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Internet Engineering Task Force (IETF)                        R. AllberyRequest for Comments: 5864                           Stanford UniversityUpdates:1183                                                 April 2010Category: Standards TrackISSN: 2070-1721DNS SRV Resource Records for AFSAbstract   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 updatesRFC1183 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5864.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Allbery                      Standards Track                    [Page 1]

RFC 5864                   DNS SRV RRs for AFS                April 2010Table of Contents1. 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:Allbery                      Standards Track                    [Page 2]

RFC 5864                   DNS SRV RRs for AFS                April 2010   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].Allbery                      Standards Track                    [Page 3]

RFC 5864                   DNS SRV RRs for AFS                April 20104.  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.  SeeSection 4.1 for more information.Allbery                      Standards Track                    [Page 4]

RFC 5864                   DNS SRV RRs for AFS                April 2010   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 beAllbery                      Standards Track                    [Page 5]

RFC 5864                   DNS SRV RRs for AFS                April 2010       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 notAllbery                      Standards Track                    [Page 6]

RFC 5864                   DNS SRV RRs for AFS                April 2010   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.Allbery                      Standards Track                    [Page 7]

RFC 5864                   DNS SRV RRs for AFS                April 20106.  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.Allbery                      Standards Track                    [Page 8]

RFC 5864                   DNS SRV RRs for AFS                April 2010   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.  References8.1.  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.Allbery                      Standards Track                    [Page 9]

RFC 5864                   DNS SRV RRs for AFS                April 2010   [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/Allbery                      Standards Track                   [Page 10]