rfc2052.txt

bind 9.3结合mysql数据库


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 override RFC 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.



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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 both http://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-



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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.

Refererences

   RFC 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.




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RFC 2052                       DNS SRV RR                   October 1996


   RFC 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.










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RFC 2052                       DNS SRV RR                   October 1996


Authors' Addresses

   Arnt Gulbrandsen
   Troll Tech
   Postboks 6133 Etterstad
   N-0602 Oslo
   Norway

   Phone: [phone]
   EMail: [e-mail]


   Paul Vixie
   Vixie Enterprises
   Star Route 159A
   Woodside, CA  94062

   Phone: [phone]
   EMail: [e-mail]
































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