AFS SRV Records: DNS SRV Resource Records for AFS

Internet Engineering Task Force (IETF)                        R. Allbery
Request for Comments: 5864                           Stanford University
Updates: 1183                                                 April 2010
Category: Standards Track
ISSN: 2070-1721


                    DNS SRV Resource Records for AFS

Abstract

   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.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5864.

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Table of Contents

   1. Overview and Rationale ..........................................2
   2. Scope ...........................................................3
   3. Requirements Notation ...........................................3
   4. DNS SRV RRs for AFS .............................................4
      4.1. Interpretation as AFS Preference Ranks .....................5
   5. Use of AFSDB RRs ................................................6
   6. Example .........................................................8
   7. Security Considerations .........................................9
   8. References ......................................................9
      8.1. Normative References .......................................9
      8.2. Informative References ....................................10

1.  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.

2.  Scope

   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:

       _<service>._<proto>.<name>

   The following values for <service> advertise servers providing AFS
   services:

   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
   RRs.

   <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
   accessible again.

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
       on.

   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
   lower-numbered priority.

   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
   values.

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
   information.

   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
   true:

   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
      preference.

   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.

6.  Example

   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.

       $ORIGIN example.com.
       @                    SOA   dns.example.com. root.example.com. (
                                    2009100201 3600 3600 604800 86400 )
                            NS    dns.example.com.
       _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
   control.

   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
   may permit.

   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.  References

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,
              October 1990.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, 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.

8.2.  Informative References

   [AFS1]     Howard, J., Kazar, M., Menees, S., Nichols, D.,
              Satyanarayanan, M., Sidebotham, R., and M. West, "Scale
              and Performance in a Distributed File System", ACM Trans.
              on Computer Systems 6(1), February 1988.

   [AFS2]     Howard, J., "An Overview of the Andrew File System", CMU-
              ITC 88-062, February 1988.

   [ARLA]     Assar Westerlund, et al., "Arla", May 2001,
              <http://www.stacken.kth.se/project/arla/html/arla.html>.

   [OPENAFS]  IBM Corporation, et al., "OpenAFS Documentation",
              April 2000, <http://docs.openafs.org/>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

Author's Address

   Russ Allbery
   Stanford University
   P.O. Box 20066
   Stanford, CA  94309
   US

   EMail: rra@stanford.edu
   URI:   http://www.eyrie.org/~eagle/