RFC2055 - WebNFS Server Specification

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　　Network Working Group B. Callaghan
Request for Comments: 2055 Sun Microsystems, Inc.
Category: Informational October 1996

WebNFS Server 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.

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.

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 . . . . . . . . . . . . . . . . . . . . . . . 7
9. Bibliography . . . . . . . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 9
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 RFC2054.

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] Srinivasan, R., "RPC: Remote Procedure Call
Protocol Specification Version 2", RFC1831,
August 1995.
http://www.internic.net/rfc/rfc1831.txt

[RFC1832] Srinivasan, R., "XDR: External Data Representation
Standard," RFC1832, August 1995.
http://www.internic.net/rfc/rfc1832.txt

[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC
Version 2", RFC1833, August 1995.
http://www.internic.net/rfc/rfc1833.txt

[RFC1094] Sun Microsystems, Inc., "Network Filesystem
Specification", RFC1094, March 1989. NFS
version 2 protocol specification.
http://www.internic.net/rfc/rfc1094.txt

[RFC1813] Sun Microsystems, Inc., "NFS Version 3 Protocol
Specification", RFC1813, June 1995. NFS version
3 protocol specification.
http://www.internic.net/rfc/rfc1813.txt

[RFC2054] Callaghan, B., "WebNFS Client Specification",
RFC2054, October 1996.
http://www.internic.net/rfc/rfc2054.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 RFC1094, RFC
1813, and Appendix A of RFC1831 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
Fax: 1-415-786-5896
EMail: brent.callaghan@eng.sun.com

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