INTERNET-DRAFT                                                Peter Koch
Expires: September 2000                           Universitaet Bielefeld
Updates: RFC 1035                                             March 2000

          A DNS RR Type for Lists of Address Prefixes (APL RR)

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
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   Comments should be sent to the author or the DNSIND WG mailing list


   The Domain Name System is primarily used to translate domain names
   into IPv4 addresses using A RRs. Several approaches exist to describe
   networks or address ranges. This document specifies a new DNS RR type
   "APL" for address prefix lists.

1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Domain names herein are for explanatory purposes only and should not
   be expected to lead to useful information in real life [RFC2606].

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2. Background

   The Domain Name System [RFC1034], [RFC1035] provides a mechanism to
   associate addresses and other internet infrastructure elements with
   hierarchically built domain names. Various types of resource records
   have been defined, especially those for IPv4 and IPv6 [RFC1886]
   addresses.  In [RFC1101] a method is described to publish information
   about the address space allocated to an organisation. In older BIND
   versions, a weak form of controlling access to zone data was
   implemented using TXT RRs describing address ranges.

   This document specifies a new RR type for address prefix lists.

3. APL RR Type

   An APL record has the DNS type of "APL" [draft: not yet applied for]
   and a numeric value of [draft:to be assigned]. The APL RR is defined
   in the IN class only. APL RRs cause no additional section processing.

4. APL RDATA format

   The RDATA section consists of zero or more strings (<apstring>) of
   the form

      |                          ADDRESSFAMILY                        |
      |             PREFIX            | N |         AFDLENGTH         |
      /                            AFDPART                            /
      |                                                               |

      ADDRESSFAMILY     16 bit unsigned value as assigned by IANA
                        (see IANA Considerations)
      PREFIX            8 bit unsigned binary coded prefix length.
                        Upper and lower bounds and interpretation of
                        this value are address family specific.
      N                 negation flag, indicates the presence of the
                        "!" character in the textual format. It has
                        the value "1" if the "!" was given, "0" else.
      AFDLENGTH         length in octets of the following address
                        family dependent part (7 bit unsigned).
      AFDPART           address family dependent part. See below.

   This document defines the AFDPARTs for address families 1 (IPv4) and

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   2 (IPv6).  Future revisions may deal with additional address

4.1. AFDPART for IPv4

   The encoding of an IPv4 address (address family 1) follows the
   encoding specified for the A RR by [RFC1035], section 3.4.1. Trailing
   zero octets MUST be ignored, regardless of the prefix length.

   PREFIX specifies the number of bits of the IPv4 address starting at
   the most significant bit. Legal values range from 0 to 32.

   An IPv4 AFDPART has a variable length of 0 to 4 octets.

4.2. AFDPART for IPv6

   The encoding of an IPv6 address (address family 2) follows the
   specification for the AAAA RR in [RFC1886], section 2.2. The 128 bit
   address is encoded in network byte order. Trailing zero octets MUST
   be ignored, regardless of the prefix length.

   PREFIX specifies the number of bits of the IPv6 address starting at
   the most significant bit. Legal values range from 0 to 128.

   An IPv6 AFDPART has a variable length of 0 to 16 octets.

5. Zone File Syntax

   The textual representation of an APL RR in a DNS zone file is as

      <owner>   IN   <TTL>   APL   {[!]afi:address/prefix}*

   The data consists of zero or more strings of the address family
   indicator <afi>, immediately followed by a colon ":", an address,
   immediately followed by the "/" character, immediately followed by a
   decimal numeric value for the prefix length. Any such string may be
   preceeded by a "!" character. The strings are separated by
   whitespace.  The <afi> is the decimal numeric value of that
   particular address family.

5.1. Textual Representation of IPv4 Addresses

   An IPv4 address in the <address> part of an <apstring> is in dotted
   quad notation, just as in an A RR.  The <prefix> has values from the
   interval 0..32 (decimal).

5.2. Textual Representation of IPv6 Addresses

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   The representation of an IPv6 address in the <address> part of an
   <apstring> follows [RFC2373], section 2.2. Legal values for <prefix>
   are from the interval 0..128 (decimal).

6. APL RR usage

   An APL RR with empty RDATA is valid and implements an empty list.
   Multiple occurences of the same <apstring> in a single APL RR are
   allowed and MUST NOT be merged by a DNS server or resolver.
   <apstrings> MUST be kept in order and MUST NOT be rearranged or

   A single APL RR may contain <apstrings> belonging to different
   address families.  The maximum number of <apstrings> is upperbounded
   by the available RDATA space.

   RRSets consisting of more than one APL RR are legal but the
   interpretation is left to the particular application.

7. Applicability Statement

   The APL RR defines a framework without specifying any particular
   meaning for the list of prefixes.  It is expected that APL RRs will
   be used in different application scenarios which have to be
   documented separately. Those scenarios may be distinguished by
   characteristic prefixes placed in front of the DNS owner name.

   An APL application specification MUST include information on

    o the characteristic prefix, if any

    o how to interpret APL RRSets consisting of more than one RR

    o how to interpret an empty APL RR

    o which address families are expected to appear in the APL RRs for
      that application

    o how to deal with APL RR list elements which belong to other
      address families, including those not yet defined

    o the exact semantics of list elements negated by the "!" character

   Possible applications include the publication of address ranges
   similar to [RFC1101], description of zones built following [RFC2317]
   and in-band access control to limit general access or zone transfer
   (AXFR) availability for zone data held in DNS servers.

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   The specification of particular application scenarios is out of the
   scope of this document.

8. Examples

   The following examples only illustrate some of the possible usages
   outlined in the previous section. None of those applications are
   hereby specified nor is it implied that any particular APL RR based
   application does exist now or will exist in the future.

      ; RFC 1101-like announcement of address ranges for foo.example
      foo.example              APL 1: !1:

      ; CIDR blocks covered by classless delegation
      42.168.192.IN-ADDR.ARPA  APL ( 1: 1:
                                   1: )

      ; Zone transfer restriction
      _axfr.sbo.example        APL 1: 1:

      ; List of address ranges for multicast
      multicast.example        APL 1:  2:FF00:0:0:0:0:0:0:0/8

   Note that since trailing zeroes are ignored in the first APL RR the
   AFDLENGTH of both <apstrings> is three.

9. Security Considerations

   Any information obtained from the DNS should be regarded as unsafe
   unless techniques specified in [RFC2535] or [TSIGRR] were used. The
   definition of a new RR type does not introduce security problems into
   the DNS, but usage of information made available by APL RRs may
   compromise security. This includes disclosure of network topology
   information and in particular the use of APL RRs to construct access
   control lists.

10. IANA Considerations

   This section is to be interpreted as following [RFC2434].

   This document does not define any new namespaces. It uses the 16 bit
   identifiers for address families maintained by IANA in

   IANA is asked to assign a numeric RR type value for APL.

11. Acknowledgements

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   The author would like to thank Mark Andrews for his review and
   constructive comments.

12. References

   [RFC1034]   Mockapetris,P., "Domain Names - Concepts and Facilities",
               RFC 1034, STD 13, November 1987

   [RFC1035]   Mockapetris,P., "Domain Names - Implementation and
               Specification", RFC 1035, STD 13, November 1987

   [RFC1101]   Mockapetris,P., "DNS Encoding of Network Names and Other
               Types", RFC 1101, April 1989

   [RFC1886]   Thomson,S., Huitema.,C., "DNS Extensions to support IP
               version 6", RFC 1886, December 1995

   [RFC2119]   Bradner,S., "Key words for use in RFCs to Indicate
               Requirement Levels", RFC 2119, BCP 14, March 1997

   [RFC2181]   Elz,R., Bush,R., "Clarifications to the DNS
               Specification", RFC 2181, July 1997

   [RFC2317]   Eidnes,H., de Groot,G., Vixie,P., "Classless IN-ADDR.ARPA
               delegation", RFC 2317, March 1998

   [RFC2373]   Hinden,R., Deering,S., "IP Version 6 Addressing
               Architecture", RFC 2373, July 1998

   [RFC2434]   Narten,T., Alvestrand,H., "Guidelines for Writing an IANA
               Considerations Section in RFCs", RFC 2434, BCP 26, October

   [RFC2535]   Eastlake,D., "Domain Name System Security Extensions", RFC
               2535, March 1999

   [RFC2606]   Eastlake,D., Panitz,A., "Reserved Top Level DNS Names",
               RFC 2606, BCP 32, June 1999

   [TSIGRR]    Vixie,P., Gudmundsson,O., Eastlake,D., Wellington,B.,
               "Secret Key Transaction Signatures for DNS (TSIG)",
               <draft-ietf-dnsind-tsig-XX.txt>, work in progress

13. Author's Address

   Peter Koch

Koch                     Expires September 2000                 [Page 6]

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   Universitaet Bielefeld
   Technische Fakultaet
   D-33594 Bielefeld
   +49 521 106 2902

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