Network Working Group C. Everhart
Internet-Draft NetApp
Intended status: Standards Track W.A. Adamson
Expires: August 02, 2012 NetApp
J. Zhang
February 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.

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Table of Contents

1. Requirements notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 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 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). The "_nfs" Service name is not limited to NFSv4; it is possible to use that prefix in naming additional Services (and their SRV records) that are also applicable to other versions of NFS (e.g., NFSv3 [RFC1813].

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 RR.

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 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 here.

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 the special directory that is populated with domain names, leading to file servers and filesystems that capture the results of SRV record lookups.

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.

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] protocol.

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. 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 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 organization.

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 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

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", March 1997.
[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.
[RFC5661] Shepler, S., Eisler, M. and D. Noveck, Editors, "Network File System (NFS) Version 4 Minor Version 1 Protocol", RFC 5661, January 2010.
[RFC1034] Mockapetris, P.V., "Domain Names - Concepts and Facilities", RFC 1034, November 1987.
[RFC1035] Mockapetris, P.V., "Domain Names - Implementation and Specification", RFC 1035, November 1987.
[RFC2782] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.
[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.
[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

[DFS] Kazar, M.L., Leverett, B.W., Anderson, O.T., Apostolides, V., Bottos, B.A., Chutani, S., Everhart, C.F., Mason, W.A., Tu, S.-T. and E.R. Zayas, "DEcorum File System Architectural Overview", Proc. USENIX Summer Conf. Anaheim, Calif., June 1990.
[AMD1] Pendry, J.-S. 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.
[AFS] Howard, J.H., "An Overview of the Andrew File System"", Proc. USENIX Winter Tech. Conf. Dallas, February 1988.
[RFC1813] Callaghan, B., Pawlowski, B. and P. Staubach, "NFS Version 3 Protocol Specification", RFC 1813, June 1995.

Authors' Addresses

Craig Everhart NetApp 800 Cranberry Woods Drive, Ste. 300 Cranberry Township, PA 16066 US Phone: +1 724 741 5101 EMail:
W.A. (Andy) Adamson NetApp 495 East Java Drive Sunnyvale, CA 94089 US Phone: +1 734 665 1204 EMail:
Jiaying Zhang Google 604 Arizona Avenue Santa Monica, CA 90401 US Phone: +1 310 309 6884 EMail: