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Network Working Group                                        E. WarnickeRequest for Comments: 4183                                 Cisco SystemsCategory: Informational                                   September 2005A Suggested Scheme for DNS Resolution of Networks and GatewaysStatus 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.  SeeRFC 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 (RFC1101 [2]).RFC 1101 suffers from a number of defects, chief amongWarnicke                     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 ofRFC 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 ofRFC 2317 to specify a series of PTR records enumerating the   subnets of a given network in theRFC 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 ofRFC 2317 to   support delegation at non-octet boundaries.2.  Generic Format of a Network Domain Name   Using the Augmented BNF ofRFC 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 inRFC 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   InRFC 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   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 Address4.1.  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:Warnicke                     Informational                      [Page 3]

RFC 4183                         DNSNET                   September 2005       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.Warnicke                     Informational                      [Page 4]

RFC 4183                         DNSNET                   September 2005       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 SupportRFC 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.Warnicke                     Informational                      [Page 5]

RFC 4183                         DNSNET                   September 2005   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.Warnicke                     Informational                      [Page 6]

RFC 4183                         DNSNET                   September 20055.  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 10.15.162.26.  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.Warnicke                     Informational                      [Page 7]

RFC 4183                         DNSNET                   September 20057.  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: eaw@cisco.comWarnicke                     Informational                      [Page 8]

RFC 4183                         DNSNET                   September 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Warnicke                     Informational                      [Page 9]