WebNFS Server Specification

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 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
   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", 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.

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