A Suggested Scheme for DNS Resolution of Networks and Gateways
RFC 4183

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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

Warnicke                     Informational                      [Page 1]
RFC 4183                         DNSNET                   September 2005

   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.

Warnicke                     Informational                      [Page 2]
RFC 4183                         DNSNET                   September 2005

   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