishment requires duplicate request
processing (the client will retransmit the request, and the
server needs to deal with a potential duplicate
non-idempotent request).
Most NFS version 3 protocol server implementations use a
cache of recent requests (called the duplicate request cache)
for the processing of duplicate non-idempotent requests. The
duplicate request cache provides a short-term memory
mechanism in which the original completion status of a
request is remembered and the operation attempted only once.
If a duplicate copy of this request is received, then the
original completion status is returned.
The duplicate-request cache mechanism has been useful in
reducing destructive side effects caused by duplicate NFS
version 3 protocol requests. This mechanism, however, does
not guarantee against these destructive side effects in all
failure modes. Most servers store the duplicate request cache
in RAM, so the contents are lost if the server crashes. The
exception to this may possibly occur in a redundant server
approach to high availability, where the file system itself
may be used to share the duplicate request cache state. Even
if the cache survives server reboots (or failovers in the
high availability case), its effectiveness is a function of
its size. A network partition can cause a cache entry to be
reused before a client receives a reply for the corresponding
request. If this happens, the duplicate request will be
processed as a new one, possibly with destructive side
effects.
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A good description of the implementation and use of a
duplicate request cache can be found in [Juszczak].
4.6 File name component handling
Server implementations of NFS version 3 protocol will
frequently impose restrictions on the names which can be
created. Many servers will also forbid the use of names that
contain certain characters, such as the path component
separator used by the server operating system. For example,
the UFS file system will reject a name which contains "/",
while "." and ".." are distinguished in UFS, and may not be
specified as the name when creating a file system object.
The exact error status values return for these errors is
specified in the description of each procedure argument. The
values (which conform to NFS version 2 protocol server
practice) are not necessarily obvious, nor are they
consistent from one procedure to the next.
4.7 Synchronous modifying operations
Data-modifying operations in the NFS version 3 protocol are
synchronous. When a procedure returns to the client, the
client can assume that the operation has completed and any
data associated with the request is now on stable storage.
4.8 Stable storage
NFS version 3 protocol servers must be able to recover
without data loss from multiple power failures (including
cascading power failures, that is, several power failures in
quick succession), operating system failures, and hardware
failure of components other than the storage medium itself
(for example, disk, nonvolatile RAM).
Some examples of stable storage that are allowable for an NFS
server include:
1. Media commit of data, that is, the modified data has
been successfully written to the disk media, for example,
the disk platter.
2. An immediate reply disk drive with battery-backed
on-drive intermediate storage or uninterruptible power
system (UPS).
3. Server commit of data with battery-backed intermediate
storage and recovery software.
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4. Cache commit with uninterruptible power system (UPS) and
recovery software.
Conversely, the following are not examples of stable
storage:
1. An immediate reply disk drive without battery-backed
on-drive intermediate storage or uninterruptible power
system (UPS).
2. Cache commit without both uninterruptible power system
(UPS) and recovery software.
The only exception to this (introduced in this protocol
revision) is as described under the WRITE procedure on the
handling of the stable bit, and the use of the COMMIT
procedure. It is the use of the synchronous COMMIT procedure
that provides the necessary semantic support in the NFS
version 3 protocol.
4.9 Lookups and name resolution
A common objection to the NFS version 3 protocol is the
philosophy of component-by-component LOOKUP by the client in
resolving a name. The objection is that this is inefficient,
as latencies for component-by-component LOOKUP would be
unbearable.
Implementation practice solves this issue. A name cache,
providing component to file-handle mapping, is kept on the
client to short circuit actual LOOKUP invocations over the
wire. The cache is subject to cache timeout parameters that
bound attributes.
4.10 Adaptive retransmission
Most client implementations use either an exponential
back-off strategy to some maximum retransmission value, or a
more adaptive strategy that attempts congestion avoidance.
Congestion avoidance schemes in NFS request retransmission
are modelled on the work presented in [Jacobson]. [Nowicki]
and [Macklem] describe congestion avoidance schemes to be
applied to the NFS protocol over UDP.
4.11 Caching policies
The NFS version 3 protocol does not define a policy for
caching on the client or server. In particular, there is no
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support for strict cache consistency between a client and
server, nor between different clients. See [Kazar] for a
discussion of the issues of cache synchronization and
mechanisms in several distributed file systems.
4.12 Stable versus unstable writes
The setting of the stable field in the WRITE arguments, that
is whether or not to do asynchronous WRITE requests, is
straightforward on a UNIX client. If the NFS version 3
protocol client receives a write request that is not marked
as being asynchronous, it should generate the RPC with stable
set to TRUE. If the request is marked as being asynchronous,
the RPC should be generated with stable set to FALSE. If the
response comes back with the committed field set to TRUE, the
client should just mark the write request as done and no
further action is required. If committed is set to FALSE,
indicating that the buffer was not synchronized with the
server's disk, the client will need to mark the buffer in
some way which indicates that a copy of the buffer lives on
the server and that a new copy does not need to be sent to
the server, but that a commit is required.
Note that this algorithm introduces a new state for buffers,
thus there are now three states for buffers. The three states
are dirty, done but needs to be committed, and done. This
extra state on the client will likely require modifications
to the system outside of the NFS version 3 protocol client.
One proposal that was rejected was the addition of a boolean
commit argument to the WRITE operation. It would be used to
indicate whether the server should do a full file commit
after doing the write. This seems as if it could be useful if
the client knew that it was doing the last write on the file.
It is difficult to see how this could be used, given existing
client architectures though.
The asynchronous write opens up the window of problems
associated with write sharing. For example: client A writes
some data asynchronously. Client A is still holding the
buffers cached, waiting to commit them later. Client B reads
the modified data and writes it back to the server. The
server then crashes. When it comes back up, client A issues a
COMMIT operation which returns with a different cookie as
well as changed attributes. In this case, the correct action
may or may not be to retransmit the cached buffers.
Unfortunately, client A can't tell for sure, so it will need
to retransmit the buffers, thus overwriting the changes from
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client B. Fortunately, write sharing is rare and the
solution matches the current write sharing situation. Without
using locking for synchronization, the behaviour will be
indeterminate.
In a high availability (redundant system) server
implementation, two cases exist which relate to the verf
changing. If the high availability server implementation
does not use a shared-memory scheme, then the verf should
change on failover, since the unsynchronized data is not
available to the second processor and there is no guarantee
that the system which had the data cached was able to flush
it to stable storage before going down. The client will need
to retransmit the data to be safe. In a shared-memory high
availability server implementation, the verf would not need
to change because the server would still have the cached data
available to it to be flushed. The exact policy regarding the
verf in a shared memory high availability implementation,
however, is up to the server implementor.
4.13 32 bit clients/servers and 64 bit clients/servers
The 64 bit nature of the NFS version 3 protocol introduces
several compatibility problems. The most notable two are
mismatched clients and servers, that is, a 32 bit client and
a 64 bit server or a 64 bit client and a 32 bit server.
The problems of a 64 bit client and a 32 bit server are easy
to handle. The client will never encounter a file that it can
not handle. If it sends a request to the server that the
server can not handle, the server should reject the request
with an appropriate error.
The problems of a 32 bit client and a 64 bit server are much
harder to handle. In this situation, the server does not have
a problem because it can handle anything that the client can
generate. However, the client may encounter a file that it
can not handle. The client will not be able to handle a file
whose size can not be expressed in 32 bits. Thus, the client
will not be able to properly decode the size of the file into
its local attributes structure. Also, a file can grow beyond
the limit of the client while the client is accessing the
file.
The solutions to these problems are left up to the individual
implementor. However, there are two common approaches used to
resolve this situation. The implementor can choose between
them or even can invent a new solution altogether.
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The most common solution is for the client to deny access to
any file whose size can not be expressed in 32 bits. This is
probably the safest, but does introduce some strange
semantics when the file grows beyond the limit of the client
while it is being access by that client. The file becomes
inaccessible even while it is being accessed.
The second solution is for the client to map any size greater
than it can handle to the maximum size that it can handle.
Effectively, it is lying to the application program. This
allows the application access as much of the file as possible
given the 32 bit offset restriction. This eliminates the
strange semantic of the file effectively disappearing after
it has been accessed, but does introduce other problems. The
client will not be able to access the entire file.
Currently, the first solution is the recommended solution.
However, client implementors are encouraged to do the best
that they can to reduce the effects of this situation.
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5.0 Appendix I: Mount protocol
The changes from the NFS version 2 protocol to the NFS version 3
protocol have required some changes to be made in the MOUNT
protocol. To meet the needs of the NFS version 3 protocol, a
new version of the MOUNT protocol has been defined. This new
protocol satisfies the requirements of the NFS version 3
protocol and addresses several other current market
requirements.
5.1 RPC Information
5.1.1 Authentication
The MOUNT service uses AUTH_NONE in the NULL procedure.
AUTH_UNIX, AUTH_SHORT, AUTH_DES, or AUTH_KERB are used for all
other procedures. Other authentication types may be supported
in the future.
5.1.2 Constants
These are the RPC constants needed to call the MOUNT service.
They are given in decimal.
PROGRAM 100005
VERSION 3
5.1.3 Transport address
The MOUNT service is normally supported over the TCP and UDP
protocols. The rpcbind daemon should be queried for the correct
transport address.
5.1.4 Sizes
const MNTPATHLEN = 1024; /* Maximum bytes in a path name */
const MNTNAMLEN = 255; /* Maximum bytes in a name */
const FHSIZE3 = 64; /* Maximum bytes in a V3 file handle */
5.1.5 Basic Data Types
typedef opaque fhandle3<FHSIZE3>;
typedef string dirpath<MNTPATHLEN>;
typedef string name<MNTNAMLEN>;
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enum mountstat3 {
MNT3_OK = 0, /* no error */
MNT3ERR_PERM = 1, /* Not owner */
MNT3ERR_NOENT = 2, /* No such file or directory */
MNT3ERR_IO = 5, /* I/O error */
MNT3ERR_ACCES = 13, /* Permission denied */
MNT3ERR_NOTDIR = 20, /* Not a directory */
MNT3ERR_INVAL = 22, /* Invalid argument */
MNT3ERR_NAMETOOLONG = 63, /* Filename too long */
MNT3ERR_NOTSUPP = 10004, /* Operation not supported */
MNT3ERR_SERVERFAULT = 10006 /* A failure on the server */
};
5.2 Server Procedures
The following sections define the RPC procedures supplied by a
MOUNT version 3 protocol server. The RPC procedure number is
given at the top of the page with the name and version. The
SYNOPSIS provides the name of the procedure, the list of the
names of the arguments, the list of the names of the results,
followed by the XDR argument declarations and results
declarations. The information in the SYNOPSIS is specified in
RPC Data Description Language as defined in [RFC1014]. The
DESCRIPTION section tells what the procedure is expected to do
and how its arguments and results are used. The ERRORS section
lists the errors returned for specific types of failures. The
IMPLEMENTATION field describes how the procedure is expected to
work and how it should be used by clients.
program MOUNT_PROGRAM {
version MOUNT_V3 {
void MOUNTPROC3_NULL(void) = 0;
mountres3 MOUNTPROC3_MNT(dirpath) = 1;
mountlist MOUNTPROC3_DUMP(void) = 2;
void MOUNTPROC3_UMNT(dirpath) = 3;
void MOUNTPROC3_UMNTALL(void) = 4;
exports MOUNTPROC3_EXPORT(void) = 5;
} = 3;
} = 100005;
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5.2.0 Procedure 0: Null - Do nothing
SYNOPSIS
void MOUNTPROC3_NULL(void) = 0;
DESCRIPTION
Procedure NULL does not do any work. It is made available
to allow server response testing and timing.
IMPLEMENTATION
It is important that this procedure do no work at all so
that it can be used to measure the overhead of processing
a service request. By convention, the NULL procedure
should never require any authentication. A server may
choose to ignore this convention, in a more secure
implementation, where responding to the NULL procedure
call acknowledges the existence of a resource to an
unauthenticated client.
ERRORS
Since the NULL procedure takes no MOUNT protocol arguments
and returns no MOUNT protocol response, it can not return
a MOUNT protocol error. However, it is possible that some
server implementations may return RPC errors based on
security and authentication requirements.
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5.2.1 Procedure 1: MNT - Add mount entry
SYNOPSIS
mountres3 MOUNTPROC3_MNT(dirpath) = 1;
struct mountres3_ok {
fhandle3 fhandle;
int auth_flavors<>;
};
union mountres3 switch (mountstat3 fhs_status) {
case MNT_OK:
mountres3_ok mountinfo;
default:
void;
};
DESCRIPTION
Procedure MNT maps a pathname on the server to a file
handle. The pathname is an ASCII string that describes a
directory on the server. If the call is successful
(MNT3_OK), the server returns an NFS version 3 protocol
file handle and a vector of RPC authentication flavors
that are supported with the client's use of the file
handle (or any file handles derived from it). The
authentication flavors are defined in Section 7.2 and
section 9 of [RFC1057].
IMPLEMENTATION
If mountres3.fhs_status is MNT3_OK, then
mountres3.mountinfo contains the file handle for the
directory and a list of acceptable authentication
flavors. This file handle may only be used in the NFS
version 3 protocol. This procedure also results in the
server adding a new entry to its mount list recording that
this client has mounted the directory. AUTH_UNIX
authentication or better is required.
ERRORS
MNT3ERR_NOENT
MNT3ERR_IO
MNT3ERR_ACCES
MNT3ERR_NOTDIR
MNT3ERR_NAMETOOLONG
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5.2.2 Procedure 2: DUMP - Return mount entries
SYNOPSIS
mountlist MOUNTPROC3_DUMP(void) = 2;
typedef struct mountbody *mountlist;
struct mountbody {
name ml_hostname;
dirpath ml_directory;
mountlist ml_next;
};
DESCRIPTION
Procedure DUMP returns the list of remotely mounted file
systems. The mountlist contains one entry for each client
host name and directory pair.
IMPLEMENTATION
This list is derived from a list maintained on the server
of clients that have requested file handles with the MNT
procedure. Entries are removed from this list only when a
client calls the UMNT or UMNTALL procedure. Entries may
become stale if a client crashes and does not issue either
UMNT calls for all of the file systems that it had
previously mounted or a UMNTALL to remove all entries that
existed for it on the server.
ERRORS
There are no MOUNT protocol errors which can be returned
from this procedure. However, RPC errors may be returned
for authentication or other RPC failures.
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5.2.3 Procedure 3: UMNT - Remove mount entry
SYNOPSIS
void MOUNTPROC3_UMNT(dirpath) = 3;
DESCRIPTION
Procedure UMNT removes the mount list entry for the
directory that was previously the subject of a MNT call
from this client. AUTH_UNIX authentication or better is
required.
IMPLEMENTATION
Typically, server implementations have maintained a list
of clients which have file systems mounted. In the past,
this list has been used to inform clients that the server
was going to be shutdown.
ERRORS
There are no MOUNT protocol errors which can be returned
from this procedure. However, RPC errors may be returned
for authentication or other RPC failures.
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5.2.4 Procedure 4: UMNTALL - Remove all mount entries
SYNOPSIS
void MOUNTPROC3_UMNTALL(void) = 4;
DESCRIPTION
Procedure UMNTALL removes all of the mount entries for
this client previously recorded by calls to MNT. AUTH_UNIX
authentication or better is required.
IMPLEMENTATION
This procedure should be used by clients when they are
recovering after a system shutdown. If the client could
not successfully unmount all of its file systems before
being shutdown or the client crashed because of a software
or hardware problem, there may be servers which still have
mount entries for this client. This is an easy way for the
client to inform all servers at once that it does not have
any mounted file systems. However, since this procedure
is generally implemented using broadcast RPC, it is only
of limited usefullness.
ERRORS
There are no MOUNT protocol errors which can be returned
from this procedure. However, RPC errors may be returned
for authentication or other RPC failures.
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5.2.5 Procedure 5: EXPORT - Return export list
SYNOPSIS
exports MOUNTPROC3_EXPORT(void) = 5;
typedef struct groupnode *groups;
struct groupnode {
name gr_name;
groups gr_next;
};
typedef struct exportnode *exports;
struct exportnode {
dirpath ex_dir;
groups ex_groups;
exports ex_next;
};
DESCRIPTION
Procedure EXPORT returns a list of all the exported file
systems and which clients are allowed to mount each one.
The names in the group list are implementation-specific
and cannot be directly interpreted by clients. These names
can represent hosts or groups of hosts.
IMPLEMENTATION
This procedure generally returns the contents of a list of
shared or exported file systems. These are the file
systems which are made available to NFS version 3 protocol
clients.
ERRORS
There are no MOUNT protocol errors which can be returned
from this procedure. However, RPC errors may be returned
for authentication or other RPC failures.
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6.0 Appendix II: Lock manager protocol
Because the NFS version 2 protocol as well as the NFS version 3
protocol is stateless, an additional Network Lock Manager (NLM)
protocol is required to support locking of NFS-mounted files.
The NLM version 3 protocol, which is used with the NFS version 2
protocol, is documented in [X/OpenNFS].
Some of the changes in the NFS version 3 protocol require a
new version of the NLM protocol. This new protocol is the NLM
version 4 protocol. The following table summarizes the
correspondence between versions of the NFS protocol and NLM
protocol.
NFS and NLM protocol compatibility
+---------+---------+
| NFS | NLM |
| Version | Version |
+===================+
| 2 | 1,3 |
+---------+---------+
| 3 | 4 |
+---------+---------+
This appendix only discusses the differences between the NLM
version 3 protocol and the NLM version 4 protocol. As in the
NFS version 3 protocol, almost all the names in the NLM version
4 protocol have been changed to include a version number. This
appendix does not discuss changes that consist solely of a name
change.
6.1 RPC Information
6.1.1 Authentication
The NLM service uses AUTH_NONE in the NULL procedure.
AUTH_UNIX, AUTH_SHORT, AUTH_DES, and AUTH_KERB are used for
all other procedures. Other authentication types may be
supported in the future.
6.1.2 Constants
These are the RPC constants needed to call the NLM service.
They are given in decimal.
PROGRAM 100021
VERSION 4
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6.1.3 Transport Address
The NLM service is normally supported over the TCP and UDP
protocols. The rpcbind daemon should be queried for the
correct transport address.
6.1.4 Basic Data Types
uint64
typedef unsigned hyper uint64;
int64
typedef hyper int64;
uint32
typedef unsigned long uint32;
int32
typedef long int32;
These types are new for the NLM version 4 protocol. They are
the same as in the NFS version 3 protocol.
nlm4_stats
enum nlm4_stats {
NLM4_GRANTED = 0,
NLM4_DENIED = 1,
NLM4_DENIED_NOLOCKS = 2,
NLM4_BLOCKED = 3,
NLM4_DENIED_GRACE_PERIOD = 4,
NLM4_DEADLCK = 5,
NLM4_ROFS = 6,
NLM4_STALE_FH = 7,
NLM4_FBIG = 8,
NLM4_FAILED = 9
};
Nlm4_stats indicates the success or failure of a call. This
version contains several new error codes, so that clients can
provide more precise failure information to applications.
NLM4_GRANTED
The call completed successfully.
NLM4_DENIED
The call failed. For attempts to set a lock, this status
implies that if the client retries the call later, it may
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succeed.
NLM4_DENIED_NOLOCKS
The call failed because the server could not allocate the
necessary resources.
NLM4_BLOCKED
Indicates that a blocking request cannot be granted
immediately. The server will issue an NLMPROC4_GRANTED
callback to the client when the lock is granted.
NLM4_DENIED_GRACE_PERIOD
The call failed because the server is reestablishing old
locks after a reboot and is not yet ready to resume normal
service.
NLM4_DEADLCK
The request could not be granted and blocking would cause
a deadlock.
NLM4_ROFS
The call failed because the remote file system is
read-only. For example, some server implementations might
not support exclusive locks on read-only file systems.
NLM4_STALE_FH
The call failed because it uses an invalid file handle.
This can happen if the file has been removed or if access
to the file has been revoked on the server.
NLM4_FBIG
The call failed because it specified a length or offset
that exceeds the range supported by the server.
NLM4_FAILED
The call failed for some reason not already listed. The
client should take this status as a strong hint not to
retry the request.
nlm4_holder
struct nlm4_holder {
bool exclusive;
int32 svid;
netobj oh;
uint64 l_offset;
uint64 l_len;
};
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This structure indicates the holder of a lock. The exclusive
field tells whether the holder has an exclusive lock or a
shared lock. The svid field identifies the process that is
holding the lock. The oh field is an opaque object that
identifies the host or process that is holding the lock. The
l_len and l_offset fields identify the region that is locked.
The only difference between the NLM version 3 protocol and
the NLM version 4 protocol is that in the NLM version 3
protocol, the l_len and l_offset fields are 32 bits wide,
while they are 64 bits wide in the NLM version 4 protocol.
nlm4_lock
struct nlm4_lock {
string caller_name<LM_MAXSTRLEN>;
netobj fh;
netobj oh;
int32 svid;
uint64 l_offset;
uint64 l_len;
};
This structure describes a lock request. The caller_name
field identifies the host that is making the request. The fh
field identifies the file to lock. The oh field is an opaque
object that identifies the host or process that is making the
request, and the svid field identifies the process that is
making the request. The l_offset and l_len fields identify
the region of the file that the lock controls. A l_len of 0
means "to end of file".
There are two differences between the NLM version 3 protocol
and the NLM version 4 protocol versions of this structure.
First, in the NLM version 3 protocol, the length and offset
are 32 bits wide, while they are 64 bits wide in the NLM
version 4 protocol. Second, in the NLM version 3 protocol,
the file handle is a fixed-length NFS version 2 protocol file
handle, which is encoded as a byte count followed by a byte
array. In the NFS version 3 protocol, the file handle is
already variable-length, so it is copied directly into the fh
field. That is, the first four bytes of the fh field are the
same as the byte count in an NFS version 3 protocol nfs_fh3.
The rest of the fh field contains the byte array from the NFS
version 3 protocol nfs_fh3.
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nlm4_share
struct nlm4_share {
string caller_name<LM_MAXSTRLEN>;
netobj fh;
netobj oh;
fsh4_mode mode;
fsh4_access access;
};
This structure is used to support DOS file sharing. The
caller_name field identifies the host making the request.
The fh field identifies the file to be operated on. The oh
field is an opaque object that identifies the host or process
that is making the request. The mode and access fields
specify the file-sharing and access modes. The encoding of fh
is a byte count, followed by the file handle byte array. See
the description of nlm4_lock for more details.
6.2 NLM Procedures
The procedures in the NLM version 4 protocol are semantically
the same as those in the NLM version 3 protocol. The only
semantic difference is the addition of a NULL procedure that
can be used to test for server responsiveness. The procedure
names with _MSG and _RES suffixes denote asynchronous
messages; for these the void response implies no reply. A
syntactic change is that the procedures were renamed to avoid
name conflicts with the values of nlm4_stats. Thus the
procedure definition is as follows.
version NLM4_VERS {
void
NLMPROC4_NULL(void) = 0;
nlm4_testres
NLMPROC4_TEST(nlm4_testargs) = 1;
nlm4_res
NLMPROC4_LOCK(nlm4_lockargs) = 2;
nlm4_res
NLMPROC4_CANCEL(nlm4_cancargs) = 3;
nlm4_res
NLMPROC4_UNLOCK(nlm4_unlockargs) = 4;
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nlm4_res
NLMPROC4_GRANTED(nlm4_testargs) = 5;
void
NLMPROC4_TEST_MSG(nlm4_testargs) = 6;
void
NLMPROC4_LOCK_MSG(nlm4_lockargs) = 7;
void
NLMPROC4_CANCEL_MSG(nlm4_cancargs) = 8;
void
NLMPROC4_UNLOCK_MSG(nlm4_unlockargs) = 9;
void
NLMPROC4_GRANTED_MSG(nlm4_testargs) = 10;
void
NLMPROC4_TEST_RES(nlm4_testres) = 11;
void
NLMPROC4_LOCK_RES(nlm4_res) = 12;
void
NLMPROC4_CANCEL_RES(nlm4_res) = 13;
void
NLMPROC4_UNLOCK_RES(nlm4_res) = 14;
void
NLMPROC4_GRANTED_RES(nlm4_res) = 15;
nlm4_shareres
NLMPROC4_SHARE(nlm4_shareargs) = 20;
nlm4_shareres
NLMPROC4_UNSHARE(nlm4_shareargs) = 21;
nlm4_res
NLMPROC4_NM_LOCK(nlm4_lockargs) = 22;
void
NLMPROC4_FREE_ALL(nlm4_notify) = 23;
} = 4;
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6.2.0 Procedure 0: NULL - Do nothing
SYNOPSIS
void NLMPROC4_NULL(void) = 0;
DESCRIPTION
The NULL procedure does no work. It is made available in
all RPC services to allow server response testing and
timing.
IMPLEMENTATION
It is important that this procedure do no work at all so
that it can be used to measure the overhead of processing
a service request. By convention, the NULL procedure
should never require any authentication.
ERRORS
It is possible that some server implementations may return
RPC errors based on security and authentication
requirements.
6.3 Implementation issues
6.3.1 64-bit offsets and lengths
Some NFS version 3 protocol servers can only support
requests where the file offset or length fits in 32 or
fewer bits. For these servers, the lock manager will have
the same restriction. If such a lock manager receives a
request that it cannot handle (because the offset or
length uses more than 32 bits), it should return the
error, NLM4_FBIG.
6.3.2 File handles
The change in the file handle format from the NFS version
2 protocol to the NFS version 3 protocol complicates the
lock manager. First, the lock manager needs some way to
tell when an NFS version 2 protocol file handle refers to
the same file as an NFS version 3 protocol file handle.
(This is assuming that the lock manager supports both NLM
version 3 protocol clients and NLM version 4 protocol
clients.) Second, if the lock manager runs the file handle
through a hashing function, the hashing function may need
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to be retuned to work with NFS version 3 protocol file
handles as well as NFS version 2 protocol file handles.
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7.0 Appendix III: Bibliography
[Corbin] Corbin, John, "The Art of Distributed
Programming-Programming Techniques for Remote
Procedure Calls." Springer-Verlag, New York, New
York. 1991. Basic description of RPC and XDR
and how to program distributed applications
using them.
[Glover] Glover, Fred, "TNFS Protocol Specification,"
Trusted System Interest Group, Work in
Progress.
[Israel] Israel, Robert K., Sandra Jett, James Pownell,
George M. Ericson, "Eliminating Data Copies in
UNIX-based NFS Servers," Uniforum Conference
Proceedings, San Francisco, CA,
February 27 - March 2, 1989. Describes two
methods for reducing data copies in NFS server
code.
[Jacobson] Jacobson, V., "Congestion Control and
Avoidance," Proc. ACM SIGCOMM `88, Stanford, CA,
August 1988. The paper describing improvements
to TCP to allow use over Wide Area Networks and
through gateways connecting networks of varying
capacity. This work was a starting point for the
NFS Dynamic Retransmission work.
[Juszczak] Juszczak, Chet, "Improving the Performance and
Correctness of an NFS Server," USENIX Conference
Proceedings, USENIX Association, Berkeley, CA,
June 1990, pages 53-63. Describes reply cache
implementation that avoids work in the server by
handling duplicate requests. More important,
though listed as a side-effect, the reply cache
aids in the avoidance of destructive
non-idempotent operation re-application --
improving correctness.
[Kazar] Kazar, Michael Leon, "Synchronization and Caching
Issues in the Andrew File System," USENIX Conference
Proceedings, USENIX Association, Berkeley, CA,
Dallas Winter 1988, pages 27-36. A description
of the cache consistency scheme in AFS.
Contrasted with other distributed file systems.
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RFC 1813 NFS Version 3 Protocol June 1995
[Macklem] Macklem, Rick, "Lessons Learned Tuning the
4.3BSD Reno Implementation of the NFS Protocol,"
Winter USENIX Conference Proceedings, USENIX
Association, Berkeley, CA, January 1991.
Describes performance work in tuning the 4.3BSD
Reno NFS implementation. Describes performance
improvement (reduced CPU loading) through
elimination of data copies.
[Mogul] Mogul, Jeffrey C., "A Recovery Protocol for Spritely
NFS," USENIX File System Workshop Proceedings,
Ann Arbor, MI, USENIX Association, Berkeley, CA,
May 1992. Second paper on Spritely NFS proposes
a lease-based scheme for recovering state of
consistency protocol.
[Nowicki] Nowicki, Bill, "Transport Issues in the Network
File System," ACM SIGCOMM newsletter Computer
Communication Review, April 1989. A brief
description of the basis for the dynamic
retransmission work.
[Pawlowski] Pawlowski, Brian, Ron Hixon, Mark Stein, Joseph
Tumminaro, "Network Computing in the UNIX and
IBM Mainframe Environment," Uniforum `89 Conf.
Proc., (1989) Description of an NFS server
implementation for IBM's MVS operating system.
[RFC1014] Sun Microsystems, Inc., "XDR: External Data
Representation Standard", RFC 1014,
Sun Microsystems, Inc., June 1987.
Specification for canonical format for data
exchange, used with RPC.
[RFC1057] Sun Microsystems, Inc., "RPC: Remote Procedure
Call Protocol Specification", RFC 1057,
Sun Microsystems, Inc., June 1988.
Remote procedure protocol specification.
[RFC1094] Sun Microsystems, Inc., "Network Filesystem
Specification", RFC 1094, Sun Microsystems, Inc.,
March 1989. NFS version 2 protocol
specification.
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RFC 1813 NFS Version 3 Protocol June 1995
[Sandberg] Sandberg, R., D. Goldberg, S. Kleiman, D. Walsh,
B. Lyon, "Design and Implementation of the Sun
Network Filesystem," USENIX Conference
Proceedings, USENIX Association, Berkeley, CA,
Summer 1985. The basic paper describing the
SunOS implementation of the NFS version 2
protocol, and discusses the goals, protocol
specification and trade-offs.
[Srinivasan] Srinivasan, V., Jeffrey C. Mogul, "Spritely
NFS: Implementation and Performance of Cache
Consistency Protocols", WRL Research Report
89/5, Digital Equipment Corporation Western
Research Laboratory, 100 Hamilton Ave., Palo
Alto, CA, 94301, May 1989. This paper analyzes
the effect of applying a Sprite-like consistency
protocol applied to standard NFS. The issues of
recovery in a stateful environment are covered
in [Mogul].
[X/OpenNFS] X/Open Company, Ltd., X/Open CAE Specification:
Protocols for X/Open Internetworking: XNFS,
X/Open Company, Ltd., Apex Plaza, Forbury Road,
Reading Berkshire, RG1 1AX, United Kingdom,
1991. This is an indispensable reference for
NFS version 2 protocol and accompanying
protocols, including the Lock Manager and the
Portmapper.
[X/OpenPCNFS] X/Open Company, Ltd., X/Open CAE Specification:
Protocols for X/Open Internetworking: (PC)NFS,
Developer's Specification, X/Open Company, Ltd.,
Apex Plaza, Forbury Road, Reading Berkshire, RG1
1AX, United Kingdom, 1991. This is an
indispensable reference for NFS version 2
protocol and accompanying protocols, including
the Lock Manager and the Portmapper.
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8. Security Considerations
Since sensitive file data may be transmitted or received
from a server by the NFS protocol, authentication, privacy,
and data integrity issues should be addressed by implementations
of this protocol.
As with the previous protocol revision (version 2), NFS
version 3 defers to the authentication provisions of the
supporting RPC protocol [RFC1057], and assumes that data
privacy and integrity are provided by underlying transport
layers as available in each implementation of the protocol.
See section 4.4 for a discussion relating to file access
permissions.
9. Acknowledgements
This description of the protocol is derived from an original
document written by Brian Pawlowski and revised by Peter
Staubach. This protocol is the result of a co-operative
effort that comprises the contributions of Geoff Arnold,
Brent Callaghan, John Corbin, Fred Glover, Chet Juszczak,
Mike Eisler, John Gillono, Dave Hitz, Mike Kupfer, Rick
Macklem, Ron Minnich, Brian Pawlowski, David Robinson, Rusty
Sandberg, Craig Schamp, Spencer Shepler, Carl Smith, Mark
Stein, Peter Staubach, Tom Talpey, Rob Thurlow, and Mark
Wittle.
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10. Authors' Addresses
Address comments related to this protocol to:
nfs3@eng.sun.com
Brent Callaghan
Sun Microsystems, Inc.
2550 Garcia Avenue
Mailstop UMTV05-44
Mountain View, CA 94043-1100
Phone: 1-415-336-1051
Fax: 1-415-336-6015
EMail: brent.callaghan@eng.sun.com
Brian Pawlowski
Network Appliance Corp.
319 North Bernardo Ave.
Mountain View, CA 94043
Phone: 1-415-428-5136
Fax: 1-415-428-5151
EMail: beepy@netapp.com
Peter Staubach
Sun Microsystems, Inc.
2550 Garcia Avenue
Mailstop UMTV05-44
Mountain View, CA 94043-1100
Phone: 1-415-336-5615
Fax: 1-415-336-6015
EMail: peter.staubach@eng.sun.com
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