Network Working Group                                        E. Warnicke
Request for Comments: 4183                                 Cisco Systems
Category: Informational                                   September 2005


    A Suggested Scheme for DNS Resolution of Networks and Gateways

Status of This Memo

  This memo provides information for the Internet community.  It does
  not specify an Internet standard of any kind.  Distribution of this
  memo is unlimited.

Copyright Notice

  Copyright (C) The Internet Society (2005).

IESG Note

  This RFC is not a candidate for any level of Internet Standard.  The
  IETF disclaims any knowledge of the fitness of this RFC for any
  purpose and notes that the decision to publish is not based on IETF
  review apart from IESG review for conflict with IETF work.  The RFC
  Editor has chosen to publish this document at its discretion.  See
  RFC 3932 [6] for more information.

Abstract

  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.

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
  host.

  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



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  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.

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

  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.

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
  octet.







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  Examples:

     10.100.2.0/26  -> 0-26.2.100.10.in-addr.arpa.
     10.20.128.0/23 -> 128-23.20.10.in-addr.arpa.
     10.192.0.0/13 -> 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
  shims.

  For example, consider an entity A that controls a network
  10.1.0.0/16.  Entity A delegates to entity B the network 10.1.0.0/18.
  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 10.1.0.0/25.  It would
  provide a domain name for this network of
  0-25.0.0-18.1.10.in-addr.arpa (in B's DNS tables).

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

  Network domain names that do not contain any maskedoctets after the
  first (leftmost) label are hereafter referred to as canonical domain
  names for that network.  0-25.0.1.10.in-addr.arpa.  is the canonical
  domain name for the network 10.1.0.0/25.

  Network domain names that 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
  0-25.0.1.10.in-addr.arpa.  Note that a network domain name represents
  the same network as the canonical domain name to which it can be
  reduced.

4.  Lookup Procedure 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:



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      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
          z-m.y.x.in-addr.arpa.

      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
          y-m.x.in-addr.arpa.

      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
      name.

  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
      x1.y1.z1.p1/q1.

      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 of the last candidate
          network was not 24 or 16 bits long, then increase the netmask
          by 1 bit and repeat steps 2-4.





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      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.

  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 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 achieved by querying
  the anycast address via SNMP.  Therefore, this document does not
  speak to providing a DNS-based mechanism for IPv6.

4.3.  Example

  Imagine we begin with the IP number 10.15.162.3.

  1.  Form a candidate network of 10.15.162.0/24.

  2.  Form a domain name 0-24.162.15.10.in-addr.arpa.

  3.  Look up the PTR records for 0-24.162.15.10.in-addr.arpa.

  4.  Suppose the lookup fails ( no PTR records returned ), then

  5.  Form a new candidate network 10.15.0.0/16.

  6.  Form a domain name 0-16.15.10.in-addr.arpa.

  7.  Look up 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 10.15.0.0/16 is subnetted into 10.15.0.0/17, 10.15.128.0/18,
       and 10.15.192.0/18.

  10.  Since 10.15.162.3 is in 10.15.128.0/18, the new candidate domain
       name is 128-18.15.10.in-addr.arpa.




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  11.  Look up 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.  0-25.160.15.10.in-addr.arpa.
       3.  128-25.160.15.10.in-addr.arpa.
       4.  0-24.161.15.10.in-addr.arpa.
       5.  162-23.15.10.in-addr.arpa.

  14.  So the network 10.15.128.0/18 is subnetted into 10.15.128.0/19,
       10.15.160.0/25, 10.15.160.128/25, 10.15.161.0/25, 10.15.162.0/
       23.

  15.  Since 10.15.162.3 is in 10.15.162.0/23, the new candidate domain
       name is 162-23.128-18.15.10.in-addr.arpa.

  16.  Look up 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.  Look up the A records for gw1.example.net.  and gw2.example.net.

  19.  Lookup returns
       1.  gw1.example.net: 10.15.162.1
       2.  gw2.example.net: 10.15.162.2

  So the 10.15.162.3 is in network 10.15.162.0/23, which has gateways
  10.15.162.1 and 10.15.162.2.














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5.  Needed DNS Entries

  The example of the lookup procedure (Section 4.3) 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  10.15.0.50
        ns1.example.org           IN  A  10.15.128.50
        ns1.example.com           IN  A  10.15.192.50

     In entity B's DNS zone files:
        128-18.15.10.in-addr.arpa.  IN  PTR
        128-19.128-18.15.10.in-addr.arpa.
        128-18.15.10.in-addr.arpa.  IN  PTR
        0-25.160.128-18.15.10.in-addr.arpa.
        128-18.15.10.in-addr.arpa.  IN  PTR
        128-25.160.128-18.15.10.in-addr.arpa.
        128-18.15.10.in-addr.arpa.  IN  PTR
        0-24.161.128-18.15.10.in-addr.arpa.
        128-18.15.10.in-addr.arpa.  IN  PTR
        162-23.128-18.15.10.in-addr.arpa.
        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 10.15.162.1
        gw2.example.net.  IN  A 10.15.162.2

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 its 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 document should be made visible to the
  public internet.

8.  Informative References

  [1]  Mockapetris, P., "Domain Names - Implementation and
       Specficication", STD 13, 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.

  [6]  Alvestrand, H., "The IESG and RFC Editor Documents: Procedures",
       BCP 92, RFC 3932, October 2004.

Author's Address

  Edward A. Warnicke
  Cisco Systems Inc.
  12515 Research Blvd., Building 4
  Austin, TX 78759
  USA

  Phone: (919) 392-8489
  EMail: [email protected]












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Acknowledgement

  Funding for the RFC Editor function is currently provided by the
  Internet Society.







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