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Network Working Group                                             A. ChiuRequest for Comments: 2755                                      M. EislerCategory: Informational                                      B. Callaghan                                                         Sun Microsystems                                                             January 2000Security Negotiation for WebNFSStatus 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 (2000).  All Rights Reserved.Abstract   This document describes a protocol for a WebNFS client [RFC2054] to   negotiate the desired security mechanism with a WebNFS server   [RFC2055] before the WebNFS client falls back to the MOUNT v3   protocol [RFC1813].  This document is provided so that people can   write compatible implementations.Table of Contents1. Introduction ..............................................22. Security Negotiation Multi-component LOOKUP ...............33  Overloaded Filehandle .....................................43.1  Overloaded NFS Version 2 Filehandle .....................53.2  Overloaded NFS Version 3 Filehandle .....................64. WebNFS Security Negotiation ...............................65. Security Considerations ...................................106. References ................................................107. Acknowledgements ..........................................108. Authors' Addresses ........................................119. Full Copyright Statement ..................................12Chiu, et al.                 Informational                      [Page 1]

RFC 2755            Security Negotiation for WebNFS         January 20001. Introduction   The MOUNT protocol is used by an NFS client to obtain the necessary   filehandle for data access.  MOUNT versions 1 and 2 [RFC1094] return   NFS version 2 filehandles, whereas MOUNT version 3 [RFC1813] returns   NFS version 3 filehandles.   Among the existing versions of the MOUNT protocol, only the MOUNT v3   provides an RPC procedure (MOUNTPROC3_MNT) which facilitates security   negotiation between an NFS v3 client and an NSF v3 server.  When this   RPC procedure succeeds (MNT3_OK) the server returns to the client an   array of security mechanisms it supports for the specified pathname,   in addition to an NFS v3 filehandle.   A security mechanism referred to in this document is a generalized   security flavor which can be an RPC authentication flavor [RFC1831]   or a security flavor referred to in the RPCSEC_GSS protocol   [RFC2203]. A security mechanism is represented as a four-octet   integer.   No RPC procedures are available for security negotiation in versions   1 or 2 of the MOUNT protocol.   The NFS mount command provides a "sec=" option for an NFS client to   specify the desired security mechanism to use for NFS transactions.   If this mount option is not specified, the default action is to use   the default security mechanism over NFS v2 mounts, or to negotiate a   security mechanism via the MOUNTPROC3_MNT procedure of MOUNT v3 and   use it over NFS v3 mounts.  In the latter, the client picks the first   security mechanism in the array returned from the server that is also   supported on the client.   As specified inRFC 2054, a WebNFS client first assumes that the   server supports WebNFS and uses the publsc filehandle as the initial   filehandle for data access, eliminating the need for the MOUNT   protocol.  The WebNFS client falls back to MOUNT if the server does   not support WebNFS.   Since a WebNFS client does not use MOUNT initially, the   MOUNTPROC3_MNT procedure of MOUNT v3 is not available for security   negotiation until the WebNFS client falls back to MOUNT.  A viable   protocol needs to be devised for the WebNFS client to negotiate   security mechanisms with the server in the absence of the   MOUNTPROC3_MNT procedure.Chiu, et al.                 Informational                      [Page 2]

RFC 2755            Security Negotiation for WebNFS         January 2000   The WebNFS security negotiation protocol must meet the following   requirements:      - Must work seamlessly with NFS v2 and v3, and the WebNFS         protocols      - Must be backward compatible with servers that do not support         this negotiation      - Minimum number of network turnarounds (latency)   This document describes the WebNFS security negotiation protocol   developed by Sun Microsystems, Inc.  Terminology and definitions from   RFCs 2054 and 2055 are used in this document.  The reader is expected   to be familiar with them.2. Security Negotiation Multi-component LOOKUP   The goal of the WebNFS security negotiation is to allow a WebNFS   client to identify a security mechanism which is used by the WebNFS   server to protect a specified path and is also supported by the   client.  The WebNFS client initiates the negotiation by sending the   WebNFS server the path. The WebNFS server responds with the array of   security mechanisms it uses to secure the specified path.  From the   array of security mechanisms the WebNFS client selects the first one   that it also supports.   Without introducing a new WebNFS request, the WebNFS security   negotiation is achieved by modifying the request and response of the   existing multi-component LOOKUP (MCL) operation [RFC2055].  Note that   the MCL operation is accomplished using the LOOKUP procedure   (NFSPROC3_LOOKUP for NFS v3 and NFSPROC_LOOKUP for NFS v2).  This and   the next sections describe how the MCL request and response are   modified to facilitate WebNFS security negotiation.   For ease of reference, the modified MCL request is henceforth   referred to as SNEGO-MCL (security negotiation multi-component   LOOKUP) request.   A multi-component LOOKUP request [RFC2055] is composed of a public   filehandle and a multi-component path:        For Canonical Path:                LOOKUP FH=0x0, "/a/b/c"Chiu, et al.                 Informational                      [Page 3]

RFC 2755            Security Negotiation for WebNFS         January 2000        For Native Path:                LOOKUP FH=0x0, 0x80 "a:b:c"   A multi-component path is either an ASCII string of slash separated   components or a 0x80 character followed by a native path.  Note that   a multi-component LOOKUP implies the use of the public filehandle in   the LOOKUP.   Similar to the MCL request, a SNEGO-MCL request consists of a public   filehandle and a pathname.  However, the pathname is uniquely   composed, as described below, to distinguish it from other pathnames.   The pathname used in a SNEGO-MCL is the regular WebNFS multi-   component path prefixed with two octets.  The first prefixed octet is   the 0x81 non-ascii character, similar to the 0x80 non-ascii character   for the native paths.  This octet represents client's indication to   negotiate security mechanisms.  It is followed by the security index   octet which stores the current value of the index into the array of   security mechanisms to be returned from the server.  The security   index always starts with one and gets incremented as negotiation   continues.  It is then followed by the pathname, either an ASCII   string of slash separated canonical components or 0x80 and a native   path.   A security negotiation multi-component LOOKUP request looks like   this:        For Canonical Path:                LOOKUP FH=0x0, 0x81 <sec-index> "/a/b/c"        For Native Path:                LOOKUP FH=0x0, 0x81 <sec-index> 0x80 "a:b:c"   In the next section we will see how the MCL response is modified for   WebNFS security negotiation.3. Overloaded Filehandle   As described inRFC2054, if a multi-component LOOKUP request   succeeds, the server responds with a valid filehandle:        LOOKUP FH=0x0, "a/b/c"                        ----------->                        <-----------                                       FH=0x3Chiu, et al.                 Informational                      [Page 4]

RFC 2755            Security Negotiation for WebNFS         January 2000   NFS filehandles are used to uniquely identify a particular file or   directory on the server and are opaque to the client.  The client   neither examines a filehandle nor has any knowledge of its contents.   Thus, filehandles make an ideal repository for the server to return   the array of security mechanisms to the client in response to a   SNEGO-MCL request.   To a successful SNEGO-MCL request the server responds, in place of   the filehandle, with an array of integers that represents the valid   security mechanisms the client must use to access the given path. A   length field is introduced to store the size (in octets) of the array   of integers.   As the filehandles are limited in size (32 octets for NFS v2 and up   to 64 octets for NFS v3), it can happen that there are more security   mechanisms than the filehandles can accommodate.  To circumvent this   problem, a one-octet status field is introduced which indicates   whether there are more security mechanisms (1 means yes, 0 means no)   that require the client to perform another SNEGO-MCL to get them.   To summarize, the response to a SNEGO-MCL request contains, in place   of the filehandle, the length field, the status field, and the array   of security mechanisms:        FH: length, status, {sec_1  sec_2 ... sec_n}   The next two sub-sections describe how NFS v2 and v3 filehandles are   "overloaded" to carry the length and status fields and the array of   security mechanisms.3.1 Overloaded NFS Version 2 Filehandle   A regular NFS v2 filehandle is defined inRFC1094 as an opaque value   occupying 32 octets:     1   2   3   4                                                32   +---+---+---+---+---+---+---+---+     +---+---+---+---+---+---+---+   |   |   |   |   |   |   |   |   | ... |   |   |   |   |   |   |   |   +---+---+---+---+---+---+---+---+     +---+---+---+---+---+---+---+   An overloaded NFS v2 filehandle looks like this:     1   2   3   4   5           8                                  32   +---+---+---+---+---+---+---+---+     +---+---+---+---+     +---+---+   | l | s |   |   |     sec_1     | ... |     sec_n     | ... |   |   |   +---+---+---+---+---+---+---+---+     +---+---+---+---+     +---+---+Chiu, et al.                 Informational                      [Page 5]

RFC 2755            Security Negotiation for WebNFS         January 2000   Note that the first four octets of an overloaded NFS v2 filehandle   contain the length octet, the status octet, and two padded octets to   make them XDR four-octet aligned.  The length octet l = 4 * n, where   n is the number of security mechanisms sent in the current overloaded   filehandle.  Apparently, an overloaded NFS v2 filehandle can carry up   to seven security mechanisms.3.2 Overloaded NFS Version 3 Filehandle   A regular NFS v3 filehandle is defined inRFC1813 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 four octet   value which describes the number of valid octets that follow:  1           4+---+---+---+---+|      len      |+---+---+---+---+  1           4                                              up to 64+---+---+---+---+---+---+---+---+---+---+---+---+     +---+---+---+---+|   |   |   |   |   |   |   |   |   |   |   |   | ... |   |   |   |   |+---+---+---+---+---+---+---+---+---+---+---+---+     +---+---+---+---+An overloaded NFS v3 filehandle looks like the following:  1           4+---+---+---+---+|      len      |+---+---+---+---+  1           4   5           8+---+---+---+---+---+---+---+---+     +---+---+---+---+| s |   |   |   |     sec_1     | ... |     sec_n     |+---+---+---+---+---+---+---+---+     +---+---+---+---+   Here, len = 4 * (n+1).  Again, n is the number of security mechanisms   contained in the current overloaded filehandle.  Three octets are   padded after the status octet to meet the XDR four-octet alignment   requirement.  An overloaded NFS v3 filehandle can carry up to fifteen   security mechanisms.4. WebNFS Security Negotiation   With the SNEGO-MCL request and the overloaded NFS v2 and v3   filehandles defined above, the following diagram depicts the WebNFS   security negotiation protocol:Chiu, et al.                 Informational                      [Page 6]

RFC 2755            Security Negotiation for WebNFS         January 2000    Client                                      Server    ------                                      ------        LOOKUP FH=0x0, 0x81 <sec-index> "path"                        ----------->                        <-----------                            FH: length, status, {sec_1  sec_2 ... sec_n}   where      0x81 represents client's indication to negotiate security      mechanisms with the server,      path is either an ASCII string of slash separated components or      0x80 and a native path,      sec-index, one octet, contains the index into the array of      security mechanisms the server uses to protect the specified path,      status, one octet, indicates whether there are more security      mechanisms (1 means yes, 0 means no) that require the client to      perform another SNEGO-MCL to get them,      length (one octet for NFS v2 and four octets for NFS v3) describes      the number of valid octets that follow,      {sec_1 sec_2 ... sec_n} represents the array of security      mechanisms.  As noted earlier, each security mechanism is      represented by a four-octet integer.   Here is an example showing the WebNFS security negotiation protocol   with NFS v2.  In the example it is assumed the server shares /export   with 10 security mechanisms {0x3900 0x3901 0x3902 ... 0x3909} on the   export, two SNEGO-MCL requests would be needed for the client to get   the complete security information:    LOOKUP FH=0x0, 0x81 0x01 "/export"                        ----------->                        <-----------        0x1c, 0x01, {0x3900 0x3901 0x3902 0x3903 0x3904 0x3905 0x3906}    LOOKUP FH=0x0, 0x81 0x08 "/export"                        ----------->                        <-----------        0x0c, 0x00, {0x3907 0x3908 0x3909}Chiu, et al.                 Informational                      [Page 7]

RFC 2755            Security Negotiation for WebNFS         January 2000   The order of the security mechanisms returned in an overloaded   filehandle implies preferences, i.e., one is more recommended than   those following it.  The ordering is the same as that returned by the   MOUNT v3 protocol.   The following shows a typical scenario which illustrates how the   WebNFS security negotiation is accomplished in the course of   accessing publicly shared filesystems.   Normally, a WebNFS client first makes a regular multi-component   LOOKUP request using the public filehandle to obtain the filehandle   for the specified path.  Since the WebNFS client does not have any   prior knowledge as to how the path is protected by the server the   default security mechanism is used in this first multi-component   LOOKUP.  If the default security mechanism does not meet server's   requirements, the server replies with the AUTH_TOOWEAK RPC   authentication error, indicating that the default security mechanism   is not valid and the WebNFS client needs to use a stronger one.   Upon receiving the AUTH_TOOWEAK error, to find out what security   mechanisms are required to access the specified path the WebNFS   client sends a SNEGO-qMCL request, using the default security   mechanism.   If the SNEGO-MCL request succeeds the server responds with the   filehandle overloaded with the array of security mechanisms required   for the specified path.  If the server does not support WebNFS   security negotiation, the SNEGO-MCL request fails with NFSERR_IO for   NFS v2 or NFS3ERR_IO for NFS v3 [RFC2055].   Depending on the size of the array of security mechanisms, the WebNFS   client may have to make more SNEGO-MCL requests to get the complete   array.   For successful SNEGO-MCL requests, the WebNFS client retrieves the   array of security mechanisms from the overloaded filehandle, selects   an appropriate one, and issues a regular multi-component LOOKUP using   the selected security mechanism to acquire the filehandle.   All subsequent NFS requests are then made using the selected security   mechanism and the filehandle.   The following depicts the scenario outlined above.  It is assumed   that the server shares /export/home as follows:        share -o sec=sec_1:sec_2:sec_3,public /export/homeChiu, et al.                 Informational                      [Page 8]

RFC 2755            Security Negotiation for WebNFS         January 2000   and AUTH_SYS is the client's default security mechanism and is not   one of {sec_1, sec_2, sec_3}.        Client                                          Server        ------                                          ------            LOOKUP FH=0x0, "/export/home"                                     AUTH_SYS                                    ----------->                                    <-----------                                                        AUTH_TOOWEAK            LOOKUP FH=0x0, 0x81 0x01 "/export/home"                                     AUTH_SYS                                    ----------->                                    <-----------                     overloaded FH: length, status, {sec_1 sec_2 sec_3}            LOOKUP FH=0x0, "/export/home"                                        sec_n                                    ----------->                                    <-----------                                                        FH = 0x01            NFS request with FH=0x01                                        sec_n                                    ----------->                                    <-----------                                                        ...   In the above scenario, the first request is a regular multi-component   LOOKUP which fails with the AUTH_TOOWEAK error.  The client then   issues a SNEGO-MCL request to get the security information.   There are WebNFS implementations that allow the public filehandle to   work with NFS protocol procedures other than LOOKUP.  For those   WebNFS implementations, if the first request is not a regular multi-   component LOOKUP and it fails with AUTH_TOOWEAK, the client should   issue a SNEGO-MCL with        0x81 0x01 "."   as the path to get the security information.Chiu, et al.                 Informational                      [Page 9]

RFC 2755            Security Negotiation for WebNFS         January 20005. Security Considerations   The reader may note that no mandatory security mechanisms are   specified in the protocol that the client must use in making SNEGO-   MCL requests.  Normally, the client uses the default security   mechanism configured on his system in the first SNEGO-MCL request.   If the default security mechanism is not valid the server replies   with the AUTH_TOOWEAK error. In this case the server does not return   the array of security mechanisms to the client.  The client can then   make another SNEGO-MCL request using a stronger security mechanism.   This continues until the client hits a valid one or has exhausted all   the supported security mechanisms.6. References   [RFC1094] Sun Microsystems, Inc., "NFS: Network File System Protocol             Specification",RFC 1094, March 1989.http://www.ietf.org/rfc/rfc1094.txt   [RFC1813] Callaghan, B., Pawlowski, B. and P. Staubach, "NFS Version             3 Protocol Specification",RFC 1813, June 1995.http://www.ietf.org/rfc/rfc1813.txt   [RFC2054] Callaghan, B., "WebNFS Client Specification",RFC 2054,             October 1996.http://www.ietf.org/rfc/rfc2054.txt   [RFC2055] Callaghan, B., "WebNFS Server Specification",RFC 2055,             October 1996.http://www.ietf.org/rfc/rfc2055.txt   [RFC2203] Eisler, M., Chiu, A. and Ling, L., "RPCSEC_GSS Protocol             Specification",RFC 2203, September 1997.http://www.ietf.org/rfc/rfc2203.txt7. Acknowledgements   This specification was extensively brainstormed and reviewed by the   NFS group of Solaris Software Division.Chiu, et al.                 Informational                     [Page 10]

RFC 2755            Security Negotiation for WebNFS         January 20008. Authors' Addresses   Alex Chiu   Sun Microsystems, Inc.   901 San Antonio Road   Palo Alto, CA 94303   Phone: +1 (650) 786-6465   EMail: alex.chiu@Eng.sun.com   Mike Eisler   Sun Microsystems, Inc.   901 San Antonio Road   Palo Alto, CA 94303   Phone: +1 (719) 599-9026   EMail: michael.eisler@Eng.sun.com   Brent Callaghan   Sun Microsystems, Inc.   901 San Antonio Road   Palo Alto, CA 94303   Phone: +1 (650) 786-5067   EMail: brent.callaghan@Eng.sun.comChiu, et al.                 Informational                     [Page 11]

RFC 2755            Security Negotiation for WebNFS         January 20009. Full Copyright Statement   Copyright (C) The Internet Society (2000).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Chiu, et al.                 Informational                     [Page 12]