    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

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    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

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

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    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

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       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.

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    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.

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       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".

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       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.

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    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.

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    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

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