Network Working Group                                        C. Everhart
Internet-Draft                                                W. Adamson
Intended status: Standards Track                                  NetApp
Expires: March 5, April 9, 2012                                          J. Zhang
                                                       September 2,
                                                         October 7, 2011

  Using DNS SRV to Specify a Global File Name Space with NFS version 4


   The NFS version 4 protocol provides a natural way mechanism for a collection of
   NFS file servers to collaborate in providing an organization-wide
   file name space.  The DNS SRV RR allows a simple and appropriate way
   for an organization to publish the root of its filesystem name space,
   even to clients that might not be intimately associated with such an
   organization.  The DNS SRV RR can be used to join these organization-
   wide file name spaces together to allow construction of a global,
   uniform NFS version 4 file name space.

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Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Proposed  Use of SRV Resource Record in DNS  . . . . . . . . . . . . . .  4
   4.  Integration with Use of NFS Version 4  . . . . . . . . . . . .  6
     4.1.  Globally-useful names: conventional mount point  . . . . .  6
     4.2.  Mount options  . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Filesystem integration issues  . . . . . . . . . . . . . .  8
     4.4.  Multicast, mDNS, and DNS-SD  . . . . . . . . . . . . . . .  8
   5.  Where is this integration carried out? . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 11
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10 11
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 13

1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Background

   The NFS

   Version 4 of the NFS protocol [RFC3530] introduced the fs_locations
   attribute.  Its use  Use of this attribute was elaborated further in the NFS
   Version 4 Minor Version 1 protocol [RFC5661], which also defined an
   extended version of the attribute as fs_locations_info.  With the
   advent of these attributes, NFS servers can cooperate to build a file
   name space that crosses server boundaries.  The fs_locations and
   fs_locations_info attributes are used as referrals, so that a file
   server may indicate to its client that the file name tree beneath a
   given name in the server is not present on itself, but is represented
   by a filesystem in some other set of servers.  The mechanism is
   general, allowing servers to describe any filesystem as being
   reachable by requests to any of a set of servers.  Thus, starting
   with a single NFS Version 4 server, using these referrals, an NFS
   Version 4 client might be able
   to could see a large name space associated with a
   collection of interrelated NFS Version 4 file servers.  An
   organization could use this capability to construct a uniform file
   name space for itself.

   An organization might wish to publish the starting point for this
   name space to its clients.  In many cases, the organization will want
   to publish this starting point to a broader set of possible clients.
   At the same time, it is useful to require clients to know only the
   smallest amount of information in order to locate the appropriate
   name space.  Simultaneously, that required information should be
   constant through the life of an organization if the clients are not
   to require reconfiguration as administrative events change, for
   instance, a server's name or address.

3.  Proposed  Use of SRV Resource Record in DNS

   Providing an organization's published filesystem name space is a
   service, and it is appropriate to use the DNS [RFC1034][RFC1035] to
   locate it.  As with the AFSDB resource record type [RFC1183], the
   client need only utter the (relatively) constant domain name provides methods for an
   organization in order to locate its filesystem name space
   discovery of that service.
   Once  This standard defines a client uses the DNS mapping from a
   domain name to locate one or more servers for the root
   of the organization's name space, it NFS filesystem(s) associated with that name; such
   filesystems are called "domain root" filesystems.  From such
   filesystems, like other NFS filesystems, an NFS client can use the
   standard NFS Version
   4 mechanisms to navigate the remainder rest of the NFS file servers for
   organization. make up the filesystem name space for the given domain.

   Such "domain root" filesystems are mounted at a conventional point in
   the NFS client namespace.  The use of this proposed mechanism results in a useful
   cross-organizational uniform cross-
   organizational file name space, just as similar to that seen in both AFS [AFS]
   [AFS][RFC5864] and DCE/DFS
   [DFS] before it.  A [DFS].  An NFS client need know only the
   domain name of the for an organization in order to locate the filesystem
   name space published by that organization.

   We propose the use of the

   The DNS SRV resource record type [RFC2782] is used to
   fulfill this function. locate "domain
   root" file servers.  The format of the DNS SRV record is as follows:

      _Service._Proto.Name TTL Class SRV Priority Weight Port Target

   In our case, we use a

   The Service name of "_nfs4._domainroot" and a
   conventional is "_nfs._domainroot".  The Protocol as of "_tcp". this
   writing could be either "_tcp" or "_sctp"; version 4 NFS is not
   defined over UDP.  Other transport protocols could be defined in the
   future, and SRV records that advertise domain root file services with
   other transport protocols would use the same Service name.  The
   Target fields give the domain names of the NFS Version 4 servers that export
   filesystems for the domain's root.  An NFS Version 4 client SHOULD may then interpret
   any of the exported root filesystems as the filesystem published by
   the organization with the given domain name.

   In order to allow the NFSv4 servers so given to export a variety of
   filesystems, those file servers SHOULD MUST export the given domain's root
   filesystems at "/.domainroot-{Name}" within their pseudo-filesystems,
   where the "{Name}" is the name of the organization as used in the SRV

   As an example, suppose a client wished to locate the root of the
   filesystem published by organization  The DNS servers
   for the domain could would publish records like

      _nfs._domainroot._tcp IN SRV 0 0 2049
      _nfs._domainroot._tcp IN SRV 1 0 2049

   The result domain names and
   indicate NFS Version 4 file servers that export the root of the
   published name space for the domain.  In accordance with
   RFC 2782 [RFC2782], these records are to be interpreted using the
   Priority and Weight field values, selecting an appropriate file
   server with which to begin a network conversation.  The two file
   servers would export filesystems that would be found at
   "/" in their pseudo-filesystems, which clients
   would mount.  Clients then carry out subsequent accesses in
   accordance with the ordinary NFS Version 4 protocol.

   We use a composite Service name (built from "_nfs4" and
   "_domainroot") so that other

   Other filesystem protocols could make use of the same "_domainroot" abstraction. "domain root"
   abstraction, but this document discusses only the SRV records
   indicating NFS servers.

4.  Integration with Use of NFS Version 4

   We expect that

   NFSv4 clients will adhering to this specification implement a special
   directory, analogous to an Automounter [AMD] [AMD1][AMD2] directory, the
   entries in which are domain names that have recently been traversed.
   When an application attempts to traverse a new name in that special
   directory, the NFSv4 client consults DNS to obtain the SRV data for
   the given name, and if successful, it mounts the indicated
   filesystem(s) in that name in the special directory.  The goal is
   that NFSv4 NFS applications will be able to lookup an organization's domain
   name in the special directory, and the NFSv4 client will be able to
   discover the filesystem that that organization publishes.  Entries in
   the special directory will be domain names, and they will each appear
   to the application as a directory name pointing to the root directory
   of the filesystem published by the organization responsible for that
   domain name.

   This functionality does not require or use any list of organizations
   that are known to provide file service, such as was required with the
   "root.afs" functionality in AFS.

   This DNS SRV record evaluation could, in principle, be done either in
   the NFSv4 client or in an NFSv4 server.  In either case, the result
   would appear the same to applications on the NFSv4 client.  This is
   discussed further in section 5 of this document.

4.1.  Globally-useful names: conventional mount point


   In order that the inter-organizational name space to be function as a
   global name space, it
   is useful for its the client-side mount point in local systems to for that name space
   must be uniform as well.
   On the same on different clients.  Conventionally, on POSIX
   machines, the name /nfs4/ SHOULD is be used so that names on one machine
   will be directly usable on any machine.  Thus, the
   published filesystem would be accessible as


   on any POSIX client.  Using this convention, "/nfs4/" is the name of
   the special directory that is populated with domain names, leading to
   file servers and filesystems that capture the results of SRV record

4.2.  Mount options

   SRV records are necessarily less complete than the information in the
   existing NFS Version 4 attributes fs_locations [RFC3530] or
   fs_locations_info [RFC5661].  For the rootpath field of fs_location,
   or the fli_fs_root of fs_locations_info, we NFS servers MUST use the "/.domainroot-
   "/.domainroot-{Name}" string.  Thus, the servers listed as targets
   for the SRV resource records should MUST export the root of the
   organization's published filesystem as the directory "/.domainroot-{Name}" "/.domainroot-
   {Name}" (for the given organization Name) in its their exported namespace. NFS
   namespaces.  For example, for organization "", the
   directory "/"
   should would be used.

   As for

   Chapter 11 of the other attributes in fs_locations_info, NFS Version 4.1 document [RFC5661] describes the recommended
   approach is for a that an NFS client should take to make its first possible contact with any
   of the referred-to servers, obtain the navigating
   fs_locations_info structure
   from that server, and use the information from that obtained
   structure as the basis for its judgment of whether it would be better
   to use a different server representative from the set of servers for
   that filesystem. information.

   The process of mounting an organization's name space should permit
   the use of what is likely to impose the lowest cost on the server.
   Thus, the NFS client SHOULD NOT insist on using a writable copy of
   the filesystem if read-only copies exist, or a zero-age copy rather
   than a copy that may be a little older.  We presume that the
   organization's file name space can be navigated to provide access to
   higher-cost properties such as writability or currency freshness as necessary,
   but that the default use when navigating to the base information for
   an organization ought to be as low-overhead as possible.

   Because of the possible distinction between read-only and read-write
   versions of a filesystem, organizations SHOULD MAY also publish the location
   of a writable instance of their domain root filesystems, and that
   NFSv4 clients SHOULD would conventionally mount that filesystem under the
   organizational domain name preceded by a period (".").  Therefore,
   when names beginning with a period are looked up under the NFSv4
   client's special directory, the SRV RR looked up in DNS uses a
   Service name of
   "_nfs4._write._domainroot", "_nfs._write._domainroot", and the any indicated server
   (or servers)
   SHOULD MUST export the writable instance at "/.domainroot-write-{Name}" "/.domainroot-
   write-{Name}" for a domain name Name.

   Extending the opening example, suppose a client wished to locate the
   read-only and read-write roots of the filesystem published by
   organization  Suppose a read-write instance were hosted
   on server, and read-only instances were on that
   server and also on server  The DNS servers for
   the domain would publish records like

      _nfs._domainroot._tcp IN SRV 0 0 2049
      _nfs._domainroot._tcp IN SRV 1 0 2049
      _nfs._write._domainroot._tcp IN SRV 0 0 2049

   The server would export filesystems at both
   "/" (the read-only instance) and
   "/" (the read-write instance).  The server need export only the "/.domainroot-" name for its read-only instance.

   The read-write version of the filesystem would be mounted (upon use)
   under "" in the special directory, and a read-only
   version would be mounted under "".  Thus,


   might be a directory in a read-only instance of the root filesystem
   of the organization "", while


   would be a writable form of that same directory.  A small benefit of
   following this convention is that names with the period prefix are
   treated as "hidden" in many operating systems, so that the visible
   name remains the lowest-overhead name.

4.3.  Filesystem integration issues

   The result of the DNS search SHOULD appear as a (pseudo-)directory in
   the client name space.  A further refinement is advisable, and SHOULD
   be deployed: RECOMMENDED: that
   only fully-qualified domain names appear as directories.  That is, in
   many environments, DNS names may be abbreviated from their fully-qualified fully-
   qualified form.  In such circumstances, multiple names might be given
   to filesystem code NFS clients that all resolve to the same DNS SRV RRs.  The
   abbreviated form SHOULD be represented in the client's name space
   cache as a symbolic link, pointing to the fully-qualified name, case-canonicalized when appropriate. case-
   folded if the client filesystem is case-sensitive.  This will allow
   pathnames obtained with, say, getcwd() to include the DNS name that
   is most likely to be usable outside the scope of any particular DNS
   abbreviation convention.

4.4.  Multicast, mDNS, and DNS-SD

   Location of the NFS domain root does not involve IP-layer broadcast,
   multicast, or anycast communication.

   It is not expected that this DNS SRV record format will be used in
   conjunction with Multicast DNS (mDNS) or DNS Service Discovery
   (DNS-SD).  While mDNS could be used to locate a local domain root via
   these SRV records, no other domain's root could be discovered.  This
   means that mDNS with DNS-SD has too little value to use in locating
   NFSv4 domain roots.

5.  Where is this integration carried out?

   Another consideration

   The NFS client is what agent should be responsible for interpreting the SRV records.  It could be done just as well by the
   NFS client or by the NFS server, though we expect that most clients
   will include this function themselves.  Using
   something like Automounter [AMD] [AMD1] [AMD2] technology, the client would be responsible for
   interprets names under a particular directory, discovering the
   appropriate filesystem to mount, and mounting it in the appropriate specified
   place in the client name space before returning control to the
   application doing a lookup.  Alternatively, one could imagine the
   existence of an NFS version 4 server that awaited similar domain-name
   lookups, then consulted the SRV records in DNS to determine the
   servers for the indicated published filesystem, and then returned
   that information as an NFS Version 4 referral.  In either case, the  The result of the DNS lookup should be
   cached (obeying TTL) so that the result could be returned more
   quickly the next time.

   NFS clients compliant to this standard MUST implement this
   functionality.  While we recognize that it would be possible to
   configure clients so that they relied on a specially-configured
   server to do their SRV lookups for them, we feel that such a
   requirement would impose unusual dependencies and vulnerabilities for
   the deployers of such clients.

6.  Security Considerations

   This functionality introduces a new reliance of NFSv4 on the
   integrity of DNS.  Spoofed  Forged SRV records in DNS could cause the NFSv4
   client to connect to the file servers of an attacker, not the file
   servers of an organization.  This is similar to attacks that can be
   made on the base NFSv4 protocol, if server names are given in
   fs_location attributes: the client can be made to connect to the file
   servers of an attacker, not the file servers intended to be the
   target for the fs_location attributes.

   If DNSSEC [RFC4033] is available, it SHOULD be used to avoid both
   such attacks.  Domain-based service principal names are a an additional
   mechanism that also apply in this case, and it would be prudent to
   use them.  They provide a mapping from the domain name that the user
   specified to names of security principals used on the NFSv4 servers
   that are indicated as the targets in the SRV records (as providing
   file service for the root filesystems).

   With domain-based service principal names, the idea is that one wants
   to authenticate {nfs, domainname, host.fqdn}, not simply {nfs,
   host.fqdn}, when the server is a domain's root file server obtained
   through a DNS SRV RR lookup that may or may not have been secure.
   The domain administrator can thus ensure that only domain root NFSv4
   servers have credentials for such domain-based service principal

   Domain-based service principal names are defined in RFCs 5178
   [RFC5178] and 5179 [RFC5179].  To make use of RFC 5178's domain-based
   names, the syntax for "domain-based-name" MUST be used with a service
   of "nfs", a domain matching the name of the organization whose root
   filesystem is being sought, and a hostname given in the target of the
   DNS SRV resource record.  Thus, in the example above, two file
   servers ( and are located as
   hosting the root filesystem for the organization  To
   communicate with, for instance, the second of the given file servers,
   GSS-API is used with the name-type of GSS_C_NT_DOMAINBASED_SERVICE
   defined in RFC 5178 and with a symbolic name of


   in order to verify that the named server ( is
   authorized to provide the root filesystem for the

   NFSv4 itself contains a facility for the negotiation of security
   mechanisms to be used between NFS clients and NFS servers.  Section
   3.3 of RFC 3530 [RFC3530] and Section 2.6 of RFC 5661 [RFC5661] both
   describe how security mechanisms are to be negotiated.  As such,
   there is no need for this document to describe how that negotiation
   is to be carried out when the NFS client contacts the NFS server for
   the specified domain root filesystem(s).

   Using SRV records to advertise the locations of NFS servers may
   expose those NFS servers to attacks.  Organizations should carefully
   consider whether they wish their DNS servers to respond
   differentially to different DNS clients, perhaps exposing their SRV
   records to only those DNS requests that originate within a given
   perimeter, in order to reduce this exposure.

7.  IANA Considerations

   This document requests the assignment of two new Service names
   without associated port numbers, each for both TCP and SCTP.  For
   both Services, the Reference is this document.

   Service name: _nfs._domainroot
   Transport Protocol(s) TCP, SCTP
   Assignee (REQUIRED)  IESG (
   Contact (REQUIRED)   IETF Chair (
   Description (REQUIRED) NFS file service for the domain root, the root
                       of the organization's published file name space
   Reference (REQUIRED)  This document
   Port Number (OPTIONAL)
   Service Code (REQUIRED for DCCP only)
   Known Unauthorized Uses (OPTIONAL)
   Assignment Notes (OPTIONAL)

   Service name: _nfs._write._domainroot
   Transport Protocol(s) TCP, SCTP
   Assignee (REQUIRED)  IESG (
   Contact (REQUIRED)   IETF Chair (
   Description (REQUIRED)  Writable file server for the NFS file service
                     for the domain root, the root of the organization's
                     published file name space
   Reference (REQUIRED)  This document
   Port Number (OPTIONAL)
   Service Code (REQUIRED for DCCP only)
   Known Unauthorized Uses (OPTIONAL)
   Assignment Notes (OPTIONAL)

8.  References

8.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3530]  Shepler, S., Callaghan, B., Robinson, D., Thurlow, R.,
              Beame, C., Eisler, M., and D. Noveck, "Network File System
              (NFS) version 4 Protocol", RFC 3530, April 2003.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC5178]  Williams, N. and A. Melnikov, "Generic Security Service
              Application Program Interface (GSS-API)
              Internationalization and Domain-Based Service Names and
              Name Type", RFC 5178, May 2008.

   [RFC5179]  Williams, N., "Generic Security Service Application
              Program Interface (GSS-API) Domain-Based Service Names
              Mapping for the Kerberos V GSS Mechanism", RFC 5179,
              May 2008.

   [RFC5661]  Shepler, S., Eisler, M., and D. Noveck, Editors, "Network
              File System (NFS) Version 4 Minor Version 1 Protocol",
              RFC 5661, January 2010.

   [RFC5864]  Allbery, R., "DNS SRV Resource Records for AFS", RFC 5864,
              April 2010.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", RFC 6335,
              August 2011.

8.2.  Informative References

   [AFS]      Howard, J., "An Overview of the Andrew File System"",
              Proc. USENIX Winter Tech. Conf. Dallas, February 1988.


   [AMD1]     Pendry, J. and N. Williams, "Amd: The 4.4 BSD Automounter
              Reference Manual", March 1991,

   [AMD2]     Crosby, M., "AMD--AutoMount Daemon", Linux Journal 1997,
              35es Article 4, March 1997.

   [DFS]      Kazar, M., Leverett, B., Anderson, O., Apostolides, V.,
              Bottos, B., Chutani, S., Everhart, C., Mason, W., Tu, S.,
              and E. Zayas, "DEcorum File System Architectural
              Overview", Proc. USENIX Summer Conf. Anaheim, Calif.,
              June 1990.

   [RFC1183]  Everhart, C., Mamakos, L., Ullmann, R., and P.
              Mockapetris, "New DNS RR Definitions", RFC 1183,
              October 1990.

Authors' Addresses

   Craig Everhart
   800 Cranberry Woods Drive, Ste. 300
   Cranberry Township, PA  16066

   Phone: +1 724 741 5101

   W.A. (Andy) Adamson
   495 East Java Drive
   Sunnyvale, CA  94089

   Phone: +1 734 665 1204

   Jiaying Zhang
   604 Arizona Avenue
   Santa Monica, CA  90401

   Phone: +1 310 309 6884