Network Working Group R. Droms (ed.)
Internet-Draft Cisco Systems
Expires: August 29, 2003 February 28, 2003
DNS Configuration options for DHCPv6
draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document describes DHCPv6 options for passing a list of
available DNS resolvers 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) [1, 6] in DHCPv6
[2].
2. Terminology
The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
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SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be
interpreted as described in RFC2119 [3].
This document uses terminology specific to IPv6 and DHCPv6 as defined
in section "Terminology" of the DHCP specification [2].
3. DNS Resolver option
The DNS Resolver option provides a list of one or more IPv6 addresses
of DNS recursive resolvers 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 Resolver 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_RESOLVERS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-resolver (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| DNS-resolver (IPv6 address) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DNS_RESOLVERS (tbd)
option-len: Length of the list of DNS resolvers in octets; must be a
multiple of 16
DNS-server: IPv6 address of DNS resolver
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.
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| searchstring |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DOMAIN_LIST (tbd)
option-len: Length of the 'searchstring' field in octets
searchstring: The specification of the list of domain names in the
Domain Search List
The list of domain names in the 'searchstring' MUST be encoded as
specified in section "Representation and use of domain names" of the
DHCPv6 specification [2].
5. Appearance of these options
The Domain Name Server option MUST NOT appear in other than the
following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request, Reply.
The Domain Search List option MUST NOT appear in other than the
following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request, Reply.
6. Security Considerations
The DNS Resolver option may be used by an intruder DHCP server to
cause DHCP clients to send DNS queries to an intruder DNS resolver.
The results of these misdirected DNS queries may be used to spoof DNS
names.
To avoid attacks through the DNS Resolver option, the DHCP client
SHOULD require DHCP authentication (see section "Authentication of
DHCP messages" in the DHCPv6 specification) before installing a list
of DNS resolvers 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
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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 the DHCPv6 specification)
prior to accepting a domain search option.
7. IANA Considerations
IANA is requested to assign an option code to these options from the
option-code space defined in section "DHCPv6 Options" of the DHCPv6
specification [2].
8. Acknowledgments
This option was originally part of the DHCPv6 specification, written
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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. Changes from draft-ietf-dhc-dhcpv6-opt-dnsconfig-02.txt
This document includes the following changes in response to comments
made during the dhc/dnsext WG last call:
o Combined RFC2119 reference and reference to DHCPv6 specification
into one "Terminology" section; added explicit normative reference
to DHCPv6 specification.
o Changed name of "Domain Name Server" option to "DNS Resolver"
option.
o Clarified and corrected filed names and descriptions of fields in
the option format diagrams.
o Reworded "Appearance of these options" for clarity; removed
Confirm from list of messages in which the options can appear.
o Clarified the type of attack that can be mounted through the
Domain Search List option by copying text from RFC3997
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 XXXX, TBD 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
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1536, October 1993.
Normative 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.
Author's Address
Ralph Droms (ed.)
Cisco Systems
250 Apollo Drive
Chelmsford, MA 01824
USA
Phone: +1 978 497 4733
EMail: rdroms@cisco.com
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