Network Working Group M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: June 1, 2001 T. Lemon
A. Gustafsson
Nominum, Inc.
December 2000
A DNS RR for Encoding DHCP Information
<draft-ietf-dnsext-dhcid-rr-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.
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Copyright Notice
Copyright (C) The Internet Society (2000). All Rights Reserved.
Abstract
A situation can arise where multiple DHCP clients request the same
DNS name from their (possibly distinct) DHCP servers. To resolve
such conflicts, 'Resolution of DNS Name Conflicts'[5] proposes
storing client identifiers in the DNS to unambiguously associate
domain names with the DHCP clients "owning" them. This memo defines
a distinct RR type for use by DHCP servers, the "DHCID" RR.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . . 3
4.1 Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
7. Appendix A: Base 64 Encoding . . . . . . . . . . . . . . . . . 4
References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 6
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 8
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1. Terminology
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 RFC 2119[1].
2. Introduction
A set of procedures to allow DHCP[2] clients and servers to
automatically update the DNS (RFC1034[3], RFC1035[4]) is proposed in
"Resolution of DNS Name Conflicts"[5].
A situation can arise where multiple DHCP clients wish to use the
same DNS name. To resolve such conflicts, Resolution of DNS Name
Conflicts[5] proposes storing client identifiers in the DNS to
unambiguously associate domain names with the DHCP clients using
them. In the interest of clarity, it would be preferable for this
DHCP information to use a distinct RR type.
This memo defines a distinct RR type for this purpose for use by
DHCP clients or servers, the "DHCID" RR.
3. The DHCID RR
The DHCID RR is defined with mnemonic DHCID and type code [TBD].
4. DHCID RDATA format
The RDATA section of a DHCID RR in transmission contains RDLENGTH
bytes of binary data. The format of this data and its
interpretation by DHCP servers and clients are described below.
DNS software should consider the RDATA section to be opaque. In DNS
master files, the RDATA is represented in base 64 (see Appendix A)
and may be divided up into any number of white space separated
substrings, down to single base 64 digits, which are concatenated to
obtain the full signature. These substrings can span lines using
the standard parenthesis. This format is identical to that used for
representing binary data in DNSSEC (RFC2535[6]).
DHCP clients or servers use the DHCID RR to associate a DHCP
client's identity with a DNS name, so that multiple DHCP clients and
servers may safely perform dynamic DNS updates to the same zone.
From the updater's perspective, the DHCID resource record consists
of a 16-bit identifier type, followed by one or more bytes
representing the actual identifier. There are two possible forms
for a DHCID RR - one that is used when the DHCP server is using the
client's link-layer address to identify it, and one that is used
when the DHCP server is using some DHCP option that the DHCP client
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sent to identify it. When the link-layer address is used as the
identifier, the first two bytes of the RRDATA are set to 0. When a
DHCP option is used as the identifier, the first two bytes of the
RRDATA contain the option number, in network byte order. The two
bytes 0xffff are reserved for future extensibility. In both cases,
the remainder of the RRDATA is the result of performing a one-way
hash across the identifier.
The details of the method used to generate the data in the RR and
the use to which a DHCP client or server may put this association
are beyond the scope of this draft, and are discussed in the
specification of the DNS update behavior, 'Resolution of DNS Name
Conflicts'[5]. This RR MUST NOT be used for any purpose other than
that detailed in the DHC document. Althought this RR contains data
that is opaque to DNS servers, the data is meaningful to DHCP
updaters. Therefore, new data formats may only be defined through
actions of the DHC Working Group.
4.1 Example
A DHCP server allocating the IPv4 address 10.0.0.1 to a client
"client.org.nil" might use the client's link-layer address to
identify the client:
client.org.nil. A 10.0.0.1
client.org.nil. DHCID AAAY KREX Igqt wYgQ o93/ yNlJ
A DHCP server allocating the IPv4 address 10.0.12.99 to a client
"chi.org.nil" might use the DHCP client identifier option to
identify the client:
chi.org.nil. A 10.0.12.99
chi.org.nil. DHCID AGGS cSLa AYjd OhGM HKD/ lJ2B
5. Security Considerations
The DHCID record as such does not introduce any new security
problems into the DNS. In order to avoid exposing private
information about DHCP clients to public scrutiny, a one-way-hash is
used to obscure all client information.
6. IANA Considerations
IANA is requested to allocate an RR type number for the DHCID record
type.
7. Appendix A: Base 64 Encoding
The following encoding technique is taken from RFC 2045[7] by N.
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Borenstein and N. Freed. It is reproduced here in an edited form
for convenience.
A 65-character subset of US-ASCII is used, enabling 6 bits to be
represented per printable character. (The extra 65th character, "=",
is used to signify a special processing function.)
The encoding process represents 24-bit groups of input bits as
output strings of 4 encoded characters. Proceeding from left to
right, a 24-bit input group is formed by concatenating 3 8-bit input
groups. These 24 bits are then treated as 4 concatenated 6-bit
groups, each of which is translated into a single digit in the base
64 alphabet.
Each 6-bit group is used as an index into an array of 64 printable
characters. The character referenced by the index is placed in the
output string.
The Base 64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. A full encoding quantum is
always completed at the end of a quantity. When fewer than 24 input
bits are available in an input group, zero bits are added (on the
right) to form an integral number of 6-bit groups. Padding at the
end of the data is performed using the '=' character. Since all
base 64 input is an integral number of octets, only the following
cases can arise: (1) the final quantum of encoding input is an
integral multiple of 24 bits; here, the final unit of encoded output
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will be an integral multiple of 4 characters with no "=" padding,
(2) the final quantum of encoding input is exactly 8 bits; here, the
final unit of encoded output will be two characters followed by two
"=" padding characters, or (3) the final quantum of encoding input
is exactly 16 bits; here, the final unit of encoded output will be
three characters followed by one "=" padding character.
References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[2] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Mar
1997.
[3] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987.
[4] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987.
[5] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients
(draft-ietf-dhc-dns-resolution-*)", July 2000.
[6] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[7] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
RFC 2045, November 1996.
Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
250 Apollo Dr.
Chelmsford, MA 01824
USA
Phone: 978.244.8498
EMail: mjs@cisco.com
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Ted Lemon
Nominum, Inc.
950 Charter St.
Redwood City, CA 94063
USA
EMail: mellon@nominum.com
Andreas Gustafsson
Nominum, Inc.
950 Charter St.
Redwood City, CA 94063
USA
EMail: gson@nominum.com
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Full Copyright Statement
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Acknowledgement
Funding for the RFC editor function is currently provided by the
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