Internet-Draft
draft-callaghan-url-nfs-00.txt
October 1, 1996
Expires April 1, 1997
NFS URL Scheme
Status of this Memo
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Abstract
A new URL scheme, "nfs:" is defined. It is used to refer
to files and directories on NFS servers. The scheme uses
the public filehandle and multi-component lookup to access
server data with a minimum of protocol overhead.
The NFS protocol provides access to shared filesystems
across networks. It is designed to be machine, operating
system, network architecture, and transport protocol independent.
The protocol currently exists in two versions: version 2 [RFC1094]
and version 3 [RFC1813], both built on ONC RPC [RFC1831] at its
associated eXternal Data Representation (XDR) [RFC1832] and
Binding Protocol [RFC1833].
Table of Contents
1. URL Syntax . . . . . . . . . . . . . . . . . . . . . . . . 2
2. URL Evaluation . . . . . . . . . . . . . . . . . . . . . . 2
3. Server Connection . . . . . . . . . . . . . . . . . . . . 2
4. NFS Version . . . . . . . . . . . . . . . . . . . . . . . 3
5. Public Filehandle . . . . . . . . . . . . . . . . . . . . 3
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5.1 NFS Version 2 Public Filehandle . . . . . . . . . . . . 3
5.2 NFS Version 3 Public Filehandle . . . . . . . . . . . . 3
6. Multi-component Lookup . . . . . . . . . . . . . . . . . . 4
6.1 Absolute vs Relative Pathname . . . . . . . . . . . . . 5
6.2 Symbolic Links . . . . . . . . . . . . . . . . . . . . . 5
7. Mount Protocol . . . . . . . . . . . . . . . . . . . . . . 6
8. Bibliography . . . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . 9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 9
11. Author's Address . . . . . . . . . . . . . . . . . . . . . 9
1. URL Syntax
An NFS URL is based on the Common Internet Scheme Syntax described
in section 3.1 of RFC 1738. It has the general form:
nfs://<host>:<port>/<url-path>
The ":<port>" part is optional. If omitted then port 2049 is
assumed. The <url-path> is also optional. If it is omitted, then
the "/" between <host>:<port> and <url-path> may also be omitted.
The <url-path> is a hierarchical directory path of the form
<directory>/<directory>/<directory>/.../<name>. The <url-path>
must consist only of characters within the US-ASCII character
set. Within a <directory> or <name> component the character "/"
is reserved and must be encoded as described in Section 2.2 of RFC
1738. If <url-path> is omitted, it must default to the path ".".
2. URL Evaluation
A client must evaluate an NFS URL by a method known as WebNFS.
This method provides easy passage through firewalls and proxy
servers, as well as using a minimum number of messages. The
WebNFS method is defined for NFS versions 2 and 3. It assumes that
the server registers on TCP or UDP port 2049 and supports the
public filehandle and multi-component lookup semantics as
described in the following sections.
3. Server Connection
The client must first attempt to create a TCP connection to <host>
using the <port> specified. If :<port> is omitted, then port 2049
will be used. If the server refuses the TCP connection, then the
client will use UDP.
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4. NFS Version
The client must first attempt to use NFS version 3. If the server
returns an RPC PROG_MISMATCH error then the client must assume
that NFS version 3 is not supported, and retry the operation with
an NFS version 2 public filehandle.
5. Public Filehandle
NFS filehandles are normally created by the server and used to
identify uniquely a particular file or directory on the server.
The client does not normally create filehandles or have any
knowledge of the contents of a filehandle.
The public filehandle is an an exception. It is an NFS filehandle
with a reserved value and special semantics that allow an initial
filehandle to be obtained. A WebNFS client uses the public
filehandle as an initial filehandle rather than using the MOUNT
protocol. Since NFS version 2 and version 3 have different
filehandle formats, the public filehandle is defined differently
for each.
The public filehandle is a zero filehandle. For NFS version 2
this is a filehandle with 32 zero octets. A version 3 public
filehandle has zero length.
5.1 NFS Version 2 Public Filehandle
A version 2 filehandle is defined in RFC 1094 as an opaque value
occupying 32 octets. A version 2 public filehandle has a zero
in each octet, i.e. all zeros.
1 32
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2 NFS Version 3 Public Filehandle
A version 3 filehandle is defined in RFC 1813 as a variable length
opaque value occupying up to 64 octets. The length of the filehandle
is indicated by an integer value contained in a 4 octet value
which describes the number of valid octets that follow. A version
3 public filehandle has a length of zero.
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+-+-+-+-+
| 0 |
+-+-+-+-+
6. Multi-component Lookup
Normally the NFS LOOKUP request (version 2 or 3) takes a directory
filehandle along with the name of a directory member, and returns
the filehandle of the directory member. If a client needs to
evaluate a pathname that contains a sequence of components, then
beginning with the directory filehandle of the first component it
must issue a series of LOOKUP requests one component at a time.
For instance, evaluation of the path "a/b/c" will generate
separate LOOKUP requests for each component of the pathname "a",
"b", and "c".
A LOOKUP request that uses the public filehandle can provide a
pathname containing multiple components. The server is expected
to evaluate the entire pathname and return a filehandle for the
final component.
For example, rather than evaluate the path "a/b/c" as:
LOOKUP FH=0x0 "a" --->
<--- FH=0x1
LOOKUP FH=0x1 "b" --->
<--- FH=0x2
LOOKUP FH=0x2 "c" --->
<--- FH=0x3
Relative to the public filehandle these three LOOKUP
requests can be replaced by a single multi-component
lookup:
LOOKUP FH=0x0 "a/b/c" --->
<--- FH=0x3
Multi-component lookup is supported only for LOOKUP requests
relative to the public filehandle.
The <url-path> of the NFS URL must be evaluated as a
multi-component lookup. This implies that the path components are
delimited by slashes and the characters that make up the path must
be in the printable US-ASCII character set.
If the <url-path> is empty, the client must send a multi-component
lookup for the pathname ".".
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6.1 Absolute vs. Relative Pathname
A pathname that begins with a slash character is considered
"absolute" and will be evaluated relative to the server's root.
A pathname that does not begin with a slash is "relative" and will
be evaluated relative to the directory with which the public
filehandle is associated.
Note that the "/" in an NFS URL that delimits the <host>:<port>
from the <url-path> is not considered part of the pathname. For
example, if the public filehandle is associated with the server's
directory "/a/b/c" then the URL:
nfs://server/d/e/f
will be evaluated with a relative multi-component lookup of the
path "d/e/f" relative to the server's directory "/a/b/c" while
the URL:
nfs://server//a/b/c/d/e/f
will locate the same file with an absolute multi-component lookup
of the path "/a/b/c/d/e/f" relative to the server's filesystem
root. Notice that a double slash is required at the beginning of
the path; the first slash is the URL delimiter between the
<host>:<port> and the <url-path> and the second slash is the first
character of <url-path>.
Not all WebNFS servers can support arbitrary use of absolute
paths. Clearly, the server must not return a filehandle if the
path identifies a file or directory that is not exported by the
server. In addition, some servers will not return a filehandle if
the path names a file or directory in an exported filesystem
different from the one that is associated with the public
filehandle.
6.2 Symbolic Links
The NFS protocol supports symbolic links, which are the
filesystem equivalent of a relative URL. If a WebNFS
client retrieves a filehandle for a symbolic link (as
indicated by the file type attribute) then it should
send a READLINK request to the server to retrieve the
path comprising the symbolic link.
This path should then be combined with the URL which referenced
the symbolic link according to the rules described in RFC 1808.
If the relative URL in the symbolic link text is to be resolved
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successfully then it must contain only ASCII characters and
conform to the syntax described in RFC 1808. Note that this
allows a symbolic link to contain an entire URL and it may specify
a scheme that is not necessarily an NFS URL.
An exception to RFC 1808 rules applies in the case of an absolute
symbolic link, where the path begins with a "/". RFC 1808
describes a method for resolving relative URLs with respect
to the base URL. Given a base URL of "nfs://s/a/b/c" that
references a symbolic link with contents "/a/b/c/d", the method
would yield a URL "nfs://s/a/b/c/d" which would be correct only
if the public filehandle were co-located with the server's
filesystem root.
If the symbolic link begins with a slash, then after resolving
a relative URL derived from the symbolic link contents according
to the method in RFC 1808, the client must insert an additional
slash in front of the path so that the server will evaluate the
path relative to the server's root, rather than the public
filehandle directory. This variation from the normal method
of resolving a relative URL applies only to handling of symbolic
links. The additional slash must not be inserted if the relative
URL was embedded in a document or other encapsulating entity.
For example, if the symbolic link is named by the URL
nfs://server/a/b
then the the following examples show how a new URL can be
formed from the symbolic link text:
c = nfs://server/a/c
c/d = nfs://server/a/c/d
../c = nfs://server/c
/c/d = nfs://server//c/d
nfs://server2/a/b = nfs://server2/a/b
7. Mount Protocol
The NFS URL may have limited use for naming files on
servers that do not support the public filehandle and
multi-component lookup.
If the server returns an NFS3ERR_STALE, NFS3ERR_INVAL, or
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NFS3ERR_BADHANDLE error in response to the client's use of
a public filehandle, then the client should attempt to resolve
the <url-path> to a filehandle using the MOUNT protocol.
Version 1 of the MOUNT protocol is described in Appendix A of
RFC 1094 and version 3 in Appendix I of RFC 1813. Version 2
of the MOUNT protocol is identical to version 1 except for
the addition of a procedure MOUNTPROC_PATHCONF which returns
POSIX pathconf information from the server.
Note that the pathname sent to the server in the MOUNTPROC_MNT
request is assumed to be a server native path, rather than
a slash-separated path described by RFC 1738. Hence, the
MOUNT protocol can reasonably be expected to map a <url-path>
to a filehandle only on servers that support slash-separated
ASCII native paths. In general, servers that do not support
WebNFS access or slash-separated ASCII native paths should not
advertise NFS URLs.
At this point the client must already have some indication
as to which version of the NFS protocol is supported on the
server. Since the filehandle format differs between NFS
versions 2 and 3, the client must select the appropriate
version of the MOUNT protocol. MOUNT versions 1 and 2 return
only NFS version 2 filehandles, whereas MOUNT version 3 returns
NFS version 3 filehandles.
Unlike the NFS service, the MOUNT service is not registered on a
well-known port. The client must use the PORTMAP service to
locate the server's MOUNT port before it can transmit a
MOUNTPROC_MNT request to retrieve the filehandle corresponding to
the requested path.
Client Server
------ ------
-------------- MOUNT port ? --------------> Portmapper
<-------------- Port=984 ------------------
------- Filehandle for /export/foo ? ----> Mountd @ port 984
<--------- Filehandle=0xf82455ce0.. ------
NFS servers commonly use a client's successful MOUNTPROC_MNT
request request as an indication that the client has "mounted"
the filesystem and may maintain this information in a file
that lists the filesystems that clients currently have mounted.
This information is removed from the file when the client
transmits an MOUNTPROC_UMNT request. Upon receiving a
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successful reply to a MOUNTPROC_MNT request, a WebNFS client
should send a MOUNTPROC_UMNT request to prevent an accumulation
of "mounted" records on the server.
8.0 Bibliography
[RFC1738] T. Berners-Lee, L. Masinter, M. McCahill,
"Uniform Resource Locators (URL)," RFC-1738,
December 1994.
http://www.internic.net/rfc/rfc1738.txt
[RFC1808] R. Fielding,
"Relative Uniform Resource Locators," RFC-1808,
June 1995
http://www.internic.net/rfc/rfc1808.txt
[RFC1831] R. Srinivasan, "RPC: Remote Procedure Call
Protocol Specification Version 2," RFC-1831,
August 1995.
http://www.internic.net/rfc/rfc1831.txt
[RFC1832] R. Srinivasan, "XDR: External Data Representation
Standard," RFC-1832, August 1995.
http://www.internic.net/rfc/rfc1832.txt
[RFC1833] R. Srinivasan, "Binding Protocols for ONC RPC
Version 2," RFC-1833, August 1995.
http://www.internic.net/rfc/rfc1833.txt
[RFC1094] Sun Microsystems, Inc., "Network Filesystem
Specification," RFC-1094, DDN Network
Information Center, SRI International, Menlo
Park, CA. NFS version 2 protocol specification.
http://www.internic.net/rfc/rfc1094.txt
[RFC1813] Sun Microsystems, Inc., "NFS Version 3 Protocol
Specification," RFC-1813, DDN Network
Information Center, SRI International, Menlo
Park, CA. NFS version 3 protocol specification.
http://www.internic.net/rfc/rfc1813.txt
[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
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specification and trade-offs.
[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.
9. Security Considerations
Since the WebNFS server features are based on NFS protocol
versions 2 and 3, the RPC based security considerations
described in RFC 1094, RFC 1831, and RFC 1832 apply here also.
Clients and servers may separately negotiate secure
connection schemes for authentication, data integrity,
and privacy.
10. Acknowledgements
This specification was extensively reviewed by the NFS
group at SunSoft and brainstormed by Michael Eisler.
11. Author's Address
Address comments related to this document to:
nfs@eng.sun.com
Brent Callaghan
Sun Microsystems, Inc.
2550 Garcia Avenue
Mailstop Mpk17-201
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Mountain View, CA 94043-1100
Phone: 1-415-786-5067
Email: brent.callaghan@eng.sun.com
Fax: 1-415-786-5896
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