A DNS Resource Record (RR) for Encoding Dynamic Host Configuration Protocol (DHCP) Information (DHCID RR)
A DNS record that stores a hashed DHCP client identity next to a name, so updaters can tell whose name it is.
Stapp, M., Lemon, T., and A. Gustafsson, "A DNS Resource Record (RR) for Encoding Dynamic Host Configuration Protocol (DHCP) Information (DHCID RR)", RFC 4701, DOI 10.17487/RFC4701, October 2006, <https://www.rfc-editor.org/info/rfc4701>.
@misc{rfc4701,
series = {Request for Comments},
number = 4701,
howpublished = {RFC 4701},
publisher = {RFC Editor},
doi = {10.17487/RFC4701},
url = {https://www.rfc-editor.org/info/rfc4701},
author = {M. Stapp and T. Lemon and A. Gustafsson},
title = {{A DNS Resource Record (RR) for Encoding Dynamic Host Configuration Protocol (DHCP) Information (DHCID RR)}},
pagetotal = 12,
year = 2006,
month = oct,
} RFC text
Contents
Network Working Group M. Stapp
Request for Comments: 4701 Cisco Systems, Inc.
Category: Standards Track T. Lemon
Nominum, Inc.
A. Gustafsson
Araneus Information Systems Oy
October 2006
A DNS Resource Record (RR) for Encoding
Dynamic Host Configuration Protocol (DHCP) Information (DHCID RR)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
It is possible for Dynamic Host Configuration Protocol (DHCP) clients
to attempt to update the same DNS Fully Qualified Domain Name (FQDN)
or to update a DNS FQDN that has been added to the DNS for another
purpose as they obtain DHCP leases. Whether the DHCP server or the
clients themselves perform the DNS updates, conflicts can arise. To
resolve such conflicts, RFC 4703 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 Resource
Record (RR) type for this purpose for use by DHCP clients and
servers: the "DHCID" RR.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. The DHCID RR ....................................................3
3.1. DHCID RDATA Format .........................................3
3.2. DHCID Presentation Format ..................................4
3.3. The DHCID RR Identifier Type Codes .........................4
3.4. The DHCID RR Digest Type Code ..............................4
3.5. Computation of the RDATA ...................................5
3.5.1. Using the Client's DUID .............................5
3.5.2. Using the Client Identifier Option ..................6
3.5.3. Using the Client's htype and chaddr .................6
3.6. Examples ...................................................6
3.6.1. Example 1 ...........................................6
3.6.2. Example 2 ...........................................7
3.6.3. Example 3 ...........................................7
4. Use of the DHCID RR .............................................8
5. Updater Behavior ................................................8
6. Security Considerations .........................................8
7. IANA Considerations .............................................9
8. Acknowledgements ................................................9
9. References ......................................................9
9.1. Normative References .......................................9
9.2. Informative References ....................................10
1. Introduction
A set of procedures to allow DHCP [7] [11] clients and servers to
automatically update the DNS ([3], [4]) is proposed in [1].
Conflicts can arise if multiple DHCP clients wish to use the same DNS
name or a DHCP client attempts to use a name added for another
purpose. To resolve such 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 obscure potentially sensitive client identifying
information, the data stored is the result of a one-way SHA-256 hash
computation. The hash includes information from the DHCP client's
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.
2. 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 [2].
3. The DHCID RR
The DHCID RR is defined with mnemonic DHCID and type code 49. The
DHCID RR is only defined in the IN class. DHCID RRs cause no
additional section processing.
3.1. DHCID RDATA Format
The RDATA section of a DHCID RR in transmission contains RDLENGTH
octets 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 2-octet identifier type, in network byte order,
followed by a 1-octet digest type, followed by one or more octets
representing the actual identifier:
< 2 octets > Identifier type code
< 1 octet > Digest type code
< n octets > Digest (length depends on digest type)
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 [8], Section 3. 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 Identifier Type Codes
The DHCID RR Identifier Type Code specifies what data from the DHCP
client's request was used as input into the hash function. The
identifier type codes are defined in a registry maintained by IANA,
as specified in Section 7. The initial list of assigned values for
the identifier type code and that type's identifier is:
+------------------+------------------------------------------------+
| Identifier Type | Identifier |
| Code | |
+------------------+------------------------------------------------+
| 0x0000 | The 1-octet 'htype' followed by 'hlen' octets |
| | of 'chaddr' from a DHCPv4 client's DHCPREQUEST |
| | [7]. |
| 0x0001 | The data octets (i.e., the Type and |
| | Client-Identifier fields) from a DHCPv4 |
| | client's Client Identifier option [10]. |
| 0x0002 | The client's DUID (i.e., the data octets of a |
| | DHCPv6 client's Client Identifier option [11] |
| | or the DUID field from a DHCPv4 client's |
| | Client Identifier option [6]). |
| 0x0003 - 0xfffe | Undefined; available to be assigned by IANA. |
| 0xffff | Undefined; RESERVED. |
+------------------+------------------------------------------------+
3.4. The DHCID RR Digest Type Code
The DHCID RR Digest Type Code is an identifier for the digest
algorithm used. The digest is calculated over an identifier and the
canonical FQDN as described in the next section.
The digest type codes are defined in a registry maintained by IANA,
as specified in Section 7. The initial list of assigned values for
the digest type codes is: value 0 is reserved, and value 1 is
SHA-256. Reserving other types requires IETF standards action.
Defining new values will also require IETF standards action to
document how DNS updaters are to deal with multiple digest types.
3.5. Computation of the RDATA
The DHCID RDATA is formed by concatenating the 2-octet identifier
type code with variable-length data.
The RDATA for all type codes other than 0xffff, which is reserved for
future expansion, is formed by concatenating the 2-octet identifier
type code, the 1-octet digest type code, and the digest value (32
octets for SHA-256).
< identifier-type > < digest-type > < digest >
The input to the digest hash function is defined to be:
digest = SHA-256(< identifier > < FQDN >)
The FQDN is represented in the buffer in the canonical wire format as
described in [9], Section 6.2. The identifier type code and the
identifier are related as specified in Section 3.3: the identifier
type code describes the source of the identifier.
A DHCPv4 updater uses the 0x0002 type code if a Client Identifier
option is present in the DHCPv4 messages and it is encoded as
specified in [6]. Otherwise, the updater uses 0x0001 if a Client
Identifier option is present, and 0x0000 if not.
A DHCPv6 updater always uses the 0x0002 type code.
3.5.1. Using the Client's DUID
When the updater is using the Client's DUID (either from a DHCPv6
Client Identifier option or from a portion of the DHCPv4 Client
Identifier option encoded as specified in [6]), the first two octets
of the DHCID RR MUST be 0x0002, in network byte order. The third
octet is the digest type code (1 for SHA-256). The rest of the DHCID
RR MUST contain the results of computing the SHA-256 hash across the
octets of the DUID followed by the FQDN.
3.5.2. Using the Client Identifier Option
When the updater is using the DHCPv4 Client Identifier option sent by
the client in its DHCPREQUEST message, the first two octets of the
DHCID RR MUST be 0x0001, in network byte order. The third octet is
the digest type code (1 for SHA-256). The rest of the DHCID RR MUST
contain the results of computing the SHA-256 hash across the data
octets (i.e., the Type and Client-Identifier fields) of the option,
followed by the FQDN.
3.5.3. Using the Client's htype and chaddr
When the updater is using the client's link-layer address as the
identifier, the first two octets of the DHCID RDATA MUST be zero.
The third octet is the digest type code (1 for SHA-256). To generate
the rest of the resource record, the updater computes a one-way hash
using the SHA-256 algorithm across a buffer containing the client's
network hardware type, link-layer address, and the FQDN data.
Specifically, the first octet 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 octets of the
'chaddr' field in the client's DHCPREQUEST message follow, in the
same order in which the octets appear in the DHCPREQUEST message.
The number of significant octets in the 'chaddr' field is specified
in the 'hlen' field of the DHCPREQUEST message. The FQDN data, as
specified above, follows.
3.6. Examples
3.6.1. Example 1
A DHCP server allocates the IPv6 address 2001:DB8::1234:5678 to a
client that included the DHCPv6 client-identifier option data 00:01:
00:06:41:2d:f1:66:01:02:03:04:05:06 in its DHCPv6 request. The
server updates the name "chi6.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 octets
to the value 0x0002, the 1-octet digest type to 1 for SHA-256, and
performing a SHA-256 hash computation across a buffer containing the
14 octets from the client-id option and the FQDN (represented as
specified in Section 3.5).
chi6.example.com. AAAA 2001:DB8::1234:5678
chi6.example.com. DHCID ( AAIBY2/AuCccgoJbsaxcQc9TUapptP69l
OjxfNuVAA2kjEA= )
If the DHCID RR type is not supported, the RDATA would be encoded
[13] as:
\# 35 ( 000201636fc0b8271c82825bb1ac5c41cf5351aa69b4febd94e8f17cd
b95000da48c40 )
3.6.2. Example 2
A DHCP server allocates the IPv4 address 192.0.2.2 to a client that
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 octets to the value 0x0001, the 1-octet digest
type to 1 for SHA-256, and performing a SHA-256 hash computation
across a buffer containing the seven octets from the client-id option
and the FQDN (represented as specified in Section 3.5).
chi.example.com. A 192.0.2.2
chi.example.com. DHCID ( AAEBOSD+XR3Os/0LozeXVqcNc7FwCfQdW
L3b/NaiUDlW2No= )
If the DHCID RR type is not supported, the RDATA would be encoded
[13] as:
\# 35 ( 0001013920fe5d1dceb3fd0ba3379756a70d73b17009f41d58bddbfcd
6a2503956d8da )
3.6.3. Example 3
A DHCP server allocating the IPv4 address 192.0.2.3 to a client with
the 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
octets to zero, the 1-octet digest type to 1 for SHA-256, and
performing an SHA-256 hash computation across a buffer containing the
1-octet 'htype' value for Ethernet, 0x01, followed by the six octets
of the Ethernet MAC address, and the domain name (represented as
specified in Section 3.5).
client.example.com. A 192.0.2.3
client.example.com. DHCID ( AAABxLmlskllE0MVjd57zHcWmEH3pCQ6V
ytcKD//7es/deY= )
If the DHCID RR type is not supported, the RDATA would be encoded
[13] as:
\# 35 ( 000001c4b9a5b249651343158dde7bcc77169841f7a4243a572b5c283
fffedeb3f75e6 )
4. Use of the DHCID RR
This RR MUST NOT be used for any purpose other than that detailed in
[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.
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 obscure the client's identity information,
a one-way hash is used. Further, 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.
However, it should be noted that an attacker that has some knowledge,
such as of MAC addresses commonly used in DHCP client identification
data, may be able to discover the client's DHCP identify by using a
brute-force attack. Even without any additional knowledge, the
number of unknown bits used in computing the hash is typically only
48 to 80.
Administrators should be wary of permitting unsecured DNS updates to
zones, whether or not they are exposed to the global Internet. Both
DHCP clients and servers SHOULD use some form of update
authentication (e.g., [12]) when performing DNS updates.
7. IANA Considerations
IANA has allocated a DNS RR type number for the DHCID record type.
This specification defines a new number-space for the 2-octet
identifier type codes associated with the DHCID RR. IANA has
established a registry of the values for this number-space. Three
initial values are assigned in Section 3.3, and the value 0xFFFF is
reserved for future use. New DHCID RR identifier type codes are
assigned through Standards Action, as defined in [5].
This specification defines a new number-space for the 1-octet digest
type codes associated with the DHCID RR. IANA has established a
registry of the values for this number-space. Two initial values are
assigned in Section 3.4. New DHCID RR digest type codes are assigned
through Standards Action, as defined in [5].
8. Acknowledgements
Many thanks to Harald Alvestrand, Ralph Droms, Olafur Gudmundsson,
Sam Hartman, Josh Littlefield, Pekka Savola, and especially Bernie
Volz for their review and suggestions.
9. References
9.1. Normative References
[1] Stapp, M. and B. Volz, "Resolution of Fully Qualified Domain
Name (FQDN) Conflicts among Dynamic Host Configuration Protocol
(DHCP) Clients", RFC 4703, October 2006.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[4] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[6] Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers
for Dynamic Host Configuration Protocol Version Four (DHCPv4)",
RFC 4361, February 2006.
9.2. Informative References
[7] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[8] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[9] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[10] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[11] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[12] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)",
RFC 2845, May 2000.
[13] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
Types", RFC 3597, September 2003.
Authors' Addresses
Mark Stapp
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
USA
Phone: 978.936.1535
EMail: mjs@cisco.com
Ted Lemon
Nominum, Inc.
950 Charter St.
Redwood City, CA 94063
USA
EMail: mellon@nominum.com
Andreas Gustafsson
Araneus Information Systems Oy
Ulappakatu 1
02320 Espoo
Finland
EMail: gson@araneus.fi
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ISO Plain Language
On this page
§1Introduction
RFC 4703 proposes a set of procedures that allow DHCP clients and servers to update the DNS automatically. Conflicts can arise if multiple DHCP clients wish to use the same DNS name, or if a DHCP client attempts to use a name that was added to the DNS for another purpose. To resolve such conflicts, RFC 4703 proposes storing client identifiers in the DNS, so that domain names can be associated unambiguously with the DHCP clients using them.
Because the DHCP information is easier to interpret when it has a resource record type of its own, this memo defines that record: the DHCID RR. 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.
To obscure potentially sensitive client identifying information, the data stored is the result of a one-way SHA-256 hash computation. The hash includes information from the DHCP client's message as well as the domain name itself, so the data stored in the DHCID RR depends on both the client identification used in the DHCP protocol interaction and the domain name. The DHCID RDATA therefore varies if a single client is associated over time with more than one name, which makes it difficult to track a client as it moves between various domain names.
When a computer gets its address over DHCP, the DNS entry for its name can be updated automatically. Two computers can then want the same name, and nothing in the DNS says which one owns it. This document defines a DNS record that holds a fingerprint of the DHCP client beside the name. The fingerprint is a one-way hash, so it does not expose the client's identity.
§3.1The DHCID RDATA Format
The DHCID RR is defined with the mnemonic DHCID and type code 49. It is only defined in the IN class, and DHCID RRs cause no additional section processing.
The RDATA section of a DHCID RR in transmission contains RDLENGTH octets of binary data. From the updater's perspective, the RDATA consists of a 2-octet identifier type, in network byte order, followed by a 1-octet digest type, followed by one or more octets representing the actual identifier.
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 RFC 3548. The data may be divided into any number of white-space-separated substrings, down to single base-64 digits, which are concatenated to form the complete RDATA, and these substrings can span lines using the standard parentheses.
The record has three parts: a 2-octet field saying where the client identifier came from, a 1-octet field naming the hash algorithm, and the hash itself. In a zone file the whole thing is written as one base-64 block, which may be split across lines.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
IDENTIFIER-TYPE | Identifier type | 2 bytes | Two octets in network byte order, naming where the client identifier came from. | |
DIGEST-TYPE | Digest type | 1 byte | One octet naming the hash algorithm. Value 1 is SHA-256. | |
DIGEST | Digest | 32 bytes | The hash value, 32 octets for SHA-256. |
§3.3Identifier and Digest Type Codes
The identifier type code says what data from the DHCP client's request was used as input into the hash function. The codes live in a registry maintained by IANA, and the initial list assigns three values.
0x0000: the 1-octethtypefollowed byhlenoctets ofchaddrfrom a DHCPv4 client's DHCPREQUEST.0x0001: the data octets, that is the Type and Client-Identifier fields, from a DHCPv4 client's Client Identifier option.0x0002: the client's DUID, that is the data octets of a DHCPv6 client's Client Identifier option, or the DUID field from a DHCPv4 client's Client Identifier option.0x0003to0xfffeare undefined and available to be assigned by IANA, and0xffffis undefined and reserved.
The digest type code is an identifier for the digest algorithm used, and the digest is calculated over an identifier and the canonical FQDN. The digest type codes live in another IANA registry. Value 0 is reserved and value 1 is SHA-256. Reserving other types requires IETF standards action, and defining new values will also require IETF standards action to document how DNS updaters are to deal with multiple digest types.
The first field says which part of the DHCP message identifies the client: the hardware address, the DHCPv4 client identifier option, or the DUID. The second field names the hash algorithm, and today only SHA-256 is assigned. Both fields are registries that IANA maintains.
- 0x0000htype and chaddr — The 1-octet htype followed by hlen octets of chaddr from a DHCPv4 client's DHCPREQUEST.
- 0x0001DHCPv4 Client Identifier option — The data octets, that is the Type and Client-Identifier fields, from a DHCPv4 client's Client Identifier option.
- 0x0002Client DUID — The client's DUID, from a DHCPv6 Client Identifier option or the DUID field of a DHCPv4 Client Identifier option.
- 0xffffReserved — Undefined and reserved.
§3.5Computation of the RDATA
The DHCID RDATA is formed by concatenating the 2-octet identifier type code with variable-length data. For all type codes other than 0xffff, which is reserved for future expansion, the RDATA is the 2-octet identifier type code, the 1-octet digest type code, and the digest value, which is 32 octets for SHA-256.
The input to the digest hash function is the identifier followed by the FQDN. The FQDN is represented in the buffer in the canonical wire format described in RFC 4034, Section 6.2, and the identifier type code describes the source of the identifier.
A DHCPv4 updater uses the 0x0002 type code if a Client Identifier option is present in the DHCPv4 messages and it is encoded as specified in RFC 4361. Otherwise, the updater uses 0x0001 if a Client Identifier option is present, and 0x0000 if not. A DHCPv6 updater always uses the 0x0002 type code.
When the identifier is the DUID, the first two octets of the DHCID RR MUST be 0x0002 in network byte order, the third octet is the digest type code, and the rest MUST contain the results of computing the SHA-256 hash across the octets of the DUID followed by the FQDN. When the identifier is the DHCPv4 Client Identifier option sent by the client in its DHCPREQUEST message, the first two octets MUST be 0x0001, the third octet is the digest type code, and the rest MUST contain the results of hashing the data octets of the option followed by the FQDN. When the identifier is the client's link-layer address, the first two octets MUST be zero, the third octet is the digest type code, and the updater computes the hash over a buffer containing the client's network hardware type as it appeared in the DHCP htype field, all of the significant octets of the chaddr field in the same order as in the DHCPREQUEST message, whose number is given by the hlen field, and then the FQDN data.
The hash is computed over the client identifier followed by the name, with the name in its canonical DNS wire form. Which identifier is used depends on the protocol: a DHCPv6 updater always uses the DUID, while a DHCPv4 updater prefers the DUID form of the client identifier option, then the raw option, and finally the hardware address.
§3.6Examples
Three examples show the record in use. In the first, a DHCP server allocates the IPv6 address 2001:DB8::1234:5678 to a client that included the DHCPv6 client-identifier option data 00:01:00:06:41:2d:f1:66:01:02:03:04:05:06 in its DHCPv6 request. The server updates the name chi6.example.com on the client's behalf and uses the client identifier option data as input, setting the two type octets to 0x0002, the digest type octet to 1 for SHA-256, and hashing the 14 octets from the option together with the FQDN.
In the second example, a server allocates 192.0.2.2 to a client that included the DHCP client-identifier option data 01:07:08:09:0a:0b:0c in its DHCP request. The name chi.example.com is updated, the two type octets are 0x0001, the digest type octet is 1, and the hash covers the seven octets from the option and the FQDN. In the third example, a server allocates 192.0.2.3 to a client with the Ethernet MAC address 01:02:03:04:05:06 using the domain name client.example.com. The two type octets are zero, the digest type octet is 1, and the hash covers the 1-octet htype value for Ethernet, 0x01, the six octets of the MAC address, and the domain name.
For servers that do not know the DHCID type, the same RDATA is written in the generic form defined in RFC 3597, which shows the length of 35 octets.
The three examples cover the three identifier types: a DHCPv6 client identifier, a DHCPv4 client identifier option, and a plain hardware address. Each produces a 35-octet record that is shown both as base-64 and in the generic form used by servers that do not know the type.
client.example.com. A 192.0.2.3
client.example.com. DHCID ( AAABxLmlskllE0MVjd57zHcWmEH3pCQ6V
ytcKD//7es/deY= )
\# 35 ( 000001c4b9a5b249651343158dde7bcc77169841f7a4243a572b5c283
fffedeb3f75e6 )§5Updater Behavior
The data in the DHCID RR allows updaters to determine whether more than one DHCP client desires to use a particular FQDN. This lets site administrators establish policy about DNS updates, but the DHCID RR does not establish any policy itself.
Updaters use data from a DHCP client's request and the domain name that the client wants to use to compute a client identity hash, and then compare that hash with 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 it has 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, which might dictate that a different name be selected, or might permit the updater to continue.
This RR MUST NOT be used for any purpose other than the one detailed in RFC 4703. Although the record contains data that is opaque to DNS servers, the data must be consistent across all entities that update and interpret it, so new data formats may only be defined through actions of the DHC Working Group, as a result of revising RFC 4703.
The record does not decide anything by itself. It lets the updater work out whether the name is already held by a different client, and then the site's own policy decides whether to rename the new client or let it take the name.
- ComputeHash the client's identifier together with the name it wants.
- CompareCheck the hash against the DHCID records already on that name.
- DecideA different hash means another client already holds the name.
- Apply policyThe record sets no policy; the site does.
§6Security Considerations
The DHCID record as such does not introduce any new security problems into the DNS. A one-way hash is used to obscure the client's identity information, and the FQDN is included in the hash computation so that it is difficult to track a client by examining the names associated with a particular hash value. The RDATA is therefore dependent on both the DHCP client identification data and on each FQDN associated with the client.
An attacker that has some knowledge, such as of MAC addresses commonly used in DHCP client identification data, may be able to discover the client's DHCP identity by using a brute-force attack. Even without any additional knowledge, the number of unknown bits used in computing the hash is typically only 48 to 80.
Administrators should be wary of permitting unsecured DNS updates to zones, whether or not they are exposed to the global Internet. Both DHCP clients and servers SHOULD use some form of update authentication, such as the mechanism in RFC 2845, when performing DNS updates.
The hash hides the client's identity from anyone reading the DNS, but it is not a secret: an attacker who can guess a hardware address can confirm it by recomputing the hash. DNS updates to these zones should also be authenticated, so that only the rightful updater can change a name.
§7IANA Considerations
IANA has allocated a DNS RR type number for the DHCID record type. This specification defines a new number space for the 2-octet identifier type codes associated with the DHCID RR, with three initial values assigned and the value 0xFFFF reserved for future use. It defines a second new number space for the 1-octet digest type codes, with two initial values assigned. New values in either space are assigned through Standards Action.
The document asks IANA to register the record type and the two code spaces it uses: one for the client identifier source and one for the hash algorithm. New codes in either space need a Standards Action.