IP: Next Generation Area Paul Vixie (ISC) INTERNET-DRAFT May, 1996 Reverse Name Lookups of Encapsulated IPv4 Addresses in IPv6 Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract [RFC1884] specified two IPv6 address forms that encapsulated IPv4 addresses. [RFC1886] specified a new DNS RR type (AAAA) to map domain names to IPv6 addresses, and also specified the form of the PTR RR owner name used to map IPv6 addresses back to the domain name of that host or interface. This memo amends [RFC1886] and gives two exceptions to its rules regarding PTR RR owner name correspondance to IPv6 addresses. Specifically, the two IPv4 encapsulation methods are exempted from [RFC1886]'s nybble mapping, and are made subject to the appropriate rules from [RFC1035] and [RFC1101]. Expires November 1996 [Page 1] INTERNET-DRAFT IPV6 PTR May 1996 1 - Overview 1.1. In [RFC1884 2.4.4] we see that addresses of the form ::D.D.D.D (which means the first 96 bits of the address are binary zero, and the last 32 bits are an IPv4 address) are used to ``tunnel IPv6 packets over IPv4 routing infrastructure.'' Later in [ibid] we see that addresses of the form ::FFFF:D.D.D.D (that is, 80 ``zero'' bits, 16 ``one'' bits, and an IPv4 address) are used to ``represent the addresses of IPv4-only nodes (those that *do not* support IPv6) as IPv6 addresses.'' 1.2. In [RFC1886 2.5] we see that an inverse name lookup for an IPv6 address is done by reversing the nybbles, formatting them as hexadecimal ASCII, and using each as a DNS label under the domain IP6.INT. Thus, to find the name associated with address ``4321:0:1:2:3:4:567:89ab,'' one would search for a PTR RR at the name: b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.IP6.INT 1.3. This leaves open the question of how to do inverse name lookups on the two IPv6 address forms which actually describe IPv4 endpoints. Should a resolver look under the IP6.INT for a nybble wise name, or under IN-ADDR.ARPA for a byte wise name? Due to format differences, there is no data oriented solution (such as CNAME RRs or replicate NS RRs) available to simply make a union in the namespace so that it does not matter which query name is used. 1.4. This memo recommends that the two IPv6 address forms which encapsulate IPv4 addresses shall follow the usual IPv4 inverse naming rules (i.e., IN-ADDR.ARPA). It is the author's view that inverse naming authority and address allocation authority must always be delegated together, and that any IPv4 address, no matter what context it is used in, has a single correct PTR RRset denoting the name(s) of the host or interface to which that address is bound. Expires November 1996 [Page 2] INTERNET-DRAFT IPV6 PTR May 1996 2 - Detail 2.1. For a mapped or tunnelled IPv4 address represented in IPv6 notation, inverse name lookups are to be done by stripping off the first 96 bits of the address, and using the last 32 bits as a raw IPv4 address. From [RFC1884]: IPv6-Tunnelled IPv4 Address | 80 bits | 16 | 32 bits | +--------------------------------------+--------------------------+ |0000..............................0000|0000| IPv4 address | +--------------------------------------+----+---------------------+ IPv6-Mapped IPv4 Address | 80 bits | 16 | 32 bits | +--------------------------------------+--------------------------+ |0000..............................0000|FFFF| IPv4 address | +--------------------------------------+----+---------------------+ 2.2. The encapsulated 32 bit IPv4 is to be broken down into four octets, and these octets are reversed, formatted in decimal ASCII, and used as labels under the IN-ADDR.ARPA domain. IPv4 Address | 8 bits | 8 bits | 8 bits | 8 bits | +----------------+----------------+----------------+----------------+ | Octet1 | Octet2 | Octet3 | Octet4 | +----------------+----------------+----------------+----------------+ IN-ADDR.ARPA Owner Name . . . . IN-ADDR . ARPA 2.3. A normal DNS query for a PTR RR is done. If the response contains an RRset matching the owner name and PTR type, then the RDATA(s) of these RRs are the names associated with the hosts or interfaces using the IPv4 address corresponding to this owner name. Multiple RRs can be present in the response PTR RRset, if this address is reachable by more than one A RR name. Thus, A RRs and PTR RRs are symmetric, while CNAME aliases are single ended. Expires November 1996 [Page 3] INTERNET-DRAFT IPV6 PTR May 1996 3 - Example IPv6 Address PTR Owner Name -------------------------------------------------- ::13.1.68.3 3.68.1.13.IN-ADDR.ARPA ::FFFF:129.144.52.38 38.52.144.129.IN-ADDR.ARPA 4 - Security Considerations None. 5 - References [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification,'' RFC 1035, USC/Information Sciences Institute, November 1987. [RFC1101] P. Mockapetris, ``DNS Encoding of Network Names and Other Types,'', RFC 1101, USC/Information Sciences Institute, April 1898. [RFC1884] R. Hinden, S. Deering, ``IP Version 6 Addressing Architecture,'' RFC 1884, Ipsilon Networks and Xerox PARC, December, 1995. [RFC1886] S. Thomson, C. Huitema, ``DNS Extensions to support IP version 6,'' RFC 1886, Bellcore and INRIA, December 1995. 6 - Acknowledgements The mailing list was instrumental in crystallizing the views presented in this document. 7 - Author's Address Paul Vixie Internet Software Consortium Star Route Box 159A Woodside, CA 94062 +1 415 747 0204 Expires November 1996 [Page 4]