Internet Engineering Task Force (IETF) R. Allbery
Request for Comments: 5864 Stanford University
Updates: 1183 April 2010
Category: Standards Track
DNS SRV Resource Records for AFS
This document specifies how to use DNS (Domain Name Service) SRV RRs
(Resource Records) to locate services for the AFS distributed file
system and how the priority and weight values of the SRV RR should be
interpreted in the server ranking system used by AFS. It updates RFC
1183 to deprecate the use of the AFSDB RR to locate AFS cell database
servers and provides guidance for backward compatibility.
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.
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Table of Contents
1. Overview and Rationale ..........................................22. Scope ...........................................................33. Requirements Notation ...........................................34. DNS SRV RRs for AFS .............................................44.1. Interpretation as AFS Preference Ranks .....................55. Use of AFSDB RRs ................................................66. Example .........................................................87. Security Considerations .........................................98. References ......................................................98.1. Normative References .......................................98.2. Informative References ....................................101. Overview and Rationale
AFS (a registered trademark of IBM Corporation) is a distributed file
system (see [AFS1] and [AFS2]). Its most widely used implementations
are [OPENAFS] and [ARLA].
AFS is organized administratively into cells. Each AFS cell consists
of one or more Volume Location Database (VLDB) servers, one or more
Protection Service (PTS) servers, and one or more file servers and
volume servers, plus possible additional services not relevant to
this document. Data stored in AFS is divided into collections of
files called volumes. An AFS protocol client, when accessing a file
within a specific AFS cell, first contacts a VLDB server for that
cell to determine the file server for the AFS volume in which that
file is located, and then contacts that file server directly to
access the file. A client may also need to contact a PTS server for
that cell to register before accessing files in that cell or to query
protection database information.
An AFS client therefore needs to determine, for a given AFS cell, the
VLDB and possibly the PTS servers for that cell. (Traditionally, the
VLDB and PTS servers are provided by the same host.) Once the client
is in contact with the VLDB server, it locates file and volume
servers through AFS protocol queries to the VLDB server. Originally,
VLDB server information was configured separately on each client in a
file called the CellServDB file. [RFC1183] specified the DNS RR
(Resource Record) AFSDB to locate VLDB servers for AFS.
Subsequent to [RFC1183], a general DNS RR was defined by [RFC2782]
for service location for any service. This DNS SRV RR has several
advantages over the AFSDB RR:
o AFSDB RRs do not support priority or ranking, leaving AFS cell
administrators without a way to indicate which VLDB servers
clients should prefer.
o AFSDB RRs do not include protocol or port information, implicitly
assuming that all VLDB servers will be contacted over the standard
port and the UDP. Future changes to the AFS protocol may require
separate VLDB server lists for UDP and TCP traffic, and some uses
of AFS, such as providing VLDB service for multiple cells from the
same systems, require use of different ports.
o Clients using AFSDB RRs must assume that VLDB and PTS services are
provided by the same host, but it may be useful to separate VLDB
servers from PTS servers.
o DNS SRV RRs are in widespread use, whereas AFSDB RRs are a little-
known and little-supported corner of the DNS protocol.
For those reasons, it is desirable to move AFS service location from
the AFSDB RR to DNS SRV RRs.
This document describes the format and use of DNS SRV RRs for AFS
service location and deprecates the AFSDB RR. It also provides
guidance for transition from the AFSDB RR to DNS SRV RRs and
recommendations for backward compatibility.
Documentation of the AFS protocol, the exact purpose and use of the
VLDB and PTS services, and other information about AFS are outside
the scope of this document.
AFSDB RRs may also be used for locating servers for the Open Software
Foundation's (OSF's) Distributed Computing Environment (DCE)
authenticated naming system, as described in [RFC1183]. Service
location for DCE servers is outside the scope of this document and is
not modified by this specification.
3. 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].
4. DNS SRV RRs for AFS
The label of a DNS SRV RR, as defined in [RFC2782], is:
The following values for <service> advertise servers providing AFS
afs3-vlserver: servers providing AFS VLDB services.
afs3-prserver: servers providing AFS PTS services.
Other AFS services, such as file and volume management services, are
located through the VLDB service and therefore do not use DNS SRV
<proto> MUST be "udp" for the current AFS protocol, which uses Rx
over UDP. Other values may be used for future revisions of the AFS
protocol supporting other protocols, such as Rx over TCP.
<name> MUST be the AFS cell name for which the identified server
provides AFS services. Clients MUST query DNS SRV RRs only for a
<name> value exactly matching the AFS cell of interest. They MUST
NOT remove leading components to search for more general DNS SRV RRs.
The AFS cell "prod.example.com" and the AFS cell "example.com" are
entirely different cells in the AFS protocol and VLDB servers for the
latter cannot provide information for the former.
NOTE: As with AFSDB RRs, this means that DNS SRV RRs can only be
used to locate AFS services for cells whose naming matches the
structure of the DNS. This is not a requirement of the AFS
protocol, but sites creating new AFS cells SHOULD use names that
follow the structure of the DNS and that result in DNS SRV RRs
under their administrative control. This both permits use of DNS
SRV RRs instead of client configuration and helps avoid naming
conflicts between separate AFS cells.
DNS SRV RRs include a priority and a weight. As defined in the DNS
SRV RR specification [RFC2782], clients MUST attempt to contact the
target host with the lowest-numbered priority they can reach. AFS
clients that use a ranked algorithm to determine which server to
contact MUST therefore assign a sufficiently distinct rank to targets
with different priorities such that targets with a higher-numbered
priority are only contacted if all targets with a lower-numbered
priority are inaccessible. See Section 4.1 for more information.
If there are multiple targets with an equal priority, the weight
value of the DNS SRV RR SHOULD be used as input to a weighted
algorithm for selecting servers. As specified by [RFC2782], larger
weights SHOULD be given a proportionately higher probability of being
selected. In the presence of records containing weights greater than
0, records with weight 0 should have a very small chance of being
selected. A weight of 0 SHOULD be used if all targets with that
priority are weighted equally. AFS clients MAY take into account
network performance and other protocol metrics along with SRV RR
weights when selecting servers, thereby possibly selecting different
servers than a system based purely on the SRV RRs would indicate.
However, such information MUST NOT override the priority information
in the SRV RR.
DNS SRV RRs, like all DNS RRs, have a time-to-live (TTL), after which
the SRV record information is no longer valid (see [RFC1034]). DNS
RRs SHOULD be discarded after their TTL, and after the DNS query
repeated. This applies to DNS SRV RRs for AFS as it does for any
other DNS RR. Any information derived from the DNS SRV RRs, such as
preference ranks, MUST be discarded when the DNS SRV RR is expired.
Implementations are not required to re-run the weighted server
selection algorithm for each call. Instead, they MAY reuse the
results of the algorithm until the DNS SRV RRs expire. Clients could
therefore use a specific server for the lifetime of the DNS SRV
records, which may affect the load distribution properties that DNS
SRV records provide. Server operators should account for this effect
when setting the TTL of those records.
AFS clients MAY remember which targets are inaccessible by that
client and ignore those targets when determining which server to
contact first. Clients that do this SHOULD have a mechanism to retry
targets that were previously inaccessible and reconsider them
according to their current priority and weight if they become
4.1. Interpretation as AFS Preference Ranks
Several AFS implementations use a ranking algorithm that assigns
numbers representing a preference rank to each server when the client
first contacts that AFS cell and then prefers the server with the
lowest rank unless that server goes down. Clients using this
algorithm SHOULD assign their ranks as follows:
1. Sort targets by priority and assign a base rank value to each
target based on its priority. Each base rank value MUST be
sufficiently different from the base rank assigned to any higher-
numbered priority so that higher-numbered targets will only be
attempted if lower-numbered targets cannot be reached. It MUST,
in other words, be farther from the base rank of any distinct
priority than any possible automatic adjustment in the rank.
When calculating base ranks, observe that the numeric value of a
priority has no meaning. Only the ordering of distinct priority
values between multiple SRV RR targets needs to be reflected in
the base ranks.
2. For each group of targets with the same priority, follow the
algorithm in [RFC2782] to order those targets. Then, assign
those targets ranks formed by incrementing the base weight for
that priority such that the first selected target has the lowest
rank, the second selected target has the next-lowest rank, and so
After assignment of ranks, the AFS client MAY then adjust the ranks
dynamically based on network performance and other protocol metrics,
provided that such adjustments are sufficiently small compared to the
difference between base ranks that they cannot cause servers with a
higher-numbered priority to be contacted instead of a server with a
The TTL of the DNS SRV RRs MUST be honored by invalidating and
regenerating the server preference ranks with new DNS information
once that TTL has expired. However, accumulated network and protocol
metrics may be retained and reapplied to the new rankings.
AFS server preference ranks are conventionally numbers between 1 and
65535. DNS SRV RR priorities are numbers between 0 and 65535.
Implementations following this algorithm could therefore encounter
problems assigning sufficiently distinct base rank values in
exceptional cases of very large numbers of DNS SRV RR targets with
different priorities. However, an AFS cell configuration with
thousands of DNS SRV RR targets for an AFS VLDB or PTS service with
meaningfully distinct priorities is highly improbable. AFS client
implementations encountering a DNS SRV RR containing targets with
more distinct priority values than can be correctly represented as
base ranks SHOULD fall back to generating ranks based solely on
priorities, ignoring other rank inputs, and disabling dynamic
adjustment of ranks. Implementations MUST be able to assign distinct
base ranks as described above for at least ten distinct priority
5. Use of AFSDB RRs
Since many AFS client implementations currently support AFSDB RRs but
do not support DNS SRV RRs, AFS cells providing DNS SRV RRs SHOULD
also provide AFSDB RRs. However, be aware that AFSDB RRs do not
provide priority or weighting information; all servers listed in
ASFDB RRs are treated as equal. AFSDB RRs also do not provide port
An AFS cell using DNS SRV RRs SHOULD therefore also provide an AFSDB
RR listing all AFS servers for which the following statements are all
o The server provides both VLDB and PTS service on the standard
ports (7003 and 7002, respectively).
o The server provides these services via Rx over UDP.
o The server either has the lowest-numbered priority of those listed
in the DNS SRV RRs or the AFS cell administrator believes it
reasonable for clients using AFSDB RRs to use this server by
The above is a default recommendation. AFS cell administrators MAY
use different lists of servers in the AFSDB RRs and DNS SRV RRs if
desired for specific effects based on local knowledge of which
clients use AFSDB RRs and which clients use DNS SRV RRs. However,
AFS servers SHOULD NOT be advertised with AFSDB RRs unless they
provide VLDB and PTS services via UDP on the standard ports. (This
falls shy of MUST NOT because it may be useful in some unusual
circumstances to advertise, via an AFSDB RR, a server that provides
only one of the two services, but be aware that such a configuration
may confuse legacy clients.)
An AFS cell SHOULD have at least one VLDB and at least one PTS server
providing service on the standard ports of 7003 and 7002,
respectively, since clients without DNS SRV RR support cannot locate
servers on non-standard ports.
Clients SHOULD query DNS SRV RRs by default but SHOULD then fall back
on AFSDB RRs if no DNS SRV RRs are found. In the absence of DNS SRV
RRs, an AFSDB RR of <subtype> 1 SHOULD be treated as equivalent to
the following pair of DNS SRV RRs:
_afs3-vlserver._udp.<cell> <ttl> IN SRV 0 0 7003 <hostname>
_afs3-prserver._udp.<cell> <ttl> IN SRV 0 0 7002 <hostname>
<cell> is the label of the AFSDB RR, <ttl> is its TTL and <hostname>
is the <hostname> value of the AFSDB RR as specified in [RFC1183].
This is the fully-qualified domain name of the server.
The following example includes TCP AFS services, separation of a PTS
server from a VLDB server, and use of non-standard ports, all
features that either assume future AFS protocol development or are
not widely supported by current clients. This example is intended to
show the range of possibilities for AFS DNS SRV RRs, not as a
practical example for an existing cell. This is a part of the zone
file for a fictional example.com domain with AFS services.
@ SOA dns.example.com. root.example.com. (
2009100201 3600 3600 604800 86400 )
_afs3-vlserver._udp SRV 0 2 7003 afsdb1.example.com.
_afs3-vlserver._udp SRV 0 4 7003 afsdb2.example.com.
_afs3-vlserver._udp SRV 1 0 65500 afsdb3.example.com.
_afs3-vlserver._tcp SRV 0 0 7003 afsdb3.example.com.
_afs3-prserver._udp SRV 0 0 7002 afsdb1.example.com.
_afs3-prserver._tcp SRV 0 0 7002 afsdb3.example.com.
@ AFSDB 1 afsdb1.example.com.
dns A 192.0.2.9
afsdb1 A 192.0.2.10
afsdb2 A 192.0.2.11
afsdb3 A 192.0.2.12
In this example, the AFS cell name is example.com.
afsdb1, afsdb2, and afsdb3 all provide VLDB service via UDP. The
first two have the same priority but have weights indicating that
afsdb1 should get about twice as many clients as afsdb2. afsdb3
should only be used for UDP VLDB service if afsdb1 and afsdb2 are not
accessible and provides that service on a non-standard port (65500).
Only one host, afsdb1, provides UDP PTS service.
afsdb3 provides a hypothetical TCP version of AFS VLDB and PTS
service on the standard ports and is the only server providing these
services over TCP for this cell. Such a TCP-based AFS protocol did
not exist at the time this document was written. This example only
shows what the record would look like in a hypothetical future if
such a protocol were developed.
An AFSDB RR is provided for backward compatibility with older
clients. It lists only afsdb1, since only that host provides both
VLDB and PTS service over UDP on the standard ports.
7. Security Considerations
Use of DNS SRV RRs for AFS service locations poses the same security
issues as the existing AFSDB RRs. Specifically, unless the integrity
and authenticity of the DNS response are checked, an attacker may
forge DNS replies and thereby direct clients at a VLDB or PTS server
under the control of the attacker. From there, the attacker may
deceive an AFS client about the volumes and file servers in a cell
and about the contents of files and directories in that cell. If the
client uses cell data in a trusted way, such as by executing programs
out of that AFS cell or using data from the cell as input to other
programs, the attacker may be able to further compromise the security
of the client and trick it into taking action under the attacker's
This attack can be prevented if the server is authenticated, since
the client can then detect a failure to authenticate the attacker's
servers and thereby detect possible impersonation. However, this
applies only to authenticated AFS access, and much AFS access is
unauthenticated. Furthermore, clients after failure to authenticate
may fall back to unauthenticated access, which the attacker's servers
Using an integrity-protected DNS system such as DNS Security (DNSSEC)
[RFC4033] can prevent this attack via DNS. However, the underlying
vulnerability is inherent in the current AFS protocol and may be
exploited in ways other than DNS forgery, such as by forging the
results of VLDB queries for an AFS cell. Addressing it properly
requires changes to the AFS protocol allowing clients to always
authenticate AFS services and discard unauthenticated data. Even in
the absence of a DNS system with integrity protection, addition of
DNS SRV RRs does not make this vulnerability more severe, only opens
another equivalent point of attack.
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1183] Everhart, C., Mamakos, L., Ullmann, R., and P.
Mockapetris, "New DNS RR Definitions", RFC 1183,