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Using DNS SRV to Specify a Global File Namespace with NFS Version 4

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6641.
Authors Craig Everhart , Andy Adamson , Jiaying Zhang
Last updated 2015-10-14 (Latest revision 2012-03-08)
Replaces draft-everhart-nfsv4-namespace-via-dns-srv
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
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Shepherd write-up Show Last changed 2011-08-17
IESG IESG state Became RFC 6641 (Proposed Standard)
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Responsible AD Martin Stiemerling
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Network Working Group                                        C. Everhart
Internet-Draft                                                W. Adamson
Intended status: Standards Track                                  NetApp
Expires: September 9, 2012                                      J. Zhang
                                                           March 8, 2012

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


   The NFS version 4 protocol provides a 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 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 file name space.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 9, 2012.

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents

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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.  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  . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Filesystem integration issues  . . . . . . . . . . . . . .  7
     4.4.  Multicast DNS  . . . . . . . . . . . . . . . . . . . . . .  8
   5.  Where is this integration carried out? . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 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

   Version 4 of the NFS protocol [RFC3530] introduced the fs_locations
   attribute.  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 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.  Use of SRV Resource Record in DNS

   Providing an organization's published filesystem name space is a
   service, and the DNS [RFC1034][RFC1035] provides methods for
   discovery of that service.  This standard defines a mapping from a
   DNS name to the NFS filesystem(s) providing the root of the
   filesystem name space associated with that DNS name; such filesystems
   are called "domain root" filesystems.  From such filesystems, like
   other NFS filesystems, an NFS client can use the standard NFS
   mechanisms to navigate the rest of the NFS file servers that make up

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   the filesystem name space for the given domain.

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

   The DNS SRV resource record type [RFC2782] is used to 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

   The Service name used is "_nfs-domainroot", in conformance with RFC
   6335 [RFC6335].  The Protocol name used is "_tcp", for NFS service
   over TCP.  Future NFS services using other protocols MUST use another
   Protocol name.  The "_udp" label MUST NOT be used to imply use of UDP
   with NFSv4, as neither RFC 3530 [RFC3530] nor RFC 5661 [RFC5661]
   defines NFSv4 over UDP.  The Target fields give the domain names of
   the NFS servers that export filesystems for the domain's root.  An
   NFS client may then interpret any of the exported root filesystems as
   the root of the filesystem published by the organization with the
   given domain name.

   The domain root service is not useful for NFS versions prior to v4,
   as the fs_locations attribute was introduced only in NFSv4 (as
   described in Section 2).

   In order to allow the NFSv4 servers so given to export a variety of
   filesystems, those file servers 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 would publish records like

      _nfs-domainroot._tcp IN SRV 0 0 2049
      _nfs-domainroot._tcp IN SRV 1 0 18204

   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

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   server with which to begin a network conversation.  The two file
   servers would export filesystems that would be found at
   "/.domainroot/" in their pseudo-filesystems, which clients
   would mount.  Clients then carry out subsequent accesses in
   accordance with the ordinary NFS Version 4 protocol.  The first
   record uses the port number 2049 assigned to NFS, and another port is
   specified for the second record; the NFS servers would provide NFS
   service at their indicated port numbers, and NFS clients would
   connect to the service via the corresponding port numbers on those
   indicated servers.

   Other filesystem protocols could make use of the same "domain root"
   abstraction, necessarily under different Service names not specified

4.  Integration with Use of NFS Version 4

   NFSv4 clients adhering to this specification implement a special
   directory, analogous to an Automounter [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 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.

   As noted in Section 3, the domain root service is not useful for NFS
   versions prior to version 4.

4.1.  Globally-useful names: conventional mount point

   In order that the inter-organizational name space function as a
   global name space, the client-side mount point for that name space
   must be the same on different clients.  Conventionally, on POSIX
   machines, the name /nfs4/ 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

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   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, NFS servers MUST use the
   "/.domainroot/{Name}" string.  Thus, the servers listed as targets
   for the SRV resource records MUST export the root of the
   organization's published filesystem as the directory "/.domainroot/
   {Name}" (for the given organization Name) in their exported NFS
   namespaces.  For example, for organization "", the
   directory "/.domainroot/" would be used.

   Chapter 11 of the NFS Version 4.1 document [RFC5661] describes the
   approach that an NFS client should take to navigating
   fs_locations_info 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.  The organization's file
   system representatives can be navigated to provide access to higher-
   cost properties such as writability or 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.

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 RECOMMENDED: 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 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.  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.

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4.4.  Multicast DNS

   Location of the NFS domain root by this SRV record is intended to be
   performed with unicast by using ordinary DNS [RFC1034][RFC1035]

   This document does not define the use of this DNS SRV record format
   in conjunction with Multicast DNS (mDNS).  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 has too
   little value to use in locating NFSv4 domain roots.

5.  Where is this integration carried out?

   The NFS client is responsible for interpreting SRV records.  Using
   something like Automounter [AMD1] [AMD2] technology, the client
   interprets names under a particular directory, discovering the
   appropriate filesystem to mount, and mounting it in the specified
   place in the client name space before returning control to the
   application doing a lookup.  The result of the DNS lookup should be
   cached (obeying TTL) so that the result could be returned more
   quickly the next time.

6.  Security Considerations

   This functionality introduces a new reliance of NFSv4 on the
   integrity of DNS.  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 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.

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   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

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   This document requests the assignment of a new Service name without
   an associated port number (as defined in RFC 6335 [RFC6335]), for
   TCP.  For this new Service, the Reference is this document.

   Service name: nfs-domainroot
   Transport Protocol(s) TCP
   Assignee (REQUIRED)  IESG (
   Contact (REQUIRED)   IETF Chair (
   Description (REQUIRED) NFS service for the domain root, the root of
                       an 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

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              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.

   [RFC1813]  Callaghan, B., Pawlowski, B., and P. Staubach, "NFS
              Version 3 Protocol Specification", RFC 1813, June 1995.

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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

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