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)
draft-ietf-dnsext-apl-rr-00.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
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Comments should be sent to the author or the DNSIND WG mailing list
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Abstract
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
families.
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
follows:
<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
aggregated.
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 1:192.168.32.0/21 !1:192.168.38.0/28
; CIDR blocks covered by classless delegation
42.168.192.IN-ADDR.ARPA APL ( 1:192.168.42.0/26 1:192.168.42.64/26
1:192.168.42.128/25 )
; Zone transfer restriction
_axfr.sbo.example APL 1:127.0.0.1/32 1:172.16.64.0/22
; List of address ranges for multicast
multicast.example APL 1:224.0.0.0/4 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
ftp://ftp.iana.org/in-notes/iana/assignments/address-family-numbers.
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
1998
[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
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Universitaet Bielefeld
Technische Fakultaet
D-33594 Bielefeld
Germany
+49 521 106 2902
<pk@TechFak.Uni-Bielefeld.DE>
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