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DNSEXT Working Group M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: May 22, 2002 T. Lemon
A. Gustafsson
Nominum, Inc.
November 21, 2001
A DNS RR for Encoding DHCP Information (DHCID RR)
<draft-ietf-dnsext-dhcid-rr-04.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
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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on May 22, 2002.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
It is possible for multiple DHCP clients to attempt to update the
same DNS FQDN as they obtain DHCP leases. Whether the DHCP server or
the clients themselves perform the DNS updates, conflicts can arise.
To resolve such conflicts, "Resolution of DNS Name Conflicts"[1]
proposes storing client identifiers in the DNS to unambiguously
associate domain names with the DHCP clients to which they refer.
This memo defines a distinct RR type for this purpose for use by
DHCP clients and servers, the "DHCID" RR.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . 4
3.2 DHCID Presentation Format . . . . . . . . . . . . . . . . . 4
3.3 The DHCID RR Type Codes . . . . . . . . . . . . . . . . . . 4
3.4 Computation of the RDATA . . . . . . . . . . . . . . . . . . 4
3.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.5.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.5.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 6
5. Updater Behavior . . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . 9
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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[2].
2. Introduction
A set of procedures to allow DHCP[3] clients and servers to
automatically update the DNS (RFC1034[4], RFC1035[5]) is proposed in
"Resolution of DNS Name Conflicts"[1].
Conflicts can arise if multiple DHCP clients wish to use the same
DNS name. To resolve such conflicts, "Resolution of DNS Name
Conflicts"[1] proposes storing client identifiers in the DNS to
unambiguously associate domain names with the DHCP clients using
them. In the interest of clarity, it is preferable for this DHCP
information to use a distinct RR type. This memo defines a distinct
RR for this purpose for use by DHCP clients or servers, the "DHCID"
RR.
In order to avoid exposing potentially sensitive identifying
information, the data stored is the result of a one-way MD5[6] hash
computation. The hash includes information from the DHCP client's
REQUEST message as well as the domain name itself, so that the data
stored in the DHCID RR will be dependent on both the client
identification used in the DHCP protocol interaction and the domain
name. This means that the DHCID RDATA will vary if a single client
is associated over time with more than one name. This makes it
difficult to 'track' a client as it is associated with various
domain names.
The MD5 hash algorithm has been shown to be weaker than the SHA-1
algorithm; it could therefore be argued that SHA-1 is a better
choice. However, SHA-1 is significantly slower than MD5. A
successful attack of MD5's weakness does not reveal the original
data that was used to generate the signature, but rather provides a
new set of input data that will produce the same signature. Because
we are using the MD5 hash to conceal the original data, the fact
that an attacker could produce a different plaintext resulting in
the same MD5 output is not significant concern.
3. The DHCID RR
The DHCID RR is defined with mnemonic DHCID and type code [TBD]. The
DHCID RR is only defined in the IN class. DHCID RRs cause no
additional section processing. The DHCID RR is not a singleton type.
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3.1 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. 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 deterministically perform dynamic DNS updates to the same zone.
From the updater's perspective, the DHCID resource record RDATA
consists of a 16-bit identifier type, in network byte order,
followed by one or more bytes representing the actual identifier:
< 16 bits > DHCP identifier used
< n bytes > MD5 digest
3.2 DHCID Presentation Format
In DNS master files, the RDATA is represented as a single block in
base 64 encoding identical to that used for representing binary data
in RFC2535[7]. The data may be divided up into any number of white
space separated substrings, down to single base 64 digits, which are
concatenated to form the complete RDATA. These substrings can span
lines using the standard parentheses.
3.3 The DHCID RR Type Codes
The type code can have one of three classes of values. The first
class contains just the value zero. This type indicates that the
remaining contents of the DHCID record encode an identifier that is
based on the client's link-layer network address.
The second class of types contains just the value 0xFFFF. This type
code is reserved for future extensibility.
The third class of types contains all the values not included in the
first two - that is, every value other than zero or 0xFFFF. Types in
this class indicate that the remaining contents of the DHCID record
encode an identifier that is based on the DHCP option whose code is
the same as the specified type. The most common value in this class
at the time of the writing of this specification is 0x3d (61
decimal), which is the DHCP option code for the Client Identifier
option [8].
3.4 Computation of the RDATA
The DHCID RDATA is formed by concatenating the two type bytes with
some variable-length identifying data.
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< type > < data >
The RDATA for all type codes other than 0xffff, which is reserved
for future expansion, is formed by concatenating the two type bytes
and a 16-byte MD5 hash value. The input to the hash function is
defined to be:
data = MD5(< identifier > < FQDN >)
The FQDN is represented in the buffer in unambiguous canonical form
as described in RFC2535[7], section 8.1. The type code and the
identifier are related as specified in Section 3.3: the type code
describes the source of the identifier.
type code identifier
0x0000 htype,hlen,chaddr from the client's DHCPREQUEST
0x0001- 'data' portion of a DHCP option from the
0xfffe client's DHCPREQUEST
0xffff RESERVED
The "Resolution of DNS Name Conflicts"[1] specification describes
the selection process that updaters follow to choose an identifier
from the information presented in a client's DHCPREQUEST message.
When the updater is using the client's link-layer address as the
identifier, the first two bytes of the DHCID RDATA MUST be zero. To
generate the rest of the resource record, the updater computes a
one-way hash using the MD5 algorithm across a buffer containing the
client's network hardware type, link-layer address, and the FQDN
data. Specifically, the first byte of the buffer contains the
network hardware type as it appeared in the DHCP 'htype' field of
the client's DHCPREQUEST message. All of the significant bytes of
the chaddr field in the client's DHCPREQUEST message follow, in the
same order in which the bytes appear in the DHCPREQUEST message. The
number of significant bytes in the 'chaddr' field is specified in
the 'hlen' field of the DHCPREQUEST message. The FQDN data, as
specified above, follows.
When the updater is using a DHCP option sent by the client in its
DHCPREQUEST message, the first two bytes of the DHCID RR MUST be the
option code of that option, in network byte order. For example, if
the DHCP client identifier option is being used, the first byte of
the DHCID RR should be zero, and the second byte should be 61
decimal. The rest of the DHCID RR MUST contain the results of
computing an MD5 hash across the payload of the option being used,
followed by the FQDN. The payload of a DHCP option consists of the
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bytes of the option following the option code and length.
3.5 Examples
3.5.1 Example 1
A DHCP server allocating the IPv4 address 10.0.0.1 to a client with
Ethernet MAC address 01:02:03:04:05:06 using domain name
"client.example.com" uses the client's link-layer address to
identify the client. The DHCID RDATA is composed by setting the two
type bytes to zero, and performing an MD5 hash computation across a
buffer containing the Ethernet MAC type byte, 0x01, the six bytes of
MAC address, and the domain name (represented as specified in
Section 3.4).
client.example.com. A 10.0.0.1
client.example.com. DHCID AAAUMru0ZM5OK/PdVAJgZ/HU
3.5.2 Example 2
A DHCP server allocates the IPv4 address 10.0.12.99 to a client
which included the DHCP client-identifier option data
01:07:08:09:0a:0b:0c in its DHCP request. The server updates the
name "chi.example.com" on the client's behalf, and uses the DHCP
client identifier option data as input in forming a DHCID RR. The
DHCID RDATA is formed by setting the two type bytes to the option
code, 0x003d, and performing an MD5 hash computation across a buffer
containing the seven bytes from the client-id option and the FQDN
(represented as specified in Section 3.4).
chi.example.com. A 10.0.12.99
chi.example.com. DHCID AD3dquu0xNqYn/4zw2FXy8X3
4. Use of the DHCID RR
This RR MUST NOT be used for any purpose other than that detailed in
"Resolution of DNS Name Conflicts"[1]. Although this RR contains
data that is opaque to DNS servers, the data must be consistent
across all entities that update and interpret this record.
Therefore, new data formats may only be defined through actions of
the DHC Working Group, as a result of revising [1].
5. Updater Behavior
The data in the DHCID RR allows updaters to determine whether more
than one DHCP client desires to use a particular FQDN. This allows
site administrators to establish policy about DNS updates. The DHCID
RR does not establish any policy itself.
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Updaters use data from a DHCP client's request and the domain name
that the client desires to use to compute a client identity hash,
and then compare that hash to the data in any DHCID RRs on the name
that they wish to associate with the client's IP address. If an
updater discovers DHCID RRs whose RDATA does not match the client
identity that they have computed, the updater SHOULD conclude that a
different client is currently associated with the name in question.
The updater SHOULD then proceed according to the site's
administrative policy. That policy might dictate that a different
name be selected, or it might permit the updater to continue.
6. 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. In order to make it
difficult to 'track' a client by examining the names associated with
a particular hash value, the FQDN is included in the hash
computation. Thus, the RDATA is dependent on both the DHCP client
identification data and on each FQDN associated with the client.
Administrators should be wary of permitting unsecured DNS updates to
zones which are exposed to the global Internet. Both DHCP clients
and servers SHOULD use some form of update authentication (e.g.,
TSIG[9]) when performing DNS updates.
7. IANA Considerations
IANA is requested to allocate an RR type number for the DHCID record
type.
References
[1] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients
(draft-ietf-dhc-dns-resolution-*)", March 2001.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997.
[3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, Mar
1997.
[4] Mockapetris, P., "Domain names - Concepts and Facilities", RFC
1034, Nov 1987.
[5] Mockapetris, P., "Domain names - Implementation and
Specification", RFC 1035, Nov 1987.
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[6] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
1992.
[7] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[8] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, Mar 1997.
[9] Vixie, P., Gudmundsson, O., Eastlake, D. and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)", RFC
2845, May 2000.
Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
250 Apollo Dr.
Chelmsford, MA 01824
USA
Phone: 978.244.8498
EMail: mjs@cisco.com
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
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
Internet organizations, except as needed for the purpose of
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
English.
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
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC editor function is currently provided by the
Internet Society.
Stapp, et. al. Expires May 22, 2002 [Page 9]
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