Internet DRAFT - draft-everhart-nfsv4-namespace-via-dns-srv
draft-everhart-nfsv4-namespace-via-dns-srv
Network Working Group C. Everhart
Internet-Draft W. Adamson
Intended status: Standards Track NetApp
Expires: March 9, 2009 J. Zhang
Google
September 5, 2008
Using DNS SRV to Specify a Global File Name Space with NFS version 4
draft-everhart-nfsv4-namespace-via-dns-srv-03.txt
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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. DNS SRV 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. This document refreshes the draft.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Proposed Use of SRV Resource Record in DNS . . . . . . . . . . 3
3.1. Deployment of the Resource Record . . . . . . . . . . . . 4
4. Integration with Use of NFS Version 4 . . . . . . . . . . . . 5
4.1. Globally-useful names: conventional mount point . . . . . 5
4.2. Mount options . . . . . . . . . . . . . . . . . . . . . . 5
4.3. File system integration issues . . . . . . . . . . . . . . 6
5. Where is this integration carried out? . . . . . . . . . . . . 7
6. Relationship to DNS NFS4ID RR . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 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
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 system 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 file system 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 file system 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 file system 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" and a conventional
Protocol of "_tcp". The Target fields give the domain names of the
NFS Version 4 servers that export root filesystems. An NFS Version 4
client SHOULD interpret any of the exported pseudo-root filesystems
as the filesystem published by the organization with the given domain
name.
Suppose a client wished to locate the root of the file system
published by organization example.net. The DNS servers for the
domain could publish records like
_nfs4._tcp IN SRV 0 0 2049 nfs1tr.example.net
_nfs4._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, 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. Subsequent accesses are carried out
in accordance with ordinary NFS Version 4 protocol.
3.1. Deployment of the Resource Record
As with any DNS resource, any server installation needs to concern
itself with the likely loads and effects of the presence of the
resource record. The answers to requests for RRs might differ
depending on what the server can tell about the client. For example,
some RRs might be returned only to those clients inside some network
perimeter (to provide an intranet service) and requests from other
clients might be denied. As the RR directs the clients to ask for
service from a given set of servers, the administrator should ensure
that the identified servers can handle the expected load.
Fortunately, the definition of the DNS SRV resource record offers a
mechanism to distribute the load to multiple servers within a
priority ordering.
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4. Integration with Use of NFS Version 4
There are at least two remaining questions: whether this DNS SRV
record evaluation is done in the NFS server or client, and also how
the domain names of the organizations are passed to client or server.
A third question is how this might produce a uniform global file name
space, and what prefix should be used for such file names.
This specification anticipates that these SRV records will most
commonly be used to define the second directory level in an inter-
organizational file name space. This directory will be populated
with domain names pointing to the file systems published for use
under those domain names. Thus, the root directory for the file
system published by example.net will effectively be mounted
underneath the example.net name in a second-level directory.
In general, a domain name will appear to a client as a directory name
pointing to the root directory of the file system published by the
organization responsible for that domain name.
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.
The name /nfs4/ SHOULD be 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
/nfs4/example.net/
on any client. Using this convention, "/nfs4/" is a mount for a
special file system that is populated with 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, we assume
that the empty string is adequate. Thus, the servers listed as
targets for the SRV resource records should export the root of the
organization's published file system as the pseudo-root in its
exported namespace.
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
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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.
We recommend, though, that 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, we recommend that the client 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.
One extension of this rule that we might choose to inherit from AFS,
though, is to give a special meaning to the domain name of an
organization preceded by a period ("."). It might be reasonable to
have names mounting the filesystem for a period-prefixed domain name
(e.g., ".example.net") attempt to mount only a read-write instance of
that organization's root filesystem, rather than permitting the use
of read-only instances of that filesystem. 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. File system integration issues
The result of the DNS search SHOULD appear as a (pseudo-)directory in
the client name space, cached for a time no longer than the RR's TTL.
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 file
system 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
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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 file system, 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
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.
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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.
Domain-based service principal names are defined in RFCs 5178
[RFC3530] and 5179 [RFC3530]. 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
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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
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.
[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,
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October 1990.
Authors' Addresses
Craig Everhart
NetApp
7301 Kit Creek Road
P.O. Box 13917
Research Triangle Park, NC 27709
US
Phone: +1 919 476 5320
Email: everhart@netapp.com
Andy Adamson
NetApp
495 East Java Drive
Sunnyvale, CA 94089
US
Phone: +1 734 665 1204
Email: andros@netapp.com
Jiaying Zhang
Google
535 W. William Street, Suite 3100
Ann Arbor, MI 48103
US
Phone: +1 734 647 4216
Email: jiayingz@umich.edu
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