Network Working Group B. Callaghan
INTERNET DRAFT
Category: Informational
Expire in six months Sun Microsystems, Inc.
October 1996
WebNFS Server Specification
<draft-callaghan-webnfs-server-00.txt>
Abstract
This document describes the specifications for a server of
WebNFS clients. WebNFS extends the semantics of versions 2
and 3 of the NFS protocols to allow clients to obtain
filehandles more easily, without recourse to the portmap or
MOUNT protocols. In removing the need for these protocols,
WebNFS clients see benefits in faster response to requests,
easy transit of firewalls and better server scalability This
description is provided to facilitate compatible
implementations of WebNFS servers.
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 2
2. TCP vs UDP . . . . . . . . . . . . . . . . . . . . . . . 2
3. Well-known Port . . . . . . . . . . . . . . . . . . . . . 2
4. Server Port Monitoring . . . . . . . . . . . . . . . . . . 3
5. Public Filehandle . . . . . . . . . . . . . . . . . . . . 3
5.1 Version 2 Public Filehandle . . . . . . . . . . . . . . 3
5.2 Version 3 Public Filehandle . . . . . . . . . . . . . . 4
6. Multi-component Lookup . . . . . . . . . . . . . . . . . . 4
6.1 Canonical Path vs. Native Path . . . . . . . . . . . . . 5
6.2 Symbolic Links . . . . . . . . . . . . . . . . . . . . . 6
6.3 Export Spanning Pathnames . . . . . . . . . . . . . . . 6
7. Location of Public Filehandle . . . . . . . . . . . . . . 7
8. Index Files . . . . . . . . . . . . . . . . . . . . . . . 8
9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 10
12. Author's Address . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
The NFS protocol provides access to shared filesystems across
networks. It is intended to be machine, operating system, network
architecture, and transport independent. The protocol currently
exists in two versions: version 2 [RFC1094] and version 3
[RFC1813], both built on Sun RPC [RFC1831] and its associated
eXternal Data Representation (XDR) [RFC1832]. This document
assumes some familiarity with the NFS protocol and underlying RPC
protocols.
WebNFS servers implement semantic extensions to both versions
of the NFS protocol to support a lightweight binding mechanism
for conventional or web browser clients that need to communicate
with NFS servers across the Internet. a WebNFS server supports
the public filehandle and multi-component lookup features
described herein, as well as meeting some additional requirements.
For a description of WebNFS client requirements, read RFC mmmm.
2. TCP vs UDP
The NFS protocol is most well known for its use of UDP which
performs acceptably on local area networks. However, on wide area
networks with error prone, high-latency connections and bandwidth
contention, TCP is well respected for its congestion control and
superior error handling. A growing number of NFS implementations
now support the NFS protocol over TCP connections.
A WebNFS client will first attempt to connect to its server
with a TCP connection. If the server refuses the connection,
the client will attempt to use UDP. All WebNFS servers should
support client use of TCP and must support UDP.
3. Well-known Port
While Internet protocols are generally identified by registered
port number assignments, RPC based protocols register a 32 bit
program number and a dynamically assigned port with the portmap
service which is registered on the well-known port 111. Since
the NFS protocol is RPC-based, NFS servers register their port
assignment with the portmap service.
NFS servers are constrained by a requirement to re-register
at the same port after a server crash and recovery so that
clients can recover simply by retransmitting an RPC request
until a response is received. This is simpler than the
alternative of having the client repeatedly check with
the portmap service for a new port assignment. NFS servers
typically achieve this port invariance by registering a
constant port assignment, 2049, for both UDP and TCP.
To avoid the overhead of contacting the server's portmap
service, and to facilitate transit through packet filtering
firewalls, WebNFS clients optimistically assume that WebNFS
servers register on port 2049. Most NFS servers use this
port assignment already, so this client optimism is well
justified.
A WebNFS server must register on UDP port 2049 and TCP
port 2049 if TCP is supported.
4. Server Port Monitoring
Some NFS servers accept requests only from reserved UDP or TCP
ports, i.e. port numbers below 1024. These "privileged" ports
are available only to Unix processes with superuser permissions.
Requests that do not originate from the range of reserved ports
are rejected. This an optimistic way of preventing direct
access to the server from user processes that may attempt to
spoof AUTH_UNIX RPC credentials.
Since WebNFS clients are not required to use reserved ports,
a WebNFS server must not check the originating port for
requests to filesystems made available to WebNFS clients.
5. Public Filehandle
The public filehandle is an NFS file handle with a reserved value
and special semantics that allow an initial filehandle to be
obtained. A WebNFS client can use the public filehandle as an
initial filehandle without 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 Version 2 Public Filehandle
A version 2 filehandle is defined in RFC1094 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 Version 3 Public Filehandle
A version 3 filehandle is defined in RFC1813 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.
+-+-+-+-+
| 0 |
+-+-+-+-+
6. Multi-component Lookup
Normally the NFS LOOKUP request (versions 2 or 3) takes a
directory file handle 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 file handle of the
first component it must issue a series of LOOKUP requests one
component at a time. For instance, evaluation of the Unix 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 file handle can provide a
pathname containing multiple components. The server is expected
to evaluate the entire pathname and return a filehandle for the
final component. The pathname syntax is assumed to be understood
by the server, but the client must not make assumptions of the
pathname syntax.
A Unix server, for instance, uses a slash "/" character to
separate components in a Unix pathname.
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.
6.1 Canonical Path vs. Native Path
If the pathname in a multi-component LOOKUP request begins with a
printable ASCII character, then it must be a canonical path.
A canonical path is a hierarchically-related, slash-separated
sequence of components, <directory>/<directory>/.../<name>.
Occurrences of the "/" character within a component will be
escaped using the escape code %2f. Non-printable ascii characters
(with values in the range 00-1F and 7f hexadecimal) will also be
escaped using the "%" character to introduce a two digit
hexadecimal code. Occurrences of the "%" character that do not
introduce an encoded character will themselves be encoded with %25.
If the first character of a canonical path is a slash, then the
canonical path must be evaluated relative to the server's root
directory. If the first character is not a slash, then the path
must be evaluated relative to the directory with which the public
filehandle is associated.
Not all WebNFS servers can support arbitrary use of absolute
paths. Clearly, the server cannot 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.
If the first character of the path is 0x80 (non-ascii) then the
following character is the first in a native path. A native path
conforms to the natural pathname syntax of the server. For
example:
Lookup for Canonical Path:
LOOKUP FH=0x0 "/a/b/c"
Lookup for Native Path:
LOOKUP FH=0x0 0x80 "a:b:c"
Other introductory characters in the range 0x81 - 0xff may
be added in future specifications. If the server receives
any character in this range that it does not understand then
it must return an error to the client: NFSERR_IO for NFS V2,
NFS3ERR_IO for NFS V3.
6.2 Symbolic Links
Servers that support symbolic links may encounter pathname
components that are symbolic links. The server is expected
to evaluate these symbolic links as a part of the normal
pathname evaluation process. This is a different semantic
from that of conventional component-at-a-time pathname evaluation
by NFS clients, where the client is expected to do the evaluation.
However, if the final component is a symbolic link, the server
must return its filehandle and let the client evaluate it.
6.3 Export Spanning Pathnames
The server may evaluate a pathname, either through a multi-component
LOOKUP or as a symbolic link embedded in a pathname, that references
a file or directory outside of the exported hierarchy.
Clearly, if the destination of the path is not in an exported
filesystem, then the server must return an error to the client.
Many NFS server implementations rely on the MOUNT protocol for
checking access to exported filesystems and NFS server does
no access checking. The NFS server assumes that the filehandle
does double duty: identifying a file as well as being a security
token. Since WebNFS clients do not normally use the MOUNT protocol,
a server that relies on MOUNT checking cannot automatically grant
access to another exported filesystem at the destination of a
spanning path. These servers must return an error.
For example: the server exports two filesystems. One is
associated with the public filehandle.
/export/this (public filehandle)
/export/that
The server receives a LOOKUP request with the public
filehandle that identifies a file or directory in the
other exported filesystem:
LOOKUP 0x0 "../that/file"
or
LOOKUP 0x0 "/export/that/file"
Even though the pathname destination is in an exported filesystem,
the server cannot return a filehandle without an assurance that
the client's use of this filehandle will be authorized.
Servers that check client access to an export on every NFS
request have more flexibility. These servers can return
filehandles for paths that span exports since the client's
use of the filehandle for the destination filesystem will
be checked by the NFS server.
7. Location of Public Filehandle
A server administrator can associate the public filehandle with
any file or directory. For instance, a WebNFS server administrator
could attach the public filehandle to the root of an anonymous FTP
archive, so that anonymous FTP pathnames could be used to identify
files in the FTP hierarchy, e.g.
# share -o ro,public /export/ftp
On servers that support spanning paths, the public filehandle need
not necessarily be attached to an exported directory, though a
successful LOOKUP relative to the public filehandle must identify
a file or directory that is exported.
For instance, if an NFS server exports a directory "/export/foo"
and the public filehandle is attached to the server's root
directory, then a LOOKUP of "export/foo" relative to the public
filehandle will return a valid file handle but a LOOKUP of
"export" will return an access error since the server's "/export"
directory is not exported.
/ (public filehandle is here)
/\
/ \
/ export (not exported)
/ /\
/ / \
/ / foo (exported)
LOOKUP 0x0 "export" (returns an error)
LOOKUP 0x0 "export/foo" (returns an filehandle)
8. Index Files
Most HTTP servers support the concept of an index file.
If a browser references a directory that contains an
index file, then the server will return the contents of
the index file rather than a directory listing.
For instance if a browser requests "a/b/c" then the
server might return the contents of "a/b/c/index.html".
A WebNFS server may choose to emulate this feature for
the benefit of clients using the NFS protocol to browse
a Web hierarchy. On receiving a multi-component lookup
for a canonical path that names a directory, the server
can check that directory for the presence of an index
file. If the file exists then the server may choose to
return the filehandle of the index file instead of the
directory. Index files are commonly called "index.html"
though the name is usually configurable.
9. Bibliography
[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
[RFCmmmm] B. Callaghan, "WebNFS Client Specification,"
RFC-mmmm, October 1996.
http://www.internic.net/rfc/rfcmmmm.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
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.
10. Security Considerations
Since the WebNFS server features are based on NFS protocol
versions 2 and 3, the RPC security considerations described
in RFC 1094, RFC 1813, and Appendix A of RFC 1831 apply
here also.
Clients and servers may separately negotiate secure
connection schemes for authentication, data integrity,
and privacy.
Implementors must note carefully the implications of
export spanning pathnames as described in section 6.3.
11. Acknowledgements
This specification was extensively reviewed by the NFS
group at SunSoft and brainstormed by Michael Eisler.
12. 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
Mountain View, CA 94043-1100
Phone: 1-415-786-5067
Email: brent.callaghan@eng.sun.com
Fax: 1-415-786-5896