Network Working Group C. Everhart
Internet-Draft W. Adamson
Intended status: Standards Track NetApp
Expires: April 29, 2010 J. Zhang
Google
October 26, 2009
Using DNS SRV to Specify a Global File Name Space with NFS version 4
draft-ietf-nfsv4-federated-fs-dns-srv-namespace-02.txt
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Abstract
The NFS version 4 protocol provides a natural way for a collection of
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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.
Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Use of SRV Resource Record in DNS . . . . . . . . . . 3
4. Integration with Use of NFS Version 4 . . . . . . . . . . . . 4
4.1. Globally-useful names: conventional mount point . . . . . 5
4.2. Mount options . . . . . . . . . . . . . . . . . . . . . . 5
4.3. Filesystem integration issues . . . . . . . . . . . . . . 7
5. Where is this integration carried out? . . . . . . . . . . . . 7
6. Relationship to DNS NFS4ID RR . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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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
With the advent of fs_locations attributes in the NFS Version 4
protocol [RFC3530], NFS servers can cooperate to build a file name
space that crosses server boundaries, as detailed in the description
of referrals in [NB0510]. With NFS Version 4 referrals, 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 [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
cross-organizational name space, just as in AFS [AFS] and DCE/DFS
[DFS] before it. A client need know only the name of the
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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". (In accordance with RFC 2782
[RFC2782], we use "_" prefix characters on the Service and Protocol
names, but we recognize that there is work in progress [Gudmundsson]
to create a registry of these names and to no longer use the "_"
prefix for some names.) 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 "/.domain-root-{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
_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 "/.domain-
root-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.
4. Integration with Use of NFS Version 4
We expect that NFSv4 clients will implement a special directory,
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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, as AFS did with its
"root.afs" functionality.
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.
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 and the proposed
fs_locations_info. For the rootpath field of fs_location, as
mentioned, we use the "/.domain-root-{Name}" string. Thus, the
servers listed as targets for the SRV resource records should export
the root of the organization's published filesystem as the directory
"/.domain-root-{Name}" (for the given organization Name) in its
exported namespace. For example, for organization "example.net", the
directory "/.domain-root-example.net" should be used.
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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 "/.domain-root-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
_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
"/.domain-root-example.net" (the read-only instance) and "/.domain-
root-write-example.net" (the read-write instance). The
nfs2ex.example.net server need export only the "/.domain-root-
example.net" name for its read-only instance.
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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
client or by the 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 lookups, then
consulted the DNS SRV records to determine the servers for the
indicated published filesystem, and then returned that information
via NFS Version 4 attributes as a referral in the way outlined by
Noveck and Burnett [NB0510]. In either case, the result of the DNS
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lookup should be cached (obeying TTL) so that the result could be
returned more quickly the next time.
We strongly suggest that this functionality be implemented by NFS
clients. 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. Relationship to DNS NFS4ID RR
This DNS use has no obvious relationship to the NFS4ID RR. The
NFS4ID RR is a mechanism to help clients and servers configure
themselves with respect to the domain strings used in "who" strings
in ACL entries and in owner and group names. The authentication/
authorization domain string of a server need have no direct
relationship to the name of the organization that is publishing a
file name space of which this server's filesystems form a part. At
the same time, it might be seen as straightforward or normal for such
a server to refer to the ownership of most of its files using a
domain string with an evident relationship to that NFS4ID-given
domain name, but this document imposes no such requirement.
7. Security Considerations
Naive use of the DNS may effectively give clients published server
referrals that are intrusive substitutes for the servers intended by
domain administrators.
It may be possible to build a trust chain by using DNSSEC [RFC4033]
to implement this function on the client, or by implementing this
function on an NFS Version 4 server that uses DNSSEC and maintaining
a trust relationship with that server. This trust chain also breaks
if the SRV interpreter accepts responses from insecure DNS zones.
Thus, it would likely be prudent also to use domain-based service
principal names for the servers for the root filesystems as indicated
as the targets of the SRV records. The idea here 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 an insecure DNS SRV RR lookup. The domain administrator can
thus ensure that only domain root NFSv4 servers have credentials for
such domain-based service principal names.
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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 should be 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.
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
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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.
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.
[Gudmundsson]
Gudmundsson, O. and A. Hoenes, "Creation of a Registry for
DNS SRV Record Service Prefixes", October 2009, <ftp://
ftp.rfc-editor.org/in-notes/internet-drafts/
draft-gudmundsson-dns-srv-iana-registry-04.txt>.
[NB0510] Noveck, D. and R. Burnett, "Next Steps for NFSv4
Migration/Replication", October 2005, <ftp://www.ietf.org/
internet-drafts/draft-noveck-nfsv4-migrep-00.txt>.
[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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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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