Internet DRAFT - draft-ietf-nfsv4-federated-fs-dns-srv-namespace
draft-ietf-nfsv4-federated-fs-dns-srv-namespace
Network Working Group C. Everhart
Internet-Draft W. Adamson
Intended status: Standards Track NetApp
Expires: September 9, 2012 J. Zhang
Google
March 8, 2012
Using DNS SRV to Specify a Global File Name Space with NFS version 4
draft-ietf-nfsv4-federated-fs-dns-srv-namespace-13.txt
Abstract
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.
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This Internet-Draft will expire on September 9, 2012.
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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",
"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
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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
RR.
As an example, suppose a client wished to locate the root of the
filesystem 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.
The result domain names nfs1tr.example.net and nfs2ex.example.net
indicate NFS Version 4 file servers that export the root of the
published name space 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
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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/example.net" 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 example.net
published filesystem would be accessible as
/nfs4/example.net/
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
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 "example.net", the
directory "/.domainroot/example.net" 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]
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.
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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
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 (nfs1tr.example.net and nfs2ex.example.net) are located as
hosting the root filesystem for the organization example.net. 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
nfs@example.net@nfs2ex.example.net
in order to verify that the named server (nfs2ex.example.net) is
authorized to provide the root filesystem 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 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 (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 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,
<http://docs.freebsd.org/info/amdref/amdref.pdf>.
[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
NetApp
800 Cranberry Woods Drive, Ste. 300
Cranberry Township, PA 16066
US
Phone: +1 724 741 5101
Email: everhart@netapp.com
W.A. (Andy) Adamson
NetApp
495 East Java Drive
Sunnyvale, CA 94089
US
Phone: +1 734 665 1204
Email: andros@netapp.com
Jiaying Zhang
Google
604 Arizona Avenue
Santa Monica, CA 90401
US
Phone: +1 310 309 6884
Email: jiayingz@google.com
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