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rfc:rfc2055

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

Callaghan Informational [Page 1] RFC 2055 WebNFS Server Specification October 1996

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 2054.

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

Callaghan Informational [Page 2] RFC 2055 WebNFS Server Specification October 1996

 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|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Callaghan Informational [Page 3] RFC 2055 WebNFS Server Specification October 1996

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

Callaghan Informational [Page 4] RFC 2055 WebNFS Server Specification October 1996

 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.

Callaghan Informational [Page 5] RFC 2055 WebNFS Server Specification October 1996

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.

Callaghan Informational [Page 6] RFC 2055 WebNFS Server Specification October 1996

 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".

Callaghan Informational [Page 7] RFC 2055 WebNFS Server Specification October 1996

 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", RFC 1831,
                 August 1995.
                 http://www.internic.net/rfc/rfc1831.txt
 [RFC1832]       Srinivasan, R., "XDR: External Data Representation
                 Standard," RFC 1832, August 1995.
                 http://www.internic.net/rfc/rfc1832.txt
 [RFC1833]       Srinivasan, R., "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, March 1989.  NFS
                 version 2 protocol specification.
                 http://www.internic.net/rfc/rfc1094.txt
 [RFC1813]       Sun Microsystems, Inc., "NFS Version 3 Protocol
                 Specification", RFC 1813, June 1995.  NFS version
                 3 protocol specification.
                 http://www.internic.net/rfc/rfc1813.txt
 [RFC2054]       Callaghan, B., "WebNFS Client Specification",
                 RFC 2054, 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.

Callaghan Informational [Page 8] RFC 2055 WebNFS Server Specification October 1996

 [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.

Callaghan Informational [Page 9] RFC 2055 WebNFS Server Specification October 1996

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

Callaghan Informational [Page 10]

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