draft-ietf-dnsext-dhcid-rr-03.txt   draft-ietf-dnsext-dhcid-rr-04.txt 
DNSEXT Working Group M. Stapp DNSEXT Working Group M. Stapp
Internet-Draft Cisco Systems, Inc. Internet-Draft Cisco Systems, Inc.
Expires: January 18, 2002 T. Lemon Expires: May 22, 2002 T. Lemon
A. Gustafsson A. Gustafsson
Nominum, Inc. Nominum, Inc.
July 20, 2001 November 21, 2001
A DNS RR for Encoding DHCP Information (DHCID RR) A DNS RR for Encoding DHCP Information (DHCID RR)
<draft-ietf-dnsext-dhcid-rr-03.txt> <draft-ietf-dnsext-dhcid-rr-04.txt>
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as other groups may also distribute working documents as
Internet-Drafts. Internet-Drafts.
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months and may be updated, replaced, or obsoleted by other documents months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 18, 2002. This Internet-Draft will expire on May 22, 2002.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved. Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract Abstract
It is possible for multiple DHCP clients to attempt to update the 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 same DNS FQDN as they obtain DHCP leases. Whether the DHCP server or
the clients themselves perform the DNS updates, conflicts can arise. the clients themselves perform the DNS updates, conflicts can arise.
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proposes storing client identifiers in the DNS to unambiguously proposes storing client identifiers in the DNS to unambiguously
associate domain names with the DHCP clients to which they refer. associate domain names with the DHCP clients to which they refer.
This memo defines a distinct RR type for this purpose for use by This memo defines a distinct RR type for this purpose for use by
DHCP clients and servers, the "DHCID" RR. DHCP clients and servers, the "DHCID" RR.
Table of Contents Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . . 3 3. The DHCID RR . . . . . . . . . . . . . . . . . . . . . . . . 3
3.1 DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . 3 3.1 DHCID RDATA format . . . . . . . . . . . . . . . . . . . . . 4
3.2 DHCID Presentation Format . . . . . . . . . . . . . . . . . 4 3.2 DHCID Presentation Format . . . . . . . . . . . . . . . . . 4
3.3 The DHCID RR Type Codes . . . . . . . . . . . . . . . . . . 4 3.3 The DHCID RR Type Codes . . . . . . . . . . . . . . . . . . 4
3.4 Computation of the RDATA . . . . . . . . . . . . . . . . . . 4 3.4 Computation of the RDATA . . . . . . . . . . . . . . . . . . 4
3.5 Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 5 3.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.6 Updater Behavior . . . . . . . . . . . . . . . . . . . . . . 5 3.5.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.7 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.5.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.7.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . 6
3.7.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . 6 5. Updater Behavior . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . 7
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 7
References . . . . . . . . . . . . . . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 8
Full Copyright Statement . . . . . . . . . . . . . . . . . . 9 Full Copyright Statement . . . . . . . . . . . . . . . . . . 9
1. Terminology 1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119[2]. document are to be interpreted as described in RFC 2119[2].
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choice. However, SHA-1 is significantly slower than MD5. A choice. However, SHA-1 is significantly slower than MD5. A
successful attack of MD5's weakness does not reveal the original successful attack of MD5's weakness does not reveal the original
data that was used to generate the signature, but rather provides a data that was used to generate the signature, but rather provides a
new set of input data that will produce the same signature. Because 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 we are using the MD5 hash to conceal the original data, the fact
that an attacker could produce a different plaintext resulting in that an attacker could produce a different plaintext resulting in
the same MD5 output is not significant concern. the same MD5 output is not significant concern.
3. The DHCID RR 3. The DHCID RR
The DHCID RR is defined with mnemonic DHCID and type code [TBD]. 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.
3.1 DHCID RDATA format 3.1 DHCID RDATA format
The RDATA section of a DHCID RR in transmission contains RDLENGTH The RDATA section of a DHCID RR in transmission contains RDLENGTH
bytes of binary data. The format of this data and its bytes of binary data. The format of this data and its
interpretation by DHCP servers and clients are described below. interpretation by DHCP servers and clients are described below.
DNS software should consider the RDATA section to be opaque. DHCP DNS software should consider the RDATA section to be opaque. DHCP
clients or servers use the DHCID RR to associate a DHCP client's 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 identity with a DNS name, so that multiple DHCP clients and servers
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From the updater's perspective, the DHCID resource record RDATA From the updater's perspective, the DHCID resource record RDATA
consists of a 16-bit identifier type, in network byte order, consists of a 16-bit identifier type, in network byte order,
followed by one or more bytes representing the actual identifier: followed by one or more bytes representing the actual identifier:
< 16 bits > DHCP identifier used < 16 bits > DHCP identifier used
< n bytes > MD5 digest < n bytes > MD5 digest
3.2 DHCID Presentation Format 3.2 DHCID Presentation Format
In DNS master files, the RDATA is represented as a single block in In DNS master files, the RDATA is represented as a single block in
base 64 encoding and may be divided up into any number of white 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 space separated substrings, down to single base 64 digits, which are
concatenated to form the complete RDATA. These substrings can span concatenated to form the complete RDATA. These substrings can span
lines using the standard parentheses. This format is identical to lines using the standard parentheses.
that used for representing binary data in RFC2535[7].
3.3 The DHCID RR Type Codes 3.3 The DHCID RR Type Codes
The type code can have one of three classes of values. The first The type code can have one of three classes of values. The first
class contains just the value zero. This type indicates that the class contains just the value zero. This type indicates that the
remaining contents of the DHCID record encode an identifier that is remaining contents of the DHCID record encode an identifier that is
based on the client's link-layer network address. based on the client's link-layer network address.
The second class of types contains just the value 0xFFFF. This type The second class of types contains just the value 0xFFFF. This type
code is reserved for future extensibility. code is reserved for future extensibility.
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first two - that is, every value other than zero or 0xFFFF. Types in first two - that is, every value other than zero or 0xFFFF. Types in
this class indicate that the remaining contents of the DHCID record this class indicate that the remaining contents of the DHCID record
encode an identifier that is based on the DHCP option whose code is 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 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 at the time of the writing of this specification is 0x3d (61
decimal), which is the DHCP option code for the Client Identifier decimal), which is the DHCP option code for the Client Identifier
option [8]. option [8].
3.4 Computation of the RDATA 3.4 Computation of the RDATA
The data following the type code (for type codes other than 0xFFFF) The DHCID RDATA is formed by concatenating the two type bytes with
is derived by running the MD5 hash algorithm across a buffer some variable-length identifying data.
containing the identifying information. The identifying information
includes some data from the DHCP client's DHCPREQUEST message, and
the FQDN which is the target of the update.
The domain name is represented in the buffer in dns wire-format as < type > < data >
described in RFC1035[5], section 3.1. The domain name MUST NOT be
compressed as described in RFC1035[5], section 4.1.4. Any uppercase The RDATA for all type codes other than 0xffff, which is reserved
alphabetic ASCII character in a label MUST be converted to lowercase for future expansion, is formed by concatenating the two type bytes
before being used to compute the hash. 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 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 identifier, the first two bytes of the DHCID RDATA MUST be zero. To
generate the rest of the resource record, the updater computes a generate the rest of the resource record, the updater computes a
one-way hash using the MD5 algorithm across a buffer containing the one-way hash using the MD5 algorithm across a buffer containing the
client's network hardware type, link-layer address, and the FQDN client's network hardware type, link-layer address, and the FQDN
data. Specifically, the first byte of the buffer contains the data. Specifically, the first byte of the buffer contains the
network hardware type as it appeared in the DHCP 'htype' field of network hardware type as it appeared in the DHCP 'htype' field of
the client's DHCPREQUEST message. All of the significant bytes of the client's DHCPREQUEST message. All of the significant bytes of
the chaddr field in the client's DHCPREQUEST message follow, in the the chaddr field in the client's DHCPREQUEST message follow, in the
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When the updater is using a DHCP option sent by the client in its 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 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 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 DHCP client identifier option is being used, the first byte of
the DHCID RR should be zero, and the second byte should be 61 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 decimal. The rest of the DHCID RR MUST contain the results of
computing an MD5 hash across the payload of the option being used, 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 followed by the FQDN. The payload of a DHCP option consists of the
bytes of the option following the option code and length. bytes of the option following the option code and length.
The "Resolution of DNS Name Conflicts"[1] specification describes 3.5 Examples
the selection process that updaters follow to choose an identifier
from the information presented in a client's DHCPREQUEST message.
3.5 Use of the DHCID RR 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 This RR MUST NOT be used for any purpose other than that detailed in
"Resolution of DNS Name Conflicts"[1]. Although this RR contains "Resolution of DNS Name Conflicts"[1]. Although this RR contains
data that is opaque to DNS servers, the data must be consistent data that is opaque to DNS servers, the data must be consistent
across all entities that update and interpret this record. across all entities that update and interpret this record.
Therefore, new data formats may only be defined through actions of Therefore, new data formats may only be defined through actions of
the DHC Working Group, as a result of revising [1]. the DHC Working Group, as a result of revising [1].
3.6 Updater Behavior 5. Updater Behavior
The data in the DHCID RR allows updaters to determine whether more The data in the DHCID RR allows updaters to determine whether more
than one DHCP client desires to use a particular FQDN. This allows than one DHCP client desires to use a particular FQDN. This allows
site administrators to establish policy about DNS updates. The DHCID site administrators to establish policy about DNS updates. The DHCID
RR does not establish any policy itself. RR does not establish any policy itself.
Updaters use data from a DHCP client's request and the domain name 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, 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 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 that they wish to associate with the client's IP address. If an
updater discovers DHCID RRs whose RDATA does not match the client updater discovers DHCID RRs whose RDATA does not match the client
identity that they have computed, the updater SHOULD conclude that a identity that they have computed, the updater SHOULD conclude that a
different client is currently associated with the name in question. different client is currently associated with the name in question.
The updater SHOULD then proceed according to the site's The updater SHOULD then proceed according to the site's
administrative policy. That policy might dictate that a different administrative policy. That policy might dictate that a different
name be selected, or it might permit the updater to continue. name be selected, or it might permit the updater to continue.
3.7 Examples 6. Security Considerations
3.7.1 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.org.nil" 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.org.nil. A 10.0.0.1
client.org.nil. DHCID AAAUMru0ZM5OK/PdVAJgZ/HU
3.7.2 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.org.nil" 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.org.nil. A 10.0.12.99
chi.org.nil. DHCID AD3dquu0xNqYn/4zw2FXy8X3
4. Security Considerations
The DHCID record as such does not introduce any new security The DHCID record as such does not introduce any new security
problems into the DNS. In order to avoid exposing private problems into the DNS. In order to avoid exposing private
information about DHCP clients to public scrutiny, a one-way hash is information about DHCP clients to public scrutiny, a one-way hash is
used to obscure all client information. In order to make it used to obscure all client information. In order to make it
difficult to 'track' a client by examining the names associated with difficult to 'track' a client by examining the names associated with
a particular hash value, the FQDN is included in the hash a particular hash value, the FQDN is included in the hash
computation. Thus, the RDATA is dependent on both the DHCP client computation. Thus, the RDATA is dependent on both the DHCP client
identification data and on each FQDN associated with the client. identification data and on each FQDN associated with the client.
Administrators should be wary of permitting unsecured DNS updates to Administrators should be wary of permitting unsecured DNS updates to
zones which are exposed to the global Internet. Both DHCP clients zones which are exposed to the global Internet. Both DHCP clients
and servers SHOULD use some form of update authentication (e.g., and servers SHOULD use some form of update authentication (e.g.,
TSIG[9]) when performing DNS updates. TSIG[9]) when performing DNS updates.
5. IANA Considerations 7. IANA Considerations
IANA is requested to allocate an RR type number for the DHCID record IANA is requested to allocate an RR type number for the DHCID record
type. type.
References References
[1] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients [1] Stapp, M., "Resolution of DNS Name Conflicts Among DHCP Clients
(draft-ietf-dhc-dns-resolution-*)", March 2001. (draft-ietf-dhc-dns-resolution-*)", March 2001.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
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