RFC 3123

Network Working Group                                            P. Koch
Request for Comments: 3123                        Universitaet Bielefeld
Category: Experimental                                         June 2001

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

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.


   The Domain Name System (DNS) is primarily used to translate domain
   names into IPv4 addresses using A RRs (Resource Records).  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].

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 [RFC2874]
   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.

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3. APL RR Type

   An APL record has the DNS type of "APL" and a numeric value of 42
   [IANA].  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 items (<apitem>) of the

      |                          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
   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.

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

   Trailing zero octets do not bear any information (e.g., there is no
   semantic difference between and 10/16) in an address
   prefix, so the shortest possible AFDLENGTH can be used to encode it.
   However, for DNSSEC [RFC2535] a single wire encoding must be used by

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   all.  Therefore the sender MUST NOT include trailing zero octets in
   the AFDPART regardless of the value of PREFIX.  This includes cases
   in which AFDLENGTH times 8 results in a value less than PREFIX.  The
   AFDPART is padded with zero bits to match a full octet boundary.

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

4.2. AFDPART for IPv6

   The 128 bit IPv6 address (address family 2) is encoded in network
   byte order (high-order byte first).

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

   With the same reasoning as in 4.1 above, the sender MUST NOT include
   trailing zero octets in the AFDPART regardless of the value of
   PREFIX.  This includes cases in which AFDLENGTH times 8 results in a
   value less than PREFIX.  The AFDPART is padded with zero bits to
   match a full octet boundary.

   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
   preceded 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 <apitem> is in dotted
   quad notation, just as in an A RR.  The <prefix> has values from the
   interval 0..32 (decimal).

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5.2. Textual Representation of IPv6 Addresses

   The representation of an IPv6 address in the <address> part of an
   <apitem> 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 occurrences of the same <apitem> in a single APL RR are
   allowed and MUST NOT be merged by a DNS server or resolver.
   <apitems> MUST be kept in order and MUST NOT be rearranged or

   A single APL RR may contain <apitems> belonging to different address
   families.  The maximum number of <apitems> is upper bounded 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

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   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.

   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.             IN APL 1: !1:

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

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

  ; List of address ranges for multicast
  multicast.example.       IN 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 <apitems> is three.

9. Security Considerations

   Any information obtained from the DNS should be regarded as unsafe
   unless techniques specified in [RFC2535] or [RFC2845] 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.

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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

   The IANA assigned numeric RR type value 42 for APL [IANA].

11. Acknowledgements

   The author would like to thank Mark Andrews, Olafur Gudmundsson, Ed
   Lewis, Thomas Narten, Erik Nordmark, and Paul Vixie for their review
   and constructive comments.

12. References

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

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

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

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

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

   [RFC2317] Eidnes, H., de Groot, G. and P. Vixie, "Classless IN-
             ADDR.ARPA delegation", BCP 20, RFC 2317, March 1998.

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

   [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
             IANA Considerations Section in RFCs", BCP 26, RFC 2434,
             October 1998.

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

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

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RFC 3123                       DNS APL RR                      June 2001

   [RFC2845] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
             "Secret Key Transaction Authentication for DNS (TSIG)", RFC
             2845, May 2000.

   [RFC2874] Crawford, M. and C. Huitema, "DNS Extensions to Support
             IPv6 Address Aggregation and Renumbering", RFC 2874, July

   [IANA]    http://www.iana.org/assignments/dns-parameters

13. Author's Address

   Peter Koch
   Universitaet Bielefeld
   Technische Fakultaet
   D-33594 Bielefeld

   Phone: +49 521 106 2902
   EMail: pk@TechFak.Uni-Bielefeld.DE

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14. Full Copyright Statement

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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