DNSEXT M. Stapp
Internet-Draft Cisco Systems, Inc.
Expires: September 1, 2006 T. Lemon
Nominum, Inc.
A. Gustafsson
Araneus Information Systems Oy
February 28, 2006
A DNS RR for Encoding DHCP Information (DHCID RR)
<draft-ietf-dnsext-dhcid-rr-12.txt>
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
It is possible for DHCP clients to attempt to update the same DNS
FQDN or attempt 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, "Resolution of DNS Name
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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.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . 5
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 . . . . . . . . . . . . . . . . . . . . . . 6
3.6.3. Example 3 . . . . . . . . . . . . . . . . . . . . . . 7
4. Use of the DHCID RR . . . . . . . . . . . . . . . . . . . . . 7
5. Updater Behavior . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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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 [6] [10] clients and servers to
automatically update the DNS (RFC 1034 [3], RFC 1035 [4]) is proposed
in "Resolution of DNS Name Conflicts" [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, "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 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.
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.
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
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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 RFC 3548 [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 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 is:
0x0000 = htype, chaddr from a DHCPv4 client's DHCPREQUEST [6].
0x0001 = The data octets (i.e., the Type and Client-Identifier
fields) from a DHCPv4 client's Client Identifier option [9].
0x0002 = The client's DUID (i.e., the data octets of a DHCPv6
client's Client Identifier option [10] or the DUID field from a
DHCPv4 client's Client Identifier option [12]).
0x0003 - 0xfffe = Available to be assigned by IANA.
0xffff = 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.
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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 unambiguous canonical form
as described in RFC 4034 [8], section 6.1. 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 [12]. 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 [12]), 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
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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 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
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
Ethernet MAC type octet, 0x01, the six octets of MAC address, and the
domain name (represented as specified in Section 3.5).
client.example.com. A 10.0.0.1
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 )
3.6.2. 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
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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 10.0.12.99
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 allocates the IPv6 address 2000::1234:5678 to a client
which 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 2000::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 )
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.
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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. And, 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., TSIG [11]) when performing DNS updates.
7. IANA Considerations
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IANA is requested to allocate 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 is
requested to establish 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 RFC
2434 [5].
This specification defines a new number-space for the 1-octet digest
type codes associated with the DHCID RR. IANA is requested to
establish 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 RFC 2434
[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 DNS Name Conflicts Among
DHCP Clients (draft-ietf-dhc-dns-resolution-*)", February 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.
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9.2. Informative References
[6] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[7] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
[8] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
[9] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
Extensions", RFC 2132, March 1997.
[10] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[11] Vixie, P., Gudmundsson, O., Eastlake, D., and B. Wellington,
"Secret Key Transaction Authentication for DNS (TSIG)",
RFC 2845, May 2000.
[12] Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers
for Dynamic Host Configuration Protocol Version Four (DHCPv4)",
RFC 4361, February 2006.
[13] Gustafsson, A., "Handling of Unknown DNS Resource Record (RR)
Types", RFC 3597, September 2003.
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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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