Network Working Group                                        C. Everhart
Internet-Draft                                                W. Adamson
Intended status: Standards Track                                  NetApp
Expires: March 30, 2009                                         J. Zhang
                                                      September 26, 2008

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

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   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.  DNS SRV can be used to join these organization-wide

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   file name spaces together to allow construction of a global, uniform
   NFS version 4 file name space.  This document refreshes the draft.

Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Proposed Use of SRV Resource Record in DNS . . . . . . . . . .  3
     3.1.  Deployment of the Resource Record  . . . . . . . . . . . .  4
   4.  Integration with Use of NFS Version 4  . . . . . . . . . . . .  5
     4.1.  Globally-useful names: conventional mount point  . . . . .  5
     4.2.  Mount options  . . . . . . . . . . . . . . . . . . . . . .  5
     4.3.  File system integration issues . . . . . . . . . . . . . .  6
   5.  Where is this integration carried out? . . . . . . . . . . . .  7
   6.  Relationship to DNS NFS4ID RR  . . . . . . . . . . . . . . . .  7
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  8
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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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].  With NFS Version 4 referrals, a file
   server may indicate to its client that the file system 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 file system name space is a
   service, and it is appropriate to use the DNS [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 file system 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
   [DFS] before it.  A client need know only the name of the

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   organization in order to locate the file system 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" and a conventional
   Protocol of "_tcp".  The Target fields give the domain names of the
   NFS Version 4 servers that export root filesystems.  An NFS Version 4
   client SHOULD interpret any of the exported pseudo-root filesystems
   as the filesystem published by the organization with the given domain

   Suppose a client wished to locate the root of the file system
   published by organization  The DNS servers for the
   domain could publish records like

      _nfs4._tcp IN SRV 0 0 2049
      _nfs4._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, 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.  Subsequent accesses are carried out
   in accordance with ordinary NFS Version 4 protocol.

3.1.  Deployment of the Resource Record

   As with any DNS resource, any server installation needs to concern
   itself with the likely loads and effects of the presence of the
   resource record.  The answers to requests for RRs might differ
   depending on what the server can tell about the client.  For example,
   some RRs might be returned only to those clients inside some network
   perimeter (to provide an intranet service) and requests from other
   clients might be denied.  As the RR directs the clients to ask for
   service from a given set of servers, the administrator should ensure
   that the identified servers can handle the expected load.
   Fortunately, the definition of the DNS SRV resource record offers a
   mechanism to distribute the load to multiple servers within a
   priority ordering.

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4.  Integration with Use of NFS Version 4

   There are at least two remaining questions: whether this DNS SRV
   record evaluation is done in the NFS server or client, and also how
   the domain names of the organizations are passed to client or server.
   A third question is how this might produce a uniform global file name
   space, and what prefix should be used for such file names.

   This specification anticipates that these SRV records will most
   commonly be used to define the second directory level in an inter-
   organizational file name space.  This directory will be populated
   with domain names pointing to the file systems published for use
   under those domain names.  Thus, the root directory for the file
   system published by will effectively be mounted
   underneath the name in a second-level directory.

   In general, a domain name will appear to a client as a directory name
   pointing to the root directory of the file system published by the
   organization responsible for that domain name.

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.
   The name /nfs4/ SHOULD be used so that names on one machine will be
   directly usable on any machine.  Thus, the published file
   system would be accessible as


   on any client.  Using this convention, "/nfs4/" is a mount for a
   special file system that is populated with 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 and the proposed
   fs_locations_info.  For the rootpath field of fs_location, we assume
   that the empty string is adequate.  Thus, the servers listed as
   targets for the SRV resource records should export the root of the
   organization's published file system as the pseudo-root in its
   exported namespace.

   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

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   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.

   We recommend, though, that 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, we recommend that the client 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.

   One extension of this rule that we might choose to inherit from AFS,
   though, is to give a special meaning to the domain name of an
   organization preceded by a period (".").  It might be reasonable to
   have names mounting the filesystem for a period-prefixed domain name
   (e.g., "") attempt to mount only a read-write instance of
   that organization's root filesystem, rather than permitting the use
   of read-only instances of that filesystem.  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.  File system integration issues

   The result of the DNS search SHOULD appear as a (pseudo-)directory in
   the client name space, cached for a time no longer than the RR's TTL.
   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 file
   system 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

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   usable outside the scope of any particular DNS abbreviation

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
   client or by the 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 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 DNS SRV records to determine the servers for the
   indicated published file system, and then returned that information
   via NFS Version 4 attributes as a referral in the way outlined by
   Noveck and Burnett [NB0510].  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.

6.  Relationship to DNS NFS4ID RR

   This DNS use has no obvious relationship to the NFS4ID RR.  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.

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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.
   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
   [RFC3530] and 5179 [RFC3530].  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.  References

8.1.  Normative References

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

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and

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              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.

8.2.  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.

   [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,

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              October 1990.

Authors' Addresses

   Craig Everhart
   7301 Kit Creek Road
   Research Triangle Park, NC  27709

   Phone: +1 919 476 5320

   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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