Internet Engineering Task Force (IETF) C. Everhart
Request for Comments: 6641 W. Adamson
Category: Standards Track NetApp
ISSN: 2070-1721 J. Zhang
Google
June 2012
Using DNS SRV to Specify a Global File Namespace with NFS Version 4
Abstract
The NFS version 4 (NFSv4) protocol provides a mechanism for a
collection of NFS file servers to collaborate in providing an
organization-wide file namespace. The DNS SRV Resource Record (RR)
allows a simple way for an organization to publish the root of its
file system namespace, 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 namespaces together to allow
construction of a global, uniform NFS file namespace.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6641.
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Copyright Notice
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Table of Contents
1. Background ......................................................3
2. Requirements Notation ...........................................3
3. Use of the SRV Resource Record in DNS ...........................3
4. Integration with Use of NFS Version 4 ...........................5
4.1. Globally Useful Names: Conventional Mount Point ............5
4.2. Mount Options ..............................................6
4.3. File System Integration Issues .............................6
4.4. Multicast DNS ..............................................7
5. Where Is This Integration Carried Out? ..........................7
6. Security Considerations .........................................7
7. IANA Considerations .............................................9
8. References ......................................................9
8.1. Normative References .......................................9
8.2. Informative References ....................................10
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1. Background
Version 4 of the NFS protocol [RFC3530] introduced the fs_locations
attribute. Use of this attribute was elaborated further in the NFSv4
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 namespace
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 file system in some other set of servers. The mechanism is
general, allowing servers to describe any file system as being
reachable by requests to any of a set of servers. Thus, starting
with a single NFSv4 server, using these referrals, an NFSv4 client
could see a large namespace associated with a collection of
interrelated NFSv4 file servers. An organization could use this
capability to construct a uniform file namespace for itself.
An organization might wish to publish the starting point for this
namespace 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 that clients know only the
smallest amount of information in order to locate the appropriate
namespace. Also, 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.
2. 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].
3. Use of the SRV Resource Record in DNS
Providing an organization's published file system namespace 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 file system(s) providing the root of the file
system namespace associated with that DNS name; such file systems are
called "domain root" file systems. From such file systems, like
other NFS file systems, an NFS client can use the standard NFS
mechanisms to navigate the rest of the NFS file servers that make up
the file system namespace for the given domain.
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Such domain root file systems are mounted at a conventional point in
the NFS client namespace. The mechanism results in a uniform cross-
organizational file namespace, similar to that seen in both AFS
[AFS][RFC5864] and Distributed Computing Environment / Distributed
File System (DCE/DFS) [DFS]. An NFS client need know only the domain
name for an organization in order to locate the file namespace
published by that organization.
The DNS SRV RR 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 file systems for the domain's root. An
NFS client may then interpret any of the exported root file systems
as the root of the file system published by the organization with the
given domain name.
The domain root service is not useful for NFS versions prior to
version 4, as the fs_locations attribute was introduced only in NFSv4
(as described in Section 1).
In order to allow the NFSv4 servers as given to export a variety of
file systems, those file servers MUST export the given domain's root
file systems at "/.domainroot/{Name}" within their pseudo-file
systems, 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 file
system published by organization example.net. The DNS servers for
the domain would publish records like
$ORIGIN example.net.
_nfs-domainroot._tcp IN SRV 0 0 2049 nfs1tr.example.net.
_nfs-domainroot._tcp IN SRV 1 0 18204 nfs2ex.example.net.
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The resulting domain names nfs1tr.example.net and nfs2ex.example.net
indicate NFSv4 file servers that export the root of the published
namespace for the example.net 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
file systems that would be found at "/.domainroot/example.net" in
their pseudo-file systems, which clients would mount. Clients then
carry out subsequent accesses in accordance with the ordinary NFSv4
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 file system protocols could make use of the same domain root
abstraction, but it is necessary to use 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 file
system(s) in that name in the special directory. The goal is that
NFSv4 applications will be able to look up an organization's domain
name in the special directory, and the NFSv4 client will be able to
discover the file system that the 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 file system 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 for the inter-organizational namespace to function as a
global file namespace, the client-side mount point for that namespace
must be the same on different clients. Conventionally, on Portable
Operating System Interface (POSIX) machines, the name "/nfs4/" is
used so that names on one machine will be directly usable on any
machine. Thus, the example.net published file system would be
accessible as
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/nfs4/example.net/
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 file systems that capture the results of SRV record
lookups.
4.2. Mount Options
SRV records are necessarily less complete than the information in the
existing NFSv4 attributes fs_locations [RFC3530] or fs_locations_info
[RFC5661]. For the rootpath field of fs_location, or the fli_fs_root
field of fs_locations_info, NFS servers MUST use the "/.domainroot/
{Name}" string. Thus, the servers listed as targets for the SRV RRs
MUST export the root of the organization's published file system as
the directory "/.domainroot/{Name}" (for the given organization Name)
in their exported NFS namespaces. For example, for organization
example.net, the directory "/.domainroot/example.net" would be used.
Section 11 of the NFSv4.1 document [RFC5661] describes the approach
that an NFS client should take to navigate fs_locations_info
information.
The process of mounting an organization's namespace 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 file
system 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 the
default use when navigating to the base information for an
organization ought to be as low-overhead as possible.
4.3. File System Integration Issues
The result of the DNS search SHOULD appear as a (pseudo-)directory in
the client namespace. 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 namespace 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 the 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, by first discovering
the appropriate file system to mount and then mounting it in the
specified place in the client namespace before returning control to
the application doing a lookup. The result of the DNS lookup should
be cached (obeying Time to Live (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, rather than the
legitimate 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 DNS Security Extensions (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 file systems).
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
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
file system is being sought, and a hostname given in the target of
the DNS SRV RR. Thus, in the example above, two file servers
(nfs1tr.example.net and nfs2ex.example.net) are located as hosting
the root file system for the organization example.net. To
communicate with, for instance, the second of the given file servers,
Generic Security Service Application Program Interface (GSS-API) is
used with the name-type of GSS_C_NT_DOMAINBASED_SERVICE defined in
RFC 5178 and with a symbolic name of
nfs@example.net@nfs2ex.example.net
in order to verify that the named server (nfs2ex.example.net) is
authorized to provide the root file system for the example.net
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 file system(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.
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7. IANA Considerations
IANA has assigned 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 (iesg@ietf.org)
Contact (REQUIRED) IETF Chair (chair@ietf.org)
Description (REQUIRED) NFS service for the domain root, the root of
an organization's published file namespace.
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",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, 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.
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[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., Ed., Eisler, M., Ed., and D. Noveck, Ed.,
"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", BCP 165,
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,
<http://docs.freebsd.org/info/amdref/amdref.pdf>.
[AMD2] Crosby, M., "AMD--AutoMount Daemon", Linux Journal,
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.
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Authors' Addresses
Craig Everhart
NetApp
800 Cranberry Woods Drive, Ste. 300
Cranberry Township, PA 16066
USA
Phone: +1 724 741 5101
EMail: everhart@netapp.com
W.A. (Andy) Adamson
NetApp
495 East Java Drive
Sunnyvale, CA 94089
USA
Phone: +1 734 665 1204
EMail: andros@netapp.com
Jiaying Zhang
Google
604 Arizona Avenue
Santa Monica, CA 90401
USA
Phone: +1 310 309 6884
EMail: jiayingz@google.com
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