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Network Working Group                                     A. GulbrandsenRequest for Comments: 2052                            Troll TechnologiesUpdates:1035,1183                                             P. VixieCategory: Experimental                                 Vixie Enterprises                                                            October 1996A DNS RR for specifying the location of services (DNS SRV)Status of this Memo   This memo defines an Experimental Protocol for the Internet   community.  This memo does not specify an Internet standard of any   kind.  Discussion and suggestions for improvement are requested.   Distribution of this memo is unlimited.Abstract   This document describes a DNS RR which specifies the location of the   server(s) for a specific protocol and domain (like a more general   form of MX).Overview and rationale   Currently, one must either know the exact address of a server to   contact it, or broadcast a question.  This has led to, for example,   ftp.whatever.com aliases, the SMTP-specific MX RR, and using MAC-   level broadcasts to locate servers.   The SRV RR allows administrators to use several servers for a single   domain, to move services from host to host with little fuss, and to   designate some hosts as primary servers for a service and others as   backups.   Clients ask for a specific service/protocol for a specific domain   (the word domain is used here in the strictRFC 1034 sense), and get   back the names of any available servers.Introductory example   When a SRV-cognizant web-browser wants to retrievehttp://www.asdf.com/   it does a lookup of      http.tcp.www.asdf.comGulbrandsen & Vixie           Experimental                      [Page 1]

RFC 2052                       DNS SRV RR                   October 1996   and retrieves the document from one of the servers in the reply.  The   example zone file near the end of the memo contains answering RRs for   this query.The format of the SRV RR   Here is the format of the SRV RR, whose DNS type code is 33:        Service.Proto.Name TTL Class SRV Priority Weight Port Target        (There is an example near the end of this document.)   Service        The symbolic name of the desired service, as defined in Assigned        Numbers or locally.        Some widely used services, notably POP, don't have a single        universal name.  If Assigned Numbers names the service        indicated, that name is the only name which is legal for SRV        lookups.  Only locally defined services may be named locally.        The Service is case insensitive.   Proto        TCP and UDP are at present the most useful values        for this field, though any name defined by Assigned Numbers or        locally may be used (as for Service).  The Proto is case        insensitive.   Name        The domain this RR refers to.  The SRV RR is unique in that the        name one searches for is not this name; the example near the end        shows this clearly.   TTL        Standard DNS meaning.   Class        Standard DNS meaning.   Priority        As for MX, the priority of this target host.  A client MUST        attempt to contact the target host with the lowest-numbered        priority it can reach; target hosts with the same priority        SHOULD be tried in pseudorandom order.  The range is 0-65535.Gulbrandsen & Vixie           Experimental                      [Page 2]

RFC 2052                       DNS SRV RR                   October 1996   Weight        Load balancing mechanism.  When selecting a target host among        the those that have the same priority, the chance of trying this        one first SHOULD be proportional to its weight.  The range of        this number is 1-65535.  Domain administrators are urged to use        Weight 0 when there isn't any load balancing to do, to make the        RR easier to read for humans (less noisy).   Port        The port on this target host of this service.  The range is        0-65535.  This is often as specified in Assigned Numbers but        need not be.   Target        As for MX, the domain name of the target host.  There MUST be        one or more A records for this name. Implementors are urged, but        not required, to return the A record(s) in the Additional Data        section.  Name compression is to be used for this field.        A Target of "." means that the service is decidedly not        available at this domain.Domain administrator advice   Asking everyone to update their telnet (for example) clients when the   first internet site adds a SRV RR for Telnet/TCP is futile (even if   desirable).  Therefore SRV will have to coexist with A record lookups   for a long time, and DNS administrators should try to provide A   records to support old clients:      - Where the services for a single domain are spread over several        hosts, it seems advisable to have a list of A RRs at the same        DNS node as the SRV RR, listing reasonable (if perhaps        suboptimal) fallback hosts for Telnet, NNTP and other protocols        likely to be used with this name.  Note that some programs only        try the first address they get back from e.g. gethostbyname(),        and we don't know how widespread this behaviour is.      - Where one service is provided by several hosts, one can either        provide A records for all the hosts (in which case the round-        robin mechanism, where available, will share the load equally)        or just for one (presumably the fastest).      - If a host is intended to provide a service only when the main        server(s) is/are down, it probably shouldn't be listed in A        records.Gulbrandsen & Vixie           Experimental                      [Page 3]

RFC 2052                       DNS SRV RR                   October 1996      - Hosts that are referenced by backup A records must use the port        number specified in Assigned Numbers for the service.   Currently there's a practical limit of 512 bytes for DNS replies.   Until all resolvers can handle larger responses, domain   administrators are strongly advised to keep their SRV replies below   512 bytes.   All round numbers, wrote Dr. Johnson, are false, and these numbers   are very round: A reply packet has a 30-byte overhead plus the name   of the service ("telnet.tcp.asdf.com" for instance); each SRV RR adds   20 bytes plus the name of the target host; each NS RR in the NS   section is 15 bytes plus the name of the name server host; and   finally each A RR in the additional data section is 20 bytes or so,   and there are A's for each SRV and NS RR mentioned in the answer.   This size estimate is extremely crude, but shouldn't underestimate   the actual answer size by much.  If an answer may be close to the   limit, using e.g. "dig" to look at the actual answer is a good idea.The "Weight" field   Weight, the load balancing field, is not quite satisfactory, but the   actual load on typical servers changes much too quickly to be kept   around in DNS caches.  It seems to the authors that offering   administrators a way to say "this machine is three times as fast as   that one" is the best that can practically be done.   The only way the authors can see of getting a "better" load figure is   asking a separate server when the client selects a server and   contacts it.  For short-lived services like SMTP an extra step in the   connection establishment seems too expensive, and for long-lived   services like telnet, the load figure may well be thrown off a minute   after the connection is established when someone else starts or   finishes a heavy job.The Port number   Currently, the translation from service name to port number happens   at the client, often using a file such as /etc/services.   Moving this information to the DNS makes it less necessary to update   these files on every single computer of the net every time a new   service is added, and makes it possible to move standard services out   of the "root-only" port range on unix.Gulbrandsen & Vixie           Experimental                      [Page 4]

RFC 2052                       DNS SRV RR                   October 1996Usage rules   A SRV-cognizant client SHOULD use this procedure to locate a list of   servers and connect to the preferred one:        Do a lookup for QNAME=service.protocol.target, QCLASS=IN,        QTYPE=SRV.        If the reply is NOERROR, ANCOUNT>0 and there is at least one SRV        RR which specifies the requested Service and Protocol in the        reply:             If there is precisely one SRV RR, and its Target is "."             (the root domain), abort.             Else, for all such RR's, build a list of (Priority, Weight,             Target) tuples             Sort the list by priority (lowest number first)             Create a new empty list             For each distinct priority level                  While there are still elements left at this priority                  level                       Select an element randomly, with probability                       Weight, and move it to the tail of the new list             For each element in the new list                  query the DNS for A RR's for the Target or use any                  RR's found in the Additional Data secion of the                  earlier SRV query.                  for each A RR found, try to connect to the (protocol,                  address, service).        else if the service desired is SMTP             skip toRFC 974 (MX).        else             Do a lookup for QNAME=target, QCLASS=IN, QTYPE=A             for each A RR found, try to connect to the (protocol,             address, service)Gulbrandsen & Vixie           Experimental                      [Page 5]

RFC 2052                       DNS SRV RR                   October 1996   Notes:      - Port numbers SHOULD NOT be used in place of the symbolic service        or protocol names (for the same reason why variant names cannot        be allowed: Applications would have to do two or more lookups).      - If a truncated response comes back from an SRV query, and the        Additional Data section has at least one complete RR in it, the        answer MUST be considered complete and the client resolver        SHOULD NOT retry the query using TCP, but use normal UDP queries        for A RR's missing from the Additional Data section.      - A client MAY use means other than Weight to choose among target        hosts with equal Priority.      - A client MUST parse all of the RR's in the reply.      - If the Additional Data section doesn't contain A RR's for all        the SRV RR's and the client may want to connect to the target        host(s) involved, the client MUST look up the A RR(s).  (This        happens quite often when the A RR has shorter TTL than the SRV        or NS RR's.)      - A future standard could specify that a SRV RR whose Protocol was        TCP and whose Service was SMTP would overrideRFC 974's rules        with regard to the use of an MX RR.  This would allow firewalled        organizations with several SMTP relays to control the load        distribution using the Weight field.      - Future protocols could be designed to use SRV RR lookups as the        means by which clients locate their servers.Fictional example   This is (part of) the zone file for asdf.com, a still-unused domain:        $ORIGIN asdf.com.        @               SOA server.asdf.com. root.asdf.com. (                            1995032001 3600 3600 604800 86400 )                        NS  server.asdf.com.                        NS  ns1.ip-provider.net.                        NS  ns2.ip-provider.net.        ftp.tcp         SRV 0 0 21 server.asdf.com.        finger.tcp      SRV 0 0 79 server.asdf.com.        ; telnet - use old-slow-box or new-fast-box if either is        ; available, make three quarters of the logins go to        ; new-fast-box.        telnet.tcp      SRV 0 1 23 old-slow-box.asdf.com.Gulbrandsen & Vixie           Experimental                      [Page 6]

RFC 2052                       DNS SRV RR                   October 1996                        SRV 0 3 23 new-fast-box.asdf.com.        ; if neither old-slow-box or new-fast-box is up, switch to        ; using the sysdmin's box and the server                        SRV 1 0 23 sysadmins-box.asdf.com.                        SRV 1 0 23 server.asdf.com.        ; HTTP - server is the main server, new-fast-box is the backup        ; (On new-fast-box, the HTTP daemon runs on port 8000)        http.tcp        SRV 0 0 80 server.asdf.com.                        SRV 10 0 8000 new-fast-box.asdf.com.        ; since we want to support bothhttp://asdf.com/ and        ;http://www.asdf.com/ we need the next two RRs as well        http.tcp.www    SRV 0 0 80 server.asdf.com.                        SRV 10 0 8000 new-fast-box.asdf.com.        ; SMTP - mail goes to the server, and to the IP provider if        ; the net is down        smtp.tcp        SRV 0 0 25 server.asdf.com.                        SRV 1 0 25 mailhost.ip-provider.net.        @               MX  0 server.asdf.com.                        MX  1 mailhost.ip-provider.net.        ; NNTP - use the IP providers's NNTP server        nntp.tcp        SRV 0 0 119 nntphost.ip-provider.net.        ; IDB is an locally defined protocol        idb.tcp         SRV  0 0 2025 new-fast-box.asdf.com.        ; addresses        server          A   172.30.79.10        old-slow-box    A   172.30.79.11        sysadmins-box   A   172.30.79.12        new-fast-box    A   172.30.79.13        ; backup A records - new-fast-box and old-slow-box are        ; included, naturally, and server is too, but might go        ; if the load got too bad        @               A   172.30.79.10                        A   172.30.79.11                        A   172.30.79.13        ; backup A RR for www.asdf.com        www             A       172.30.79.10        ; NO other services are supported        *.tcp           SRV  0 0 0 .        *.udp           SRV  0 0 0 .   In this example, a telnet connection to "asdf.com." needs an SRV   lookup of "telnet.tcp.asdf.com." and possibly A lookups of "new-   fast-box.asdf.com." and/or the other hosts named.  The size of the   SRV reply is approximately 365 bytes:      30 bytes general overhead      20 bytes for the query string, "telnet.tcp.asdf.com."      130 bytes for 4 SRV RR's, 20 bytes each plus the lengths of "new-Gulbrandsen & Vixie           Experimental                      [Page 7]

RFC 2052                       DNS SRV RR                   October 1996        fast-box", "old-slow-box", "server" and "sysadmins-box" -        "asdf.com" in the query section is quoted here and doesn't        need to be counted again.      75 bytes for 3 NS RRs, 15 bytes each plus the lengths of        "server", "ns1.ip-provider.net." and "ns2" - again, "ip-        provider.net." is quoted and only needs to be counted once.      120 bytes for the 6 A RR's mentioned by the SRV and NS RR's.RefererencesRFC 1918: Rekhter, Y., Moskowitz, R., Karrenberg, D., de Groot, G.,        and E.  Lear, "Address Allocation for Private Internets",RFC 1918, February 1996.RFC 1916 Berkowitz, H., Ferguson, P, Leland, W. and P. Nesser,        "Enterprise Renumbering: Experience and Information        Solicitation",RFC 1916, February 1996.RFC 1912 Barr, D., "Common DNS Operational and Configuration        Errors",RFC 1912, February 1996.RFC 1900: Carpenter, B., and Y. Rekhter, "Renumbering Needs Work",RFC 1900, February 1996.RFC 1920: Postel, J., "INTERNET OFFICIAL PROTOCOL STANDARDS",        STD 1,RFC 1920, March 1996.RFC 1814: Gerich, E., "Unique Addresses are Good",RFC 1814, June             1995.RFC 1794: Brisco, T., "DNS Support for Load Balancing", April 1995.RFC 1713: Romao, A., "Tools for DNS debugging", November 1994.RFC 1712: Farrell, C., Schulze, M., Pleitner, S., and D. Baldoni,        "DNS Encoding of Geographical Location",RFC 1712, November        1994.RFC 1706: Manning, B. and R. Colella, "DNS NSAP Resource Records",RFC 1706, October 1994.RFC 1700: Reynolds, J., and J. Postel, "ASSIGNED NUMBERS",        STD 2,RFC 1700, October 1994.RFC 1183: Ullmann, R., Mockapetris, P., Mamakos, L., and        C. Everhart, "New DNS RR Definitions",RFC 1183, November        1990.Gulbrandsen & Vixie           Experimental                      [Page 8]

RFC 2052                       DNS SRV RR                   October 1996RFC 1101: Mockapetris, P., "DNS encoding of network names and other        types",RFC 1101, April 1989.RFC 1035: Mockapetris, P., "Domain names - implementation and        specification", STD 13,RFC 1035, November 1987.RFC 1034: Mockapetris, P., "Domain names - concepts and        facilities", STD 13,RFC 1034, November 1987.RFC 1033: Lottor, M., "Domain administrators operations guide",RFC 1033, November 1987.RFC 1032: Stahl, M., "Domain administrators guide",RFC 1032,        November 1987.RFC 974: Partridge, C., "Mail routing and the domain system",        STD 14,RFC 974, January 1986.Security Considerations   The authors believes this RR to not cause any new security problems.   Some problems become more visible, though.      - The ability to specify ports on a fine-grained basis obviously        changes how a router can filter packets.  It becomes impossible        to block internal clients from accessing specific external        services, slightly harder to block internal users from running        unautorised services, and more important for the router        operations and DNS operations personnel to cooperate.      - There is no way a site can keep its hosts from being referenced        as servers (as, indeed, some sites become unwilling secondary        MXes today).  This could lead to denial of service.      - With SRV, DNS spoofers can supply false port numbers, as well as        host names and addresses.  The authors do not see any practical        effect of this.   We assume that as the DNS-security people invent new features, DNS   servers will return the relevant RRs in the Additional Data section   when answering an SRV query.Gulbrandsen & Vixie           Experimental                      [Page 9]

RFC 2052                       DNS SRV RR                   October 1996Authors' Addresses   Arnt Gulbrandsen   Troll Tech   Postboks 6133 Etterstad   N-0602 Oslo   Norway   Phone: +47 22646966   EMail: agulbra@troll.no   Paul Vixie   Vixie Enterprises   Star Route 159A   Woodside, CA  94062   Phone: (415) 747-0204   EMail: paul@vix.comGulbrandsen & Vixie           Experimental                     [Page 10]