Network Working Group E. Warnicke Internet-Draft Cisco Systems Expires: August 2, 2003 February 2003 DNS Resolution of Networks and Gateways draft-warnicke-network-dns-resolution-01.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http:// www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 2, 2003. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. Abstract This document specifies a method given an IP address to perform a series of DNS lookups to determine the network that contains that IP address, the netmask of that network, and the gateway(s) on that network. This method allows for variable length subnet masks, delegation of subnets on non-octet boundaries, and multiple gateways per subnet. Warnicke Expires August 2, 2003 [Page 1] Internet-Draft DNS Resolution of Networks and Gateways February 2003 1. Introduction As a variety of new devices are introduced to the network, many of them not traditional workstations or routers, there are some applications arising which require the participation of the first hop router on the edge in the provision of some network service for a host. There are some mechanisms provided for the resolution of gateways for classful networks in RFC 1035 [1], and subnets in 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 in commonly deployed in the internet. Since the writing of RFC 1101 mechanisms for dealing with classless networks in DNS have arisen, for example RFC 2317 [3]. This document describes a mechanism of using the notation similar to 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 allow for delegation at non-octet boundaries. Warnicke Expires August 2, 2003 [Page 2] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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( "1" / "2" ) DIGIT ) / "3" ("1" / "2" ) decimaloctet = ( *1("1") DIGIT DIGIT ) / ( "2" ( "1" / "2" / "3" / "4" ) DIGIT ) / ( "2" "5" ( "1" / "2" /"3" / "4" / "5" ) ) 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. Warnicke Expires August 2, 2003 [Page 3] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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. 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 which 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 which 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 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 0-25.0.1.10.in-addr.arpa. Note that a network domain name represents the same network as the canonical domain name to which it can be reduced. Warnicke Expires August 2, 2003 [Page 4] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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: 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.z.1.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 ) Warnicke Expires August 2, 2003 [Page 5] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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 for the last candidate network was not 24 of 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. 7. Perform a DNS A record lookup for the domain name of the gateway to determine the IP number of the gateway. 4.2 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. Lookup 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. 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 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 Warnicke Expires August 2, 2003 [Page 6] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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. 0-25.160.15.10.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. Lookup the PTR records for 162-23.128-18.15.10.in-addr.arpa. 17. Lookup returns: 1. gw1.example.com. 2. gw2.example.com. 18. Lookup the A records for gw1.example.com. and gw2.example.com. 19. Lookup returns 1. gw1.example.com: 10.15.162.1 Warnicke Expires August 2, 2003 [Page 7] Internet-Draft DNS Resolution of Networks and Gateways February 2003 2. gw2.example.com: 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 Expires August 2, 2003 [Page 8] Internet-Draft DNS Resolution of Networks and Gateways February 2003 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: ;; provide entries for the subnets of 10.15.0.0/16 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. ;; delegate a shim zone for each of the subnets 0-17.15.10.in-addr.arpa. IN NS 10.15.0.50 128-18.15.10.in-addr.arpa. IN NS 10.15.128.50 192-18.15.10.in-addr.arpa. IN NS 10.15.192.50 Warnicke Expires August 2, 2003 [Page 9] Internet-Draft DNS Resolution of Networks and Gateways February 2003 In entity B's DNS zone files: ;; provide entries for the subnets of 10.15.128.0/18 128-18.15.10.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. ;; provide entries pointing to non in-addr.arpa hostnames for ;; terminal network 162-23.128-18.15.10.in-addr.arpa. IN PTR gw1.example.com. 162-23.128-18.15.10.in-addr.arpa. IN PTR gw2.example.com. ;; provide A records for the gateways. gw1.example.com. IN A 10.15.162.1 gw2.example.com. IN A 10.15.162.2 Warnicke Expires August 2, 2003 [Page 10] Internet-Draft DNS Resolution of Networks and Gateways February 2003 6. 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 internet. Warnicke Expires August 2, 2003 [Page 11] Internet-Draft DNS Resolution of Networks and Gateways February 2003 References [1] Mockapetris, P., "DOMAIN NAMES - IMPLEMENTATION AND 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", November 1997. Author's Address Edward A. Warnicke Cisco Systems Inc. 7025 Kit Creek Road PO Box 14987 Research Triangle Park, NC 27709-4987 US Phone: (919) 392-8489 EMail: eaw@cisco.com Warnicke Expires August 2, 2003 [Page 12] Internet-Draft DNS Resolution of Networks and Gateways February 2003 Appendix A. Acknowledgements The author gratefully acknowledges the assistance of Murry Gavin and Josh Littlefield in reviewing this document. Warnicke Expires August 2, 2003 [Page 13] Internet-Draft DNS Resolution of Networks and Gateways February 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property 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; neither does it represent that it has made any effort to identify any such rights. 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