RFC2224 - NFS URL Scheme

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　　Network Working Group B. Callaghan
Request for Comments: 2224 Sun Microsystems, Inc.
Category: Informational October 1997

NFS URL Scheme

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1997). All Rights Reserved.

Abstract

A new URL scheme, 'nfs' is defined. It is used to refer to files and
directories on NFS servers using the general URL syntax defined in
RFC1738, "Uniform Resource Locators (URL)".

This scheme uses the public filehandle and multi-component lookup
[RFC2054, RFC2055] 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 . . . . . . . . . . . . . . . . . . . . . . 2
5. Public Filehandle . . . . . . . . . . . . . . . . . . . 3
5.1 NFS Version 2 Public Filehandle . . . . . . . . . . . 3
5.2 NFS Version 3 Public Filehandle . . . . . . . . . . . 3
6. Multi-component Lookup . . . . . . . . . . . . . . . . . 3
6.1 Absolute vs Relative Pathname . . . . . . . . . . . . 4
6.2 Symbolic Links . . . . . . . . . . . . . . . . . . . . 5
7. Mount Protocol . . . . . . . . . . . . . . . . . . . . . 6
8. Bibliography . . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . 8

10. BNF for NFS URL Scheme . . . . . . . . . . . . . . . . . 9
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . 10
12. Author's Address . . . . . . . . . . . . . . . . . . . . 10
13. Full Copyright Statement . . . . . . . . . . . . . . . . 11

1. URL Syntax

An NFS URL is based on the Common Internet Scheme Syntax described in
section 3.1 of RFC1738. 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.

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 RFC1738. If <url-
path> is omitted or consists solely of "/", it must default to the
path ".".

2. URL Evaluation

A client must evaluate an NFS URL by a method known as WebNFS
[RFC2054, RFC2055]. 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.

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 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 NFS 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 (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. Characters not in the "unreserved"
set (see BNF description below) may be included in pathname
components but must be escaped.

If the <url-path> is empty or consists solely of "/", the client must
send a multi-component lookup for the pathname ".".

6.1 Absolute vs. Relative Pathname

A multi-component 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 initial "/" that introduces the <url-path> of an NFS
URL must not be passed to the server for multi-component lookup since
the pathname is to be evaluated relative to the public filehandle
directory. 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 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.

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 RFC1808. If the
relative URL in the symbolic link text is to be resolved successfully
then it must contain only ASCII characters and conform to the syntax
described in RFC1808. 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.

A symbolic link path that begins with a slash will be evaluated as an
absolute path relative to the directory associated with the public
filehandle which may not be the same as the server's system root
directory. If symbolic links with absolute paths are to be evaluated
correctly on both client and server then the public filehandle must
be associated with the system root directory.

For example, if the symbolic link is named by the URL

nfs://server/a/b

then 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
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 RFC1813. 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 RFC1738. 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 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] Berners-Lee, T., Masinter, L. and M. McCahill,
"Uniform Resource Locators (URL)," RFC1738,
December 1994.

[RFC1808] Fielding, R., "Relative Uniform Resource Locators,"
RFC1808, June 1995.

[RFC1831] Srinivasan, R., "RPC: Remote Procedure Call
Protocol Specification Version 2," RFC1831,
August 1995.

[RFC1832] Srinivasan, R., "XDR: External Data Representation
Standard," RFC1832, August 1995.

[RFC1833] Srinivasan, R., "Binding Protocols for ONC RPC
Version 2," RFC1833, August 1995.

[RFC1094] Sun Microsystems, Inc., "Network Filesystem
Specification," RFC1094, March 1989.

[RFC1813] Callaghan, B., Pawlowski, B. and P. Staubach,
"NFS Version 3 Protocol Specification," RFC1813,
June 1995.

[RFC2054] Callaghan, B., "WebNFS Client Specification,"
RFC2054, October 1996.

[RFC2055] Callaghan, B., "WebNFS Server Specification,"
RFC2055, October 1996.

[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
the NFS 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 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 RFC1094,
RFC1831, and RFC1832 apply here also.

Server implementors should be careful when implementing multi-
component lookup that the client cannot obtain unauthorized access to
files or directories. For example: a path that includes multiple
occurrences of "../" may locate a filesystem outside of the exported
filesystem associated with the public filehandle.

Clients and servers may separately negotiate secure connection
schemes for authentication, data integrity, and privacy.

10. BNF for NFS URL Scheme

The syntax of the NFS URL is a subset of the Generic URL syntax
described in RFC1738. An NFS URL does not include user or password
components, nor does it recognize "?" query or "#" fragment
components.
nfsURL = "nfs" ":" relativeURL
relativeURL = net_path | abs_path | rel_path
net_path = "//" hostport [ abs_path ]
abs_path = "/" rel_path
rel_path = [ path_segments ]

hostport = host [ ":" port ]
host = hostname | hostnumber
hostname = *( domainlabel "." ) toplabel
domainlabel = alphanum | alphanum *( alphanum | "-" ) alphanum
toplabel = alpha | alpha *( alphanum | "-" ) alphanum
hostnumber = 1*digit "." 1*digit "." 1*digit "." 1*digit
port = *digit

url-path = [ "/" ] path_segments
path_segments = segment *( "/" segment )
segment = *pchar
pchar = unreserved | escaped | ":" | "@" | "&" | "=" | "+"

reserved = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+"
unreserved = alpha | digit | mark
mark = "$" | "-" | "_" | "." | "!" | "~" |
"*" | "'" | "(" | ")" | ","

escaped = "%" hex hex
hex = digit | "A" | "B" | "C" | "D" | "E" | "F" |
"a" | "b" | "c" | "d" | "e" | "f"

alphanum = alpha | digit
alpha = lowalpha | upalpha

lowalpha = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" |
"j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" |
"s" | "t" | "u" | "v" | "w" | "x" | "y" | "z"
upalpha = "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" |
"J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" |
"S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z"
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
"8" | "9"

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 RFCto:

brent@eng.sun.com

Brent Callaghan
Sun Microsystems, Inc.
Mailstop Mpk17-201,
901 San Antonio Road,
Palo Alto, California 94303

Phone: 1-415-786-5067
EMail: brent.callaghan@eng.sun.com
Fax: 1-415-786-5896

13. Full Copyright Statement

Copyright (C) The Internet Society (1997). All Rights Reserved.

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