Network Working Group                                        C. Everhart
Internet-Draft                                                W. Adamson
Intended status: Standards Track                                  NetApp
Expires: November 26, 2010                                      J. Zhang
                                                            May 25, 2010

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


   The NFS version 4 protocol provides a natural way 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 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.

Status of this Memo

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   This Internet-Draft will expire on November 26, 2010.

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   carefully, as they describe your rights and restrictions with respect
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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
   5.  Where is this integration carried out? . . . . . . . . . . . .  8
   6.  Relationship to DNS NFS4ID RR  . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Requirements notation

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

2.  Background

   With the advent of fs_locations attributes in the NFS Version 4
   protocol [RFC3530], NFS servers can cooperate to build a file name
   space that crosses server boundaries, as detailed in the description
   of referrals in [NB0510] and as further developed in the NFS Version
   4 Minor Version 1 protocol [RFC5661].  With NFS Version 4 referrals,
   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 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 for an
   organization in order to locate its filesystem name space service.
   Once a client uses the DNS to locate one or more servers for the root
   of the organization's name space, it can use the standard NFS Version
   4 mechanisms to navigate the remainder of the NFS servers for that
   organization.  The use of this proposed mechanism results in a useful
   cross-organizational name space, just as in AFS [AFS] and DCE/DFS

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   [DFS] before it.  A client need know only the name of the
   organization in order to locate the filesystem name space published
   by that organization.

   We propose the use of the DNS SRV resource record type [RFC2782] to
   fulfill this function.  The format of the DNS SRV record is as

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

   In our case, we use a Service name of "_nfs4._domainroot" and a
   conventional Protocol of "_tcp".  (In accordance with RFC 2782
   [RFC2782], we use "_" prefix characters on the Service and Protocol
   names, but we recognize that there is work in progress [Gudmundsson]
   to create a registry of these names and to no longer use the "_"
   prefix for some names.)  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 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 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 publish records like

      _nfs4._domainroot._tcp IN SRV 0 0 2049
      _nfs4._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 filesystem protocols could make use of

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   the same "_domainroot" abstraction.

4.  Integration with Use of NFS Version 4

   We expect that NFSv4 clients will implement a special directory,
   analogous to an Automounter [AMD] 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 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, as AFS did with its
   "root.afs" functionality.

   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.

4.1.  Globally-useful names: conventional mount point

   For the inter-organizational name space to be a global name space, it
   is useful for its mount point in local systems to be uniform as well.
   On POSIX machines, the name /nfs4/ SHOULD 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 use the "/.domainroot-

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   {Name}" string.  Thus, the servers listed as targets for the SRV
   resource records should export the root of the organization's
   published filesystem as the directory "/.domainroot-{Name}" (for the
   given organization Name) in its exported namespace.  For example, for
   organization "", the directory "/"
   should be used.

   As for the other attributes in fs_locations_info, the recommended
   approach is for a client to make its first possible contact with any
   of the referred-to servers, obtain the 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.

   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 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 also publish the
   location of a writable instance of their root filesystems, and that
   NFSv4 clients SHOULD 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", and the indicated server (or servers)
   SHOULD export the writable instance at "/.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

      _nfs4._domainroot._tcp IN SRV 0 0 2049
      _nfs4._domainroot._tcp IN SRV 1 0 2049
      _nfs4._write._domainroot._tcp IN SRV 0 0 2049

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   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: that only fully-qualified domain names appear as
   directories.  That is, in many environments, DNS names may be
   abbreviated from their fully-qualified form.  In such circumstances,
   multiple names might be given to filesystem code 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.  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.

5.  Where is this integration carried out?

   Another consideration 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] technology, the client would be responsible for
   interpreting names under a particular directory, discovering the
   appropriate filesystem to mount, and mounting it in the appropriate
   place in the client name space before returning control to the

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   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
   result of the DNS lookup should be cached (obeying TTL) so that the
   result could be returned more quickly the next time.

   We strongly suggest that this functionality be implemented by NFS
   clients.  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.  Yet even if it is desirable to deploy this
   functionality on the NFS client side, it may be valuable as a
   transition aid for a site to be able to deploy it on the NFS server
   side: it may be easier for them to install it on special NFS servers
   rather than install it on all their NFS clients.  Thus, from an
   implementation standpoint, NFS clients SHOULD implement the
   functionality, and NFS servers MAY implement it.

6.  Relationship to DNS NFS4ID RR

   This DNS use has no obvious relationship to the NFS4ID RR [Mesta].
   The NFS4ID RR is a mechanism to help clients and servers configure
   themselves with respect to the domain strings used in "who" strings
   in ACL entries and in owner and group names.  The authentication/
   authorization domain string of a server need have no direct
   relationship to the name of the organization that is publishing a
   file name space of which this server's filesystems form a part.  At
   the same time, it might be seen as straightforward or normal for such
   a server to refer to the ownership of most of its files using a
   domain string with an evident relationship to that NFS4ID-given
   domain name, but this document imposes no such requirement.

7.  Security Considerations

   Naive use of the DNS may effectively give clients published server
   referrals that are intrusive substitutes for the servers intended by
   domain administrators.

   It may be possible to build a trust chain by using DNSSEC [RFC4033]
   to implement this function on the client, or by implementing this
   function on an NFS Version 4 server that uses DNSSEC and maintaining
   a trust relationship with that server.  This trust chain also breaks
   if the SRV interpreter accepts responses from insecure DNS zones.

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   Thus, it would likely be prudent also to use domain-based service
   principal names for the servers for the root filesystems as indicated
   as the targets of the SRV records.  The idea here 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 an insecure DNS SRV RR lookup.  The domain administrator can
   thus ensure that only domain root NFSv4 servers have credentials for
   such domain-based service principal names.

   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 should be 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

8.  IANA Considerations

   This document does not raise actions for IANA, but it may require
   cosmetic changes if the in-progress work in the Gudmundsson/Hoenes
   draft [Gudmundsson] is adopted.

9.  References

9.1.  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.

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   [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.

9.2.  Informative References

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

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

   [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.

              Gudmundsson, O. and A. Hoenes, "Creation of a Registry for
              DNS SRV Record Service Prefixes", October 2009, <ftp://

   [Mesta]    Mesta, R., "A DNS RR for NFSv4 ID Domains", October 2005,

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   [NB0510]   Noveck, D. and R. Burnett, "Next Steps for NFSv4
              Migration/Replication", October 2005, <

   [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

   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

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