draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txt   draft-ietf-dhc-dhcpv6-opt-dnsconfig-04.txt 
Network Working Group R. Droms (ed.) Dynamic Host Configuration Group R. Droms (ed.)
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Expires: August 29, 2003 February 28, 2003 Expires: February 22, 2004 August 24, 2003
DNS Configuration options for DHCPv6 DNS Configuration options for DHCPv6
draft-ietf-dhc-dhcpv6-opt-dnsconfig-03.txt draft-ietf-dhc-dhcpv6-opt-dnsconfig-04.txt
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Abstract Abstract
This document describes DHCPv6 options for passing a list of This document describes DHCPv6 options for passing a list of
available DNS resolvers and a domain search list to a client. available DNS recursive name servers and a domain search list to a
client.
1. Introduction 1. Introduction
This document describes two options for passing configuration This document describes two options for passing configuration
information related to Domain Name Service (DNS) [1, 6] in DHCPv6 information related to Domain Name Service (DNS) (RFC 1034 [6] and
[2]. RFC 1035 [1]) in DHCPv6 (RFC 3315 [2]).
2. Terminology 2. Terminology
The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, The key words MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL in this document are to be
interpreted as described in RFC2119 [3]. interpreted as described in RFC2119 [3].
This document uses terminology specific to IPv6 and DHCPv6 as defined Throughout this document, unless otherwise specified, the acronym
in section "Terminology" of the DHCP specification [2]. DHCP refers to DHCP as specified in RFC 3315.
3. DNS Resolver option This document uses terminology specific to IPv6 and DHCP as defined
in section "Terminology" of RFC 3315.
The DNS Resolver option provides a list of one or more IPv6 addresses 3. DNS Recursive Name Server option
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: The DNS Recursive Name Server option provides a list of one or more
IPv6 addresses of DNS recursive 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
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 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 | | OPTION_DNS_SERVERS | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| DNS-resolver (IPv6 address) | | DNS-recursive-name-server (IPv6 address) |
| | | |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
| DNS-resolver (IPv6 address) | | DNS-recursive-name-server (IPv6 address) |
| | | |
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... | | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DNS_RESOLVERS (tbd) option-code: OPTION_DNS_SERVERS (tbd)
option-len: Length of the list of DNS resolvers in octets; must be a option-len: Length of the list of DNS recursive name
multiple of 16 servers in octets; must be a multiple of 16
DNS-server: IPv6 address of DNS resolver DNS-recursive-name-server: IPv6 address of DNS recursive name server
4. Domain Search List option 4. Domain Search List option
The Domain Search List option specifies the domain search list the The Domain Search List option specifies the domain search list the
client is to use when resolving hostnames with DNS. This option does client is to use when resolving hostnames with DNS. This option does
not apply to other name resolution mechanisms. not apply to other name resolution mechanisms.
The format of the Domain Search List option is: The format of the Domain Search List option is:
0 1 2 3 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 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 | | OPTION_DOMAIN_LIST | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| searchstring | | searchlist |
| ... | | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
option-code: OPTION_DOMAIN_LIST (tbd) option-code: OPTION_DOMAIN_LIST (tbd)
option-len: Length of the 'searchstring' field in octets option-len: Length of the 'searchlist' field in octets
searchstring: The specification of the list of domain names in the searchlist: The specification of the list of domain names in the
Domain Search List Domain Search List
The list of domain names in the 'searchstring' MUST be encoded as The list of domain names in the 'searchlist' MUST be encoded as
specified in section "Representation and use of domain names" of the specified in section "Representation and use of domain names" of RFC
DHCPv6 specification [2]. 3315.
5. Appearance of these options 5. Appearance of these options
The Domain Name Server option MUST NOT appear in other than the The DNS Recursive Name Server option MUST NOT appear in other than
following messages: Solicit, Advertise, Request, Renew, Rebind, the following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request, Reply. Information-Request, Reply.
The Domain Search List option MUST NOT appear in other than the The Domain Search List option MUST NOT appear in other than the
following messages: Solicit, Advertise, Request, Renew, Rebind, following messages: Solicit, Advertise, Request, Renew, Rebind,
Information-Request, Reply. Information-Request, Reply.
6. Security Considerations 6. Security Considerations
The DNS Resolver option may be used by an intruder DHCP server to The DNS Recursive Name Server option may be used by an intruder DHCP
cause DHCP clients to send DNS queries to an intruder DNS resolver. server to cause DHCP clients to send DNS queries to an intruder DNS
The results of these misdirected DNS queries may be used to spoof DNS recursive name server. The results of these misdirected DNS queries
names. may be used to spoof DNS names.
To avoid attacks through the DNS Resolver option, the DHCP client To avoid attacks through the DNS Recursive Name Server option, the
SHOULD require DHCP authentication (see section "Authentication of DHCP client SHOULD require DHCP authentication (see section
DHCP messages" in the DHCPv6 specification) before installing a list "Authentication of DHCP messages" in RFC 3315) before installing a
of DNS resolvers obtained through authenticated DHCP . list of DNS recursive name servers obtained through authenticated
DHCP .
The Domain Search List option may be used by an intruder DHCP server The Domain Search List option may be used by an intruder DHCP server
to cause DHCP clients to search through invalid domains for to cause DHCP clients to search through invalid domains for
incompletely specified domain names. The results of these incompletely specified domain names. The results of these misdirected
misdirected searches may be used to spoof DNS names. Note that searches may be used to spoof DNS names. Note that support for
support for DNSSEC [4] will not avert this attack, because the DNSSEC [4] will not avert this attack, because the resource records
resource records in the invalid domains may be legitimately signed. in the invalid domains may be legitimately signed.
The degree to which a host is vulnerable to attack via an invalid The degree to which a host is vulnerable to attack via an invalid
domain search option is determined in part by DNS resolver behavior. domain search option is determined in part by DNS resolver behavior.
RFC1535 [7] contains a discussion of security weaknesses related to RFC1535 [7] contains a discussion of security weaknesses related to
implicit as well as explicit domain searchlists, and provides implicit as well as explicit domain searchlists, and provides
recommendations relating to resolver searchlist processing. Section recommendations relating to resolver searchlist processing. Section 6
6 of RFC1536 [5] also addresses this vulnerability, and recommends of RFC1536 [5] also addresses this vulnerability, and recommends that
that resolvers: resolvers:
1. Use searchlists only when explicitly specified; no implicit 1. Use searchlists only when explicitly specified; no implicit
searchlists should be used. searchlists should be used.
2. Resolve a name that contains any dots by first trying it as an 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 FQDN and if that fails, with the names in the searchlist
appended. appended.
3. Resolve a name containing no dots by appending with the 3. Resolve a name containing no dots by appending with the
searchlist right away, but once again, no implicit searchlists searchlist right away, but once again, no implicit searchlists
skipping to change at page 4, line 39 skipping to change at page 4, line 46
that: that:
1. Hosts implementing the domain search option SHOULD also implement 1. Hosts implementing the domain search option SHOULD also implement
the searchlist recommendations of RFC1536, section 6. the searchlist recommendations of RFC1536, section 6.
2. Where DNS parameters such as the domain searchlist or DNS servers 2. Where DNS parameters such as the domain searchlist or DNS servers
have been manually configured, these parameters SHOULD NOT be have been manually configured, these parameters SHOULD NOT be
overridden by DHCP. overridden by DHCP.
3. A host SHOULD require the use of DHCP authentication (see section 3. A host SHOULD require the use of DHCP authentication (see section
"Authentication of DHCP messages" in the DHCPv6 specification) "Authentication of DHCP messages" in RFC 3315) prior to accepting
prior to accepting a domain search option. a domain search option.
7. IANA Considerations 7. IANA Considerations
IANA is requested to assign an option code to these options from the IANA is requested to assign an option code to the DNS Recursive Name
option-code space defined in section "DHCPv6 Options" of the DHCPv6 Server option and to the Domain Search List option from the DHCP
specification [2]. option code space defined in section "IANA Considerations" of RFC
3315.
8. Acknowledgments 8. Acknowledgments
This option was originally part of the DHCPv6 specification, written This option was originally part of the DHCPv6 specification, written
by Jim Bound, Mike Carney, Charlie Perkins, Ted Lemon, Bernie Volz by Jim Bound, Mike Carney, Charlie Perkins, Ted Lemon, Bernie Volz
and Ralph Droms. and Ralph Droms.
The analysis of the potential attack through the domain search list The analysis of the potential attack through the domain search list
is taken from the specification of the DHCPv4 Domain Search option, is taken from the specification of the DHCPv4 Domain Search option,
RFC3397 [8]. RFC3397 [8].
Thanks to Rob Austein, Alain Durand, Peter Koch, Tony Lindstrom and Thanks to Rob Austein, Alain Durand, Peter Koch, Tony Lindstrom and
Pekka Savola for their contributions to this document. 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 Normative References
[1] Mockapetris, P., "Domain names - implementation and [1] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[2] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R. [2] Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
Droms (ed.), "Dynamic Host Configuration Protocol for IPv6 Droms (ed.), "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC XXXX, TBD 2003. (DHCPv6)", RFC 3315, May 2003.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
[4] Eastlake, D., "Domain Name System Security Extensions", RFC [4] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999. 2535, March 1999.
[5] Kumar, A., Postel, J., Neuman, C., Danzig, P. and S. Miller, [5] Kumar, A., Postel, J., Neuman, C., Danzig, P. and S. Miller,
"Common DNS Implementation Errors and Suggested Fixes", RFC "Common DNS Implementation Errors and Suggested Fixes", RFC
1536, October 1993. 1536, October 1993.
Normative References Informative References
[6] Mockapetris, P., "Domain names - concepts and facilities", STD [6] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987. 13, RFC 1034, November 1987.
[7] Gavron, E., "A Security Problem and Proposed Correction With [7] Gavron, E., "A Security Problem and Proposed Correction With
Widely Deployed DNS Software", RFC 1535, October 1993. Widely Deployed DNS Software", RFC 1535, October 1993.
[8] Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol [8] Aboba, B. and S. Cheshire, "Dynamic Host Configuration Protocol
(DHCP) Domain Search Option", RFC 3397, November 2002. (DHCP) Domain Search Option", RFC 3397, November 2002.
Author's Address Author's Address
Ralph Droms (ed.) Ralph Droms (ed.)
Cisco Systems Cisco Systems
250 Apollo Drive 1414 Massachusetts Ave.
Chelmsford, MA 01824 Boxboro, MA 01719
USA USA
Phone: +1 978 497 4733 Phone: +1 978 936 1674
EMail: rdroms@cisco.com EMail: rdroms@cisco.com
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