Network Working Group C. Everhart
Internet-Draft W. Adamson
Intended status: Standards Track NetApp
Expires: March 5, 2012 J. Zhang
Google
September 2, 2011
Using DNS SRV to Specify a Global File Name Space with NFS version 4
draft-ietf-nfsv4-federated-fs-dns-srv-namespace-08.txt
Abstract
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. The DNS SRV RR can be used to join these organization-
wide file name spaces together to allow construction of a global,
uniform NFS version 4 file name space.
Status of this Memo
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This Internet-Draft will expire on March 5, 2012.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Proposed 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 . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Filesystem integration issues . . . . . . . . . . . . . . 8
5. Where is this integration carried out? . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
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
The NFS Version 4 protocol [RFC3530] introduced the fs_locations
attribute. Its use 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 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 filesystem name space is a
service, and it is appropriate to use the DNS [RFC1034][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 filesystem 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
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cross-organizational name space, just as in AFS [AFS] and DCE/DFS
[DFS] before it. A client need know only the name of the
organization in order to locate the filesystem 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
follows:
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
In our case, we use a Service name of "_nfs4._domainroot" and a
conventional Protocol of "_tcp". The Target fields give the domain
names of the NFS Version 4 servers that export filesystems for the
domain's root. An NFS Version 4 client SHOULD interpret any of the
exported root filesystems as the filesystem published by the
organization with the given domain name.
In order to allow the NFSv4 servers so given to export a variety of
filesystems, those file servers SHOULD 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 could publish records like
$ORIGIN example.net.
_nfs4._domainroot._tcp IN SRV 0 0 2049 nfs1tr.example.net.
_nfs4._domainroot._tcp IN SRV 1 0 2049 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
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.
We use a composite Service name (built from "_nfs4" and
"_domainroot") so that other filesystem protocols could make use of
the same "_domainroot" abstraction.
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4. Integration with Use of NFS Version 4
We expect that NFSv4 clients will implement a special directory,
analogous to an Automounter [AMD] 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.
This functionality does not require or use any list of organizations
that are known to provide file service, such as was required with the
"root.afs" functionality in AFS.
This DNS SRV record evaluation could, in principle, be done either in
the NFSv4 client or in an NFSv4 server. In either case, the result
would appear the same to applications on the NFSv4 client. This is
discussed further in section 5 of this document.
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.
On POSIX machines, the name /nfs4/ SHOULD 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
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, we use the "/.domainroot-
{Name}" string. Thus, the servers listed as targets for the SRV
resource records should export the root of the organization's
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published filesystem as the directory "/.domainroot-{Name}" (for the
given organization Name) in its exported namespace. For example, for
organization "example.net", the directory "/.domainroot-example.net"
should be used.
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
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.
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. 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.
Because of the possible distinction between read-only and read-write
versions of a filesystem, organizations SHOULD also publish the
location of a writable instance of their root filesystems, and that
NFSv4 clients SHOULD mount that filesystem under the organizational
domain name preceded by a period ("."). Therefore, when names
beginning with a period are looked up under the NFSv4 client's
special directory, the SRV RR looked up in DNS uses a Service name of
"_nfs4._write._domainroot", and the indicated server (or servers)
SHOULD export the writable instance at "/.domainroot-write-{Name}"
for a domain name Name.
Extending the opening example, suppose a client wished to locate the
read-only and read-write roots of the filesystem published by
organization example.net. Suppose a read-write instance were hosted
on server nfs1tr.example.net, and read-only instances were on that
server and also on server nfs2ex.example.net. The DNS servers for
the domain would publish records like
$ORIGIN example.net.
_nfs4._domainroot._tcp IN SRV 0 0 2049 nfs1tr.example.net.
_nfs4._domainroot._tcp IN SRV 1 0 2049 nfs2ex.example.net.
_nfs4._write._domainroot._tcp IN SRV 0 0 2049 nfs1tr.example.net.
The nfs1tr.example.net server would export filesystems at both
"/.domainroot-example.net" (the read-only instance) and
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"/.domainroot-write-example.net" (the read-write instance). The
nfs2ex.example.net server need export only the "/.domainroot-
example.net" name for its read-only instance.
The read-write version of the filesystem would be mounted (upon use)
under ".example.net" in the special directory, and a read-only
version would be mounted under "example.net". Thus,
/nfs4/example.net/users
might be a directory in a read-only instance of the root filesystem
of the organization "example.net", while
/nfs4/.example.net/users
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. Filesystem integration issues
The result of the DNS search SHOULD appear as a (pseudo-)directory in
the client name space. 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 filesystem 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 usable outside the scope of any particular
DNS abbreviation convention.
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
NFS client or by the NFS 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
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lookups, then consulted the SRV records in DNS to determine the
servers for the indicated published filesystem, and then returned
that information as an NFS Version 4 referral. 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.
NFS clients compliant to this standard MUST implement this
functionality. 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. Security Considerations
This functionality introduces a new reliance of NFSv4 on the
integrity of DNS. Spoofed 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 a 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 (nfs1tr.example.net and nfs2ex.example.net) are located as
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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.
7. IANA Considerations
None.
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.
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,
<http://docs.freebsd.org/info/amdref/amdref.pdf>.
[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.
[RFC1183] Everhart, C., Mamakos, L., Ullmann, R., and P.
Mockapetris, "New DNS RR Definitions", RFC 1183,
October 1990.
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
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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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