A Suggested Scheme for DNS Resolution of Networks and Gateways

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Author Edward Warnicke 
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DNS Extentions Working Group                                 E. Warnicke
Internet-Draft                                             Cisco Systems
Expires: April 1, 2006                                      October 2005

     A Suggested Scheme for DNS Resolution of Networks and Gateways

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of section 3 of RFC 3667.  By submitting this Internet-Draft, each
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   RFC 3668.

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   This document suggests a method of using DNS to determine the network
   that contains a specified IP address, the netmask of that network,
   and the address(es) of first-hop routers(s) on that network.  This
   method supports variable length subnet masks, delegation of subnets
   on non-octet boundaries, and multiple routers per subnet.

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1.  Introduction

   As a variety of new devices are introduced to the network, many of
   them not traditional workstations or routers, there are requirements
   that the first-hop router provide some network service for a host.
   It may be necessary for a third party server in the network to
   request some service related to the host from the first-hop router(s)
   for that host.  It would be useful to have a standard mechanism for
   such a third party device to find the first-hop router(s) for that

   DNS-based mechanisms have been defined for the resolution of router
   addresses for classful networks (RFC 1035 [1]) and of subnets (RFC
   1101 [2]).  RFC 1101 suffers from a number of defects, chief among
   which are that it does not support variable length subnet masks,
   which are commonly deployed in the Internet.  The present document
   defines DNS-based mechanisms to cure these defects.

   Since the writing of RFC 1101, DNS mechanisms for dealing with
   classless networks have been defined, for example RFC 2317 [3].  This
   document describes a mechanism that uses notation similar to that of
   RFC 2317 to specify a series of PTR records enumerating the subnets
   of a given network in the RFC 2317 notation.  This lookup process
   continues until the contents of the PTR records are not an
   in-addr.arpa.-derived domain name.  These terminal PTR record values
   are treated as the hostname(s) of the router(s) on that network.
   This RFC also specifies an extension to the method of RFC 2317 to
   support delegation at non-octet boundaries.

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2.  Generic format of a network domain name

   Using the Augmented BNF of RFC 2234 [4] we can describe a generic
   domain name for a network as follows:
      networkdomainname = maskedoctet "." *( decimaloctet / maskedoctet
      ".") "in-addr.arpa."
      maskedoctet = decimaloctet "-" mask
      mask = 1*2DIGIT ; representing a decimal integer value in the
                      ; range 1-32
      decimaloctet = 1*3DIGIT ; representing a decimal integer value in
                              ; the range 0 through 255

   The by way of reference an IPv4 CIDR notation network address would
   be written
      IPv4CIDR = decimaloctet "." decimaloctet "." decimaloctet "."
      decimaloctet "/" mask

   A "-" is used as a delimiter in a maskedoctet instead of a "/" as in
   RFC 2317 out of concern about compatibility with existing DNS
   servers, many of which do not consider "/" to be a valid character in
   a hostname.

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3.  Non-octet boundary delegation

   In RFC 2317 there is no mechanism for non-octet boundary delegation.
   Networks would be represented as being part of the domain of the next

   Examples:  -> 0- -> 128-23.20.10.in-addr.arpa. -> 192-13.10.in-addr.arpa.

   In the event that the entity subnetting does not actually own the
   network being subnetted on an octet break, a mechanism needs to be
   available to allow for the specification of those subnets.  The
   mechanism is to allow the use of maskedoctet labels as delegation

   For example, consider an entity A which controls a network
   16.  Entity A delegates to entity B the network  In
   order to avoid having to update entries for entity B whenever entity
   B updates subnetting, entity A delegates the 0-18.1.10.in-addr.arpa
   domain ( with an NS record in A's DNS tables as usual ) to entity B.
   Entity B then subnets off  It would provide a domain
   name for this network of 0-25.0.0-18.1.10.in-addr.arpa ( in B's DNS

   In order to speak about the non-octet boundary case more easily it is
   useful to define a few terms.

   Network domain names which do not contain any maskedoctets after the
   first ( leftmost ) label are hereafter referred to as canonical
   domain names for that network.  0-  is the
   canonical domain name for the network

   Network domain names which do contain maskedoctet labels after the
   first ( leftmost ) label can be reduced to a canonical domain name by
   dropping all maskedoctet labels after the first ( leftmost ) label.
   They are said to be reducible to the canonical network domain name.
   So for example 0-25.0.0-18.1.10.in-addr.arpa.  is reducible to
   0-  Note that a  network domain name
   represents the same network as the canonical domain name to which it
   can be reduced.

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4.  Lookup proceedure for a network given an IP address

4.1  Procedure

   1.  Take the initial IP address x.y.z.w and create a candidate
       network by assuming a 24 bit subnet mask.  Thus the initial
       candidate network is x.y.z.0/24.
   2.  Given a candidate network of the form x.y.z.n/m create an
       in-addr.arpa candidate domain name:
       1.  If the number of mask bits m is greater than or equal to 24
           but less than or equal to 32 then the candidate domain name
           is n-m.z.y.x.in-addr.arpa.
       2.  If the number of mask bits m is greater than or equal to 16
           but less than 24 then the candidate domain name is
       3.  If the number of mask bits m is greater than or equal to 8
           but less than 16 then the candidate domain name is
       4.  The notion of fewer than 8 mask bits is not reasonable.
   3.  Perform a DNS lookup for a PTR record for the candidate domain
   4.  If the PTR records returned from looking up the candidate domain
       name  are of the form of a domain name for a network as defined
       previously (Section 2), then for each PTR record reduce that
       returned domain name to the canonical form
       p1-q1.z1.y1.x1.in-addr.arpa.  This represents a network
       1.  If one of the x1.y1.z1.p1/q1 subnets contains the original IP
           address x.y.z.w then the PTR record return becomes the new
           candidate domain name.  Repeat steps 3-4.
       2.  If none of the x1.y1.z1.p1/q1 subnets contain the original IP
           address x.y.z.w then this process has failed.
   5.  If the PTR record(s) for the candidate network is not of the form
       of a network domain name then they are presumed to be the
       hostname(s) of the gateway(s) for the subnet being resolved.
   6.  If the PTR lookup fails ( no PTR  records are returned ).
       1.  If no candidate network PTR lookup for this IP address has
           succeeded in the past and the netmask for the last candidate
           network was 24 or 16 bits long then presume a netmask of 8
           fewer bits for the candidate network and repeat steps 2-4.
       2.  If no candidate network PTR lookup for this IP address has
           succeeded in the past and the netmask fo the last candidate
           network was not 24 or 16 bits long, then increase the netmask
           by 1 bit and repeat steps 2-4.
       3.  If a candidate network PTR lookup for this IP address has
           succeeded in the past or the netmask of the last candidate
           network was 32 bits then this process has failed.

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   7.  Perform a DNS A record lookup for the domain name of the gateway
       to determine the IP number of the gateway.

4.2  IPv6 support

   RFC 3513 [5] requires all IPv6 unicast addresses that do not begin
   with binary 000 have a 64 bit interface id.  From the point of view
   of identifying the last hop router for an IPv6 unicast address this
   means that almost all hosts may be considered to live on a /64
   subnet.  Given the requirement for an that for any subnet there must
   be an anycast address for the routers on that subnet, the process
   described for IPv4 in this document can just as easily be acheived by
   querying the anycast address via SNMP.  Therefore this draft does not
   speak to providing a DNS based mechanism for IPv6.

4.3  Example

   Imagine we begin with the IP number
   1.  Form a candidate network of
   2.  Form a domain name 0-
   3.  Lookup the PTR records for 0-
   4.  Suppose the lookup fails ( no PTR records returned ), then
   5.  Form a new candidate network
   6.  Form a domain name 0-16.15.10.in-addr.arpa.
   7.  Lookup the PTR records for 0-16.15.10.in-addr.arpa.
   8.  Lookup returns:
        1.  0-17.15.10.in-addr.arpa.
        2.  128-18.15.10.in-addr.arpa.
        3.  192-18.15.10.in-addr.arpa.
   9.  So is subnetted into,,
   10.  Since is in, the new candidate domain
        name is 128-18.15.10.in-addr.arpa.
   11.  Lookup the PTR records for 128-18.15.10.in-addr.arpa.
   12.  Lookup returns
        1.  128-19.128-18.15.10.in-addr.arpa.
        2.  0-25.160.128-18.15.10.in-addr.arpa.
        3.  128-25.160.128-18.15.10.in-addr.arpa.
        4.  0-24.161.128-18.15.10.in-addr.arpa.
        5.  162-23.128-18.15.10.in-addr.arpa.
   13.  The canonical network domains for these returned records are
        1.  128-19.15.10.in-addr.arpa.
        2.  0-
        3.  128-
        4.  0-
        5.  162-23.15.10.in-addr.arpa.
   14.  So the network is subnetted into,,,,

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   15.  Since is in the new candidate domain
        name is 162-23.128-18.15.10.in-addr.arpa.
   16.  Lookup the PTR records for 162-23.128-18.15.10.in-addr.arpa.
   17.  Lookup returns:
        1.  gw1.example.net.
        2.  gw2.example.net.
   18.  Lookup the A records for gw1.example.net.  and gw2.example.net.
   19.  Lookup returns
        1.  gw1.example.net:
        2.  gw2.example.net:
   So the is in network which has gateways and

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5.  Needed DNS Entries

   The example in the Lookup procedure (Section 4) section would require
   DNS records as follows:
      In entity A's DNS zone files:
         0-16.15.10.in-addr.arpa.  IN  PTR 0-17.15.10.in-addr.arpa.
         0-16.15.10.in-addr.arpa.  IN  PTR 128-18.15.10.in-addr.arpa.
         0-16.15.10.in-addr.arpa.  IN  PTR 192-18.15.10.in-addr.arpa.
         0-17.15.10.in-addr.arpa.  IN  NS ns1.example.org
         128-18.15.10.in-addr.arpa.  IN  NS ns1.example.net
         192-18.15.10.in-addr.arpa.  IN  NS ns1.example.com
         ns1.example.net           IN  A
         ns1.example.org           IN  A
         ns1.example.com           IN  A
      In entity B's DNS zone files:
         128-18.15.10.in-addr.arpa.  IN  PTR
         128-18.15.10.in-addr.arpa.  IN  PTR
         128-18.15.10.in-addr.arpa.  IN  PTR
         128-18.15.10.in-addr.arpa.  IN  PTR
         128-18.15.10.in-addr.arpa.  IN  PTR
         162-23.128-18.15.10.in-addr.arpa.  IN  PTR gw1.example.net.
         162-23.128-18.15.10.in-addr.arpa.  IN  PTR gw2.example.net.
         gw1.example.net.  IN  A
         gw2.example.net.  IN  A

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6.  Alternate Domain Suffix

   Proper functioning of this method may required the cooperation of
   upstream network providers.  Not all upstream network providers may
   wish to implement this method.  If an upstream provider does not wish
   to implement this method, the method may still be used with an
   alternate domain suffix.

   For example, if the upstream network provider of example.com did not
   wish to provide glue records in their branch of the in-addr.arpa.
   domain, then example.com might elect to use the suffix
   in-addr.example.com as an alternate domain suffix for that purpose.

   For this reason implementations of clients intending to use this
   method should use in-addr.arpa.  as the default suffix, but allow for
   configuration of an alternate suffix.

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7.  Security Considerations

   Any revelation of information to the public internet about the
   internal structure of your network may make it easier for nefarious
   persons to mount diverse attacks upon a network.  Consequently care
   should be exercised in deciding which ( if any ) of the DNS resource
   records described in this draft should be made visible to the public

8  References

        SPECIFICATION", RFC 1035, November 1987.

   [2]  Mockapetris, P., "DNS Encoding of Network Names and Other
        Types", RFC 1101, April 1989.

   [3]  Eidnes, H., de Groot, G. and P. Vixie, "Classless IN-ADDR.ARPA
        delegation", RFC 2317, March 1998.

   [4]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [5]  Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
        Addressing Architecture", RFC 3513, April 2003.

Author's Address

   Edward A. Warnicke
   Cisco Systems Inc.
   7025-6 Kit Creek Road
   Research Triangle Park, NC  27709

   Phone: (919) 392-8489
   EMail: eaw@cisco.com

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