DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)
Two DHCPv6 options that tell a host which DNS servers to use and which domain names to try.
Droms, R., Ed., "DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646, DOI 10.17487/RFC3646, December 2003, <https://www.rfc-editor.org/info/rfc3646>.
@misc{rfc3646,
series = {Request for Comments},
number = 3646,
howpublished = {RFC 3646},
publisher = {RFC Editor},
doi = {10.17487/RFC3646},
url = {https://www.rfc-editor.org/info/rfc3646},
author = {R. Droms},
title = {{DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6)}},
pagetotal = 7,
year = 2003,
month = dec,
} RFC text
Contents
Network Working Group R. Droms, Ed.
Request for Comments: 3646 Cisco Systems
Category: Standards Track December 2003
DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)
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 (2003). All Rights Reserved.
Abstract
This document describes Dynamic Host Configuration Protocol for IPv6
(DHCPv6) options for passing a list of available DNS recursive name
servers and a domain search list to a client.
1. Introduction
This document describes two options for passing configuration
information related to Domain Name Service (DNS) (RFC 1034 [6] and
RFC 1035 [1]) in DHCPv6 (RFC 3315 [2]).
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 BCP 14, RFC 2119 [3].
Throughout this document, unless otherwise specified, the acronym
DHCP refers to DHCP for IPv6 (DHCPv6) as specified in RFC 3315.
This document uses terminology specific to IPv6 and DHCP as defined
in section "Terminology" of RFC 3315.
3. DNS Recursive Name Server option
The DNS Recursive Name Server option provides a list of one or more
IPv6 addresses of DNS recursive name servers to which a client's DNS
resolver MAY send DNS queries [1]. The DNS servers are listed in the
order of preference for use by the client resolver.
The format of the DNS Recursive Name Server option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_DNS_SERVERS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-recursive-name-server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-recursive-name-server (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DNS_SERVERS (23)
option-len: Length of the list of DNS recursive name
servers in octets; must be a multiple of
16
DNS-recursive-name-server: IPv6 address of DNS recursive name server
4. Domain Search List option
The Domain Search List option specifies the domain search list the
client is to use when resolving hostnames with DNS. This option does
not apply to other name resolution mechanisms.
The format of the Domain Search List option is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION_DOMAIN_LIST | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| searchlist |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DOMAIN_LIST (24)
option-len: Length of the 'searchlist' field in octets
searchlist: The specification of the list of domain names in the
Domain Search List
The list of domain names in the 'searchlist' MUST be encoded as
specified in section "Representation and use of domain names" of RFC
3315.
5. Appearance of these options
The DNS Recursive Name Server option MUST NOT appear in any other
than the following messages: Solicit, Advertise, Request, Renew,
Rebind, Information-Request, and Reply.
The Domain Search List option MUST NOT appear in any other than the
following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request, and Reply.
6. Security Considerations
The DNS Recursive Name Server option may be used by an intruder DHCP
server to cause DHCP clients to send DNS queries to an intruder DNS
recursive name server. The results of these misdirected DNS queries
may be used to spoof DNS names.
To avoid attacks through the DNS Recursive Name Server option, the
DHCP client SHOULD require DHCP authentication (see section
"Authentication of DHCP messages" in RFC 3315) before installing a
list of DNS recursive name servers obtained through authenticated
DHCP.
The Domain Search List option may be used by an intruder DHCP server
to cause DHCP clients to search through invalid domains for
incompletely specified domain names. The results of these
misdirected searches may be used to spoof DNS names. Note that
support for DNSSEC [4] will not avert this attack, because the
resource records in the invalid domains may be legitimately signed.
The degree to which a host is vulnerable to attack via an invalid
domain search option is determined in part by DNS resolver behavior.
RFC1535 [7] contains a discussion of security weaknesses related to
implicit as well as explicit domain searchlists, and provides
recommendations relating to resolver searchlist processing. Section
6 of RFC1536 [5] also addresses this vulnerability, and recommends
that resolvers:
1. Use searchlists only when explicitly specified; no implicit
searchlists should be used.
2. Resolve a name that contains any dots by first trying it as an
FQDN and if that fails, with the names in the searchlist appended.
3. Resolve a name containing no dots by appending with the searchlist
right away, but once again, no implicit searchlists should be
used.
In order to minimize potential vulnerabilities it is recommended
that:
1. Hosts implementing the domain search option SHOULD also implement
the searchlist recommendations of RFC1536, section 6.
2. Where DNS parameters such as the domain searchlist or DNS servers
have been manually configured, these parameters SHOULD NOT be
overridden by DHCP.
3. A host SHOULD require the use of DHCP authentication (see section
"Authentication of DHCP messages" in RFC 3315) prior to accepting
a domain search option.
7. IANA Considerations
IANA has assigned an option code to the DNS Recursive Name Server
option (23) and to the Domain Search List option (24) from the DHCP
option code space defined in section "IANA Considerations" of RFC
3315.
8. Acknowledgements
This option was originally part of the DHCPv6 specification, written
by Jim Bound, Mike Carney, Charlie Perkins, Ted Lemon, Bernie Volz
and Ralph Droms.
The analysis of the potential attack through the domain search list
is taken from the specification of the DHCPv4 Domain Search option,
RFC3397 [8].
Thanks to Rob Austein, Alain Durand, Peter Koch, Tony Lindstrom and
Pekka Savola for their contributions to this document.
9. References
9.1. Normative References
[1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[2] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, May 2003.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[4] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[5] Kumar, A., Postel, J., Neuman, C., Danzig, P. and S. Miller,
"Common DNS Implementation Errors and Suggested Fixes", RFC
1536, October 1993.
9.2. Informative References
[6] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[7] Gavron, E., "A Security Problem and Proposed Correction With
Widely Deployed DNS Software", RFC 1535, October 1993.
[8] Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol
(DHCP) Domain Search Option", RFC 3397, November 2002.
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Author's Address
Ralph Droms, Editor
Cisco Systems
1414 Massachusetts Ave.
Boxboro, MA 01719
USA
Phone: +1 978 936 1674
EMail: rdroms@cisco.com
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ISO Plain Language
On this page
§1What the Two Options Do
This RFC defines two options for DHCPv6, the configuration protocol that lets a host on an IPv6 network get its address and its settings from a server. Both options carry Domain Name Service (DNS) information to the client.
The DNS Recursive Name Server option gives the client a list of one or more IPv6 addresses of DNS recursive name servers that its resolver may send queries to. The servers are listed in the order of preference for use by the client resolver. The Domain Search List option gives the client the list of domain names to try when it resolves a host name with DNS. It applies to DNS only, and not to other name resolution mechanisms.
Throughout the document, unless otherwise specified, the acronym DHCP refers to DHCP for IPv6 as specified in RFC 3315, and the terminology specific to IPv6 and DHCP defined in that document is used. The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY and OPTIONAL are to be interpreted as described in BCP 14, RFC 2119.
When a computer joins an IPv6 network, DHCPv6 can tell it its address and other settings. This RFC adds two DNS-related settings to that handover: which DNS servers to ask, and which domain names to try when a lookup fails.
§3The DNS Recursive Name Server Option
The option has a two-octet option code, OPTION_DNS_SERVERS, which is 23, followed by a two-octet option length. The option length gives the length of the list of DNS recursive name servers in octets, and it must be a multiple of 16, because each entry in the list is a 16-octet IPv6 address. The rest of the option is the list of those addresses.
A client's DNS resolver MAY send DNS queries to the servers on that list. The order in which the addresses appear is the order of preference for use by the client resolver, so the first entry is the one the resolver should try first.
This option is a list of IPv6 addresses. The list length must be a multiple of 16 bytes, since each server takes one IPv6 address. The servers are listed in the order the resolver should try them.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
OPTION-CODE | Option code | 2 bytes | 23 | OPTION_DNS_SERVERS, which is 23. |
OPTION-LEN | Option length | 2 bytes | Length of the server list in octets; must be a multiple of 16. | |
DNS-SERVER | DNS recursive name server | 16 bytes | The IPv6 address of one recursive name server. Further addresses follow in the same way. |
§4The Domain Search List Option
This option has a two-octet option code, OPTION_DOMAIN_LIST, which is 24, followed by a two-octet option length giving the length of the searchlist field in octets. The searchlist field holds the list of domain names in the Domain Search List.
The list of domain names in the searchlist MUST be encoded as specified in the section on Representation and use of domain names of RFC 3315, which is the encoding DHCPv6 uses for domain names in its other options.
The option specifies the domain search list the client is to use when resolving hostnames with DNS. It does not apply to other name resolution mechanisms, so a client that resolves names in some other way, such as by consulting a local file, is unaffected by it.
This option holds the domain names to try, written in the same encoding DHCPv6 uses everywhere else for domain names. When the resolver looks up a short name, it can try each of these suffixes in turn.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
OPTION-CODE | Option code | 2 bytes | 24 | OPTION_DOMAIN_LIST, which is 24. |
OPTION-LEN | Option length | 2 bytes | Length of the searchlist field in octets. | |
SEARCHLIST | Searchlist | 4 bytes | The list of domain names, encoded as specified in RFC 3315. |
§5Which Messages Carry the Options
Both options are restricted to a fixed set of DHCPv6 messages, so that they are never exchanged in a context where they would not be understood.
The DNS Recursive Name Server option MUST NOT appear in any message other than Solicit, Advertise, Request, Renew, Rebind, Information-Request and Reply. The Domain Search List option MUST NOT appear in any message other than the same seven: Solicit, Advertise, Request, Renew, Rebind, Information-Request and Reply.
The two options travel only in the DHCPv6 messages used to set up and refresh a lease. They must not appear in any other message type.
§6Security Considerations
The DNS Recursive Name Server option may be used by an intruder DHCP server to cause DHCP clients to send DNS queries to an intruder DNS recursive name server. The results of these misdirected queries may be used to spoof DNS names. To avoid attacks through this option, the DHCP client SHOULD require DHCP authentication before it installs a list of DNS recursive name servers obtained through authenticated DHCP.
The Domain Search List option may be used in the same way, to cause DHCP clients to search through invalid domains for incompletely specified domain names, and the results of these misdirected searches may be used to spoof DNS names. The RFC notes that support for DNSSEC will not avert this attack, because the resource records in the invalid domains may be legitimately signed.
How vulnerable a host is to an invalid domain search option is determined in part by DNS resolver behaviour. The RFC points to the recommendations in section 6 of RFC 1536: use search lists only when they are explicitly specified, and never an implicit search list; resolve a name that contains any dots by first trying it as a fully qualified domain name and, if that fails, with the names in the search list appended; and resolve a name containing no dots by appending the search list right away, while again not using any implicit search list.
To minimise the potential vulnerabilities, the RFC recommends three things: hosts that implement the domain search option SHOULD also implement the search list recommendations of RFC 1536; where DNS parameters such as the domain search list or the DNS servers have been manually configured, these parameters SHOULD NOT be overridden by DHCP; and a host SHOULD require the use of DHCP authentication prior to accepting a domain search option.
A rogue DHCP server could point your computer at its own DNS server, or at domain names that are not yours, and use the answers to pretend to be someone else's site. Signing the DNS answers does not help, because the rogue server can sign its own records properly. The fix is to require the DHCP exchange itself to be authenticated.
§7Option Codes and Their Origin
IANA has assigned an option code from the DHCP option code space defined in the IANA Considerations section of RFC 3315 to each of the two options: 23 for the DNS Recursive Name Server option and 24 for the Domain Search List option.
The RFC records in its acknowledgements that these options were originally part of the DHCPv6 specification, written by Jim Bound, Mike Carney, Charlie Perkins, Ted Lemon, Bernie Volz and Ralph Droms, and that the analysis of the potential attack through the domain search list is taken from the specification of the DHCPv4 Domain Search option, RFC 3397.
Each option has a number that identifies it on the wire: 23 for the list of DNS servers and 24 for the domain search list. Both were part of the original DHCPv6 specification before they were split out into this document.
- 23OPTION_DNS_SERVERS — A list of IPv6 addresses of DNS recursive name servers.
- 24OPTION_DOMAIN_LIST — The domain search list the client uses when resolving hostnames with DNS.