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Versions: 00                                                            
Dynamic Host Congiguration                                      T. Chown
Internet-Draft                                 University of Southampton
Expires: August 9, 2004                                        S. Venaas
                                                               C. Strauf
                                           JOIN (University of Muenster)
                                                        February 9, 2004

               IPv4 and IPv6 Dual-Stack Issues for DHCPv6

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://

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   This Internet-Draft will expire on August 9, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). All Rights Reserved.


   A node may have support for communications using IPv4 and/or IPv6
   protocols.  Such a node may wish to obtain IPv4 and/or IPv6
   configuration settings via the Dynamic Host Configuration Protocol
   (DHCP).   The original version of DHCP [1] designed for IPv4 has now
   been complemented by a new DHCPv6 [4] for IPv6. This document
   describes issues identified with dual IP version DHCP interactions.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Configuration scenarios  . . . . . . . . . . . . . . . . . . .  3
   3.  Dual-stack issues  . . . . . . . . . . . . . . . . . . . . . .  4
   3.1 Handling multiple responses  . . . . . . . . . . . . . . . . .  4
   3.2 Multiple interfaces  . . . . . . . . . . . . . . . . . . . . .  4
   3.3 DNS load balancing . . . . . . . . . . . . . . . . . . . . . .  5
   3.4 DNS search path issues . . . . . . . . . . . . . . . . . . . .  5
   3.5 Administrative management  . . . . . . . . . . . . . . . . . .  5
   3.6 DHCP option variations . . . . . . . . . . . . . . . . . . . .  5
   3.7 Security issues  . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Potential solutions  . . . . . . . . . . . . . . . . . . . . .  6
   4.1 Separate DHCP servers  . . . . . . . . . . . . . . . . . . . .  6
   4.2 Single DHCPv6 server . . . . . . . . . . . . . . . . . . . . .  6
   4.3 Administrative and other areas . . . . . . . . . . . . . . . .  7
   5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
       Normative References . . . . . . . . . . . . . . . . . . . . .  7
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  8
       Intellectual Property and Copyright Statements . . . . . . . .  9

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1. Introduction

   The original specification of the Dynamic Host Configuration Protocol
   (DHCP) was made with only IPv4 in mind. That specification has been
   subsequently revised, up to the latest version of DHCP [1].    With
   the arrival of IPv6, a new DHCP specification for IPv6 has been
   designed, and published as DHCPv6 [4].

   These protocols allow nodes to communicate via IPv4 or IPv6 to
   retrieve configuration settings for operation in a managed
   environment.  While an IPv6 node may acquire address-related
   configuration settings via IPv6 stateless address autoconfiguration
   [2], such a node may wish to use stateless DHCPv6 [5] for other
   administratively configured options (e.g. DNS, NTP).

   In early IPv6 deployments, a dual-stack mode of operation is
   typically used. There will thus be nodes that require both IPv4 and
   IPv6 configuration settings.   This document discusses issues with
   obtaining such settings in a dual-stack environment.

   In this document, we refer to a "DHCP server" as a server
   implementing the original DHCP [1], and a "DHCPv6 server" as a server
   implementing DHCPv6 [4] or its stateless subset.

2. Configuration scenarios

   For a node in an IPv4-only or IPv6-only environment, the choice of
   DHCP server is a straightforward one; a DHCP server for IPv4, or a
   DHCPv6 server for IPv6.

   In a dual-stack environment a node in a managed environment will need
   to obtain both IPv4 and IPv6 configuration settings, e.g.

   o  IPv4 address

   o  IPv6 address

   o  NTP server

   o  DNS server

   o  NIS server

   o  DNS search path

   While the format of address settings will be IP-specific, the node
   may equally well acquire IPv4 or IPv6 addresses for some settings,
   e.g. for DNS or NTP, if those services are available via IPv4 or IPv6

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   transport.   Currently, a DHCP server returns IPv4 data, while a
   DHCPv6 server returns IPv6 data.

   It is worth noting that in an IPv4 environment, with a DHCP server,
   the choice of whether to use DHCP is made by the node. In an IPv6
   environment, the use of the managed and other bits in the Router
   Advertisement can tell the node whether or not to use DHCPv6. It is
   perhaps not clear whether a dual-stack node should do DHCP for IPv4
   if Managed and OtherConfig flags in the Router Advertisement are both
   off; it seems most appropriate that the decision to use DHCP for IPv4
   or not should be as if the host was IPv4-only.

3. Dual-stack issues

   In this section we list issues that have been raised to date related
   to dual-stack DHCP operation.

3.1 Handling multiple responses

   The general question is how to handle configuration information that
   may be gathered from multiple sources.   Where those sources are DHCP
   and DHCPv6 servers (which may be two physical nodes or two servers
   running on the same node) the client node needs to know whether to
   use the most recent data, or whether to perform some merger or union
   of the responses by certain rules.  A node may choose to ask a DHCPv6
   server and only use a DHCP server if no response is received.

   Merging is possible, but is likely to be complex.  There could be
   some priority, so that if both DHCP and DHCPv6 servers offer a value,
   only one is used.   Or the node could choose to store and use both,
   in some order of its choosing.

   A node may also obtain information from other sources, e.g. a manual
   configuration file (e.g. /etc/resolv.conf for DNS data on many Unix
   systems).  A node configured manually to use an IPv6 DNS server via
   such manual configuration may lose that configuration if it then uses
   DHCP to obtain IPv4 settings if in a dual-stack environment; that
   IPv4 configuration may then overwrite the manual IPv6 DNS setting
   with new IPv4 settings from the DHCP response.

3.2 Multiple interfaces

   A node may have multiple interfaces and run IPv4 and IPv6 on
   different interfaces.  A question then is whether the settings are
   per interface or per node?   DHCPv6 introduces the idea of a DHCP
   Unique Indentifer (DUID) which does not yet exist for DHCP; some
   effort is being made to retrofit the concept to DHCP [6].

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   Per interface settings can be complex because a client node needs to
   know from which interface system settings like NTP server came from.
   And it may not be apparent which setting should be used, if e.g. an
   NTP server option is received on multiple interfaces, potentially
   over different protocols.

3.3 DNS load balancing

   In some cases it is preferable to list DNS server information in an
   ordered way per node for load balancing, giving different responses
   to different clients.   Responses from different DHCP and DHCPv6
   servers may make such configuration problematic.

3.4 DNS search path issues

   The DNS search path may vary for administrative reasons.  For
   example, a site under the domain foo.com chooses to place an early
   IPv6 deployment under the subdomain ipv6.foo.com, until it is
   confident of offering a full dual-stack service under its main
   domain.   The subtlety here is that the DNS search path then affects
   choice of protocol used, e.g. IPv6 for nodes in ipv6.foo.com.

3.5 Administrative management

   In some deployments, the IPv4 and IPv6 services may not be
   administered by the same organisation or people, e.g. in a community
   wireless environment. This poses problems for consistency of data
   offered by either DHCP version.

3.6 DHCP option variations

   Some options in DHCP are not available in DHCPv6 and vice-versa. Some
   IP-version limitations naturally apply, e.g. only IPv6 addresses can
   be in an IPv6 NTP option. The DHCP and DHCPv6 option numbers may be

   A site administrator may wish to configure all their dual-stack nodes
   with (say) two NTP servers, one of which has an IPv4 address, the
   other an IPv6 address.  In this case it may be desirable for an NTP
   option to carry a list of addresses, where some may be IPv4 and some
   may be IPv6.  In general one could consider having DHCPv6 options
   that can carry mix of IPv4 and IPv6 addresses.

3.7 Security issues

   At this stage in the formation of this draft no specific security
   issues have been raised.  The authors welcome comments on this,
   should such issues exist.

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   While there is a specification for authentiation for DHCP messages
   [3], the standard seems to have very few, if any, implementations.
   Thus DHCP and DHCPv6 servers are still liable to be spoofed.   Adding
   an additional protocol may give an extra avenue for attack, should an
   attacker perhaps spoof a DHCPv6 server but not a DHCP server.

4. Potential solutions

   While this document did not originally intend to have solutions in
   its scope, we discuss potential solution spaces in brief here in
   order to provoke some discussion of the issues.   If separate
   solution document(s) emerge, these notes may be removed from this
   document; alternatively this document could be expanded to become a
   best practice guide.  Comments on this are welcomed.

4.1 Separate DHCP servers

   One solution is to run separate DHCP and DHCPv6 servers.  These may
   or may not be run on the same physical node.

   In this approach, some best practice guidance is required for how
   multiple responses are handled or merged.   Administrators have the
   onus to maintain consistency (e.g. scripts may generate common DHCP
   and DHCPv6 configuration files).

   In some cases, inconsistencies may not matter.  In a simple case, an
   NTP server will give the same time whether accessed by IPv4 or IPv6.
   Even if different recursive DNS servers are offered via DHCP or
   DHCPv6, those name servers will provide the same response to a given
   query. The order of DNS servers in a node's configuration is not
   important, unless DNS load balancing is required.

   In the case of separate servers, there are some options like DNS
   search path, that aren't used in a specific IP protocol context.

   It is worth noting that there has been little effort to date to agree
   a common method for IPv6 nodes to acquire non-address settings via
   DHCPv6 because in most dual-stack environments a node will acquire
   its DNS settings via DHCP and query a local (perhaps dual-stack)

4.2 Single DHCPv6 server

   There is an argument for not having to configure and operate both
   DHCP and DHCPv6 servers.  The use of both servers may also lead to
   some redundancy in the information served. Thus one solution may be
   to modify DHCPv6 to be able to return IPv4 information.   This
   solution is hinted at in the DHCPv6 [4] specification: "If there is

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   sufficient interest and demand, integration can be specified in a
   document that extends DHCPv6 to carry IPv4 addresses and
   configuration information." This solution may allow DHCP for IPv4 to
   be completely replaced by DHCPv6 with additional IPv4 information
   options, for dual-stack nodes.

   This approach may require the listing of a mix of IPv4 and IPv6
   addresses for an option.  This should be considered when new IPv6
   options are introduced.

   One problem with this approach is that the client node may then be
   IPv6-only and receiving IPv4 configuration settings that it does not
   want or be able to meaningfully handle.

4.3 Administrative and other areas

   There are also administrative issues or best practice that could be
   promoted.   For example, it may be recommended that sites do not
   split their DNS name space for IPv6-specific testbeds.

   It may be worth considering whether separate manual configuration
   files should be kept for IPv4 and IPv6 settings, e.g. separate /etc/
   resolv.conf files for DNS settings on Unix systems.  However, this
   seems a complex solution that should be better solved by other more
   generalised methods.

   Some differences in DHCP and DHCPv6 may not be reconciled, but may
   not need to be, e.g. different ways to assign addresses by DUID in
   DHCPv6, or the non-aligned option numbers for DHCP and DHCPv6.

5. Summary

   There are a number of issues in the operation of DHCP and DHCPv6
   servers for nodes in dual-stack environments that should be
   clarified.   While some differences in the protocols may not be
   reconciled, there may not be a need to do so.  However, for general
   operation some best practice should be agreed, the principle choice
   being whether separate DHCP and DHCPv6 servers should be maintained
   by a site, or whether DHCPv6 should be extended to carry IPv4
   configuration settings for dual-stack nodes.

6. Security Considerations

   There are no security considerations in this problem statemement per
   se, as it does not propose a new protocol.

Normative References

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   [1]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
        March 1997.

   [2]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

   [3]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [4]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
        Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
        RFC 3315, July 2003.

   [5]  Droms, R., "A Guide to Implementing Stateless DHCPv6 Service",
        draft-ietf-dhc-dhcpv6-stateless-01 (work in progress), October

   [6]  Lemon, T., "Node-Specific Client Identifiers for DHCPv4",
        draft-ietf-dhc-3315id-for-v4-00 (work in progress), October

Authors' Addresses

   Tim Chown
   University of Southampton
   School of Electronics and Computer Science
   Southampton, Hampshire  SO17 1BJ
   United Kingdom

   EMail: tjc@ecs.soton.ac.uk

   Stig Venaas
   Trondheim  NO 7465

   EMail: venaas@uninett.no

   Christian Strauf
   JOIN (University of Muenster)
   Roentgenstr. 9-13
   Muenster  D-48149

   EMail: strauf@uni-muenster.de

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   Funding for the RFC Editor function is currently provided by the
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