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Network Working Group                                           T. ChownRequest for Comments: 4477                     University of SouthamptonCategory: Informational                                        S. Venaas                                                                 UNINETT                                                               C. Strauf                                      Clausthal University of Technology                                                                May 2006Dynamic Host Configuration Protocol (DHCP):IPv4 and IPv6 Dual-Stack IssuesStatus of This Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   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 (RFC 2131) designed for IPv4   has now been complemented by a new DHCPv6 (RFC 3315) for IPv6.  This   document describes issues identified with dual IP version DHCP   interactions, the most important aspect of which is how to handle   potential problems in clients processing configuration information   received from both DHCPv4 and DHCPv6 servers.  The document makes a   recommendation on the general strategy on how best to handle such   issues and identifies future work to be undertaken.Chown, et al.                Informational                      [Page 1]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006Table of Contents1. Introduction ....................................................32. Configuration Scenarios .........................................33. Dual-Stack Issues ...............................................43.1. Handling Multiple Responses ................................43.2. Different Administrative Management ........................53.3. Multiple Interfaces ........................................53.4. DNS Load Balancing .........................................53.5. DNS Search Path Issues .....................................53.6. Protocol Startup Sequence ..................................63.7. DHCP Option Variations .....................................63.8. Security Issues ............................................64. Potential Solutions .............................................74.1. Separate DHCP Servers ......................................74.2. Single DHCPv6 Server .......................................84.3. Optimising for Failure with Lists of Addresses .............94.4. Administrative and Other Areas ............................105. Summary ........................................................106. Security Considerations ........................................127. Acknowledgements ...............................................128. Informative References .........................................12Chown, et al.                Informational                      [Page 2]

RFC 4477                DHCP: Dual-Stack Issues                 May 20061.  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   (respectively) 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, such as DNS or 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.   There is a general multihoming issue to be solved for DHCP.  A host   might simultaneously be connected to multiple networks managed by   multiple parties.  Also, IPv4 and IPv6 might be managed by separate   parties.  While these issues are touched on in this document, here we   focus on the specific issues for operating DHCP in a mixed (typically   dual-stack) IPv4 and IPv6 environment within a single administrative   domain.   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 [5].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, such as the   following:   o  IPv4 address   o  IPv6 address   o  NTP serverChown, et al.                Informational                      [Page 3]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   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,   such as for DNS or NTP, if those services are available via IPv4 or   IPv6 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 offer a hint to the node whether or not to use full   DHCPv6 or its stateless variant.  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 were IPv4-only.3.  Dual-Stack Issues   In this section, we list issues that have been raised to date,   related to dual-stack DHCP operation.   It has been noted from comments that the first four, and possibly   five, subsections here may also be viewed as multihoming issues.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 method for merging lists of   addresses, for options that carry such information, may also be   required.  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.  Merging issues are further discussed   later in this document.Chown, et al.                Informational                      [Page 4]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   A node may also obtain information from other sources, such as a   manual configuration file (for example, /etc/resolv.conf for DNS data   on many UNIX systems).  A node configured manually to use an IPv6 DNS   server may lose that configuration if it is in a dual-stack   environment and uses DHCP to obtain IPv4 settings; the new IPv4   settings from the DHCP response may then overwrite the manual IPv6   DNS setting.3.2.  Different Administrative Management   In some deployments, the IPv4 and IPv6 services may not be   administered by the same organisation or people, such as in a   community wireless environment.  This poses problems for consistency   of data offered by either DHCP version.   There may also be different connectivity for the protocols, and the   client may gain advantage from knowing which 'administrative domain'   is supplying which information.  A client may need to use different   received information depending on which connectivity is being used.   In the example of the community wireless environment, the question of   which connectivity is 'better' is a separate issue.3.3.  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.   Per-interface settings can be complex because a client node needs to   know which interface system settings, like NTP server, came from.   And it may not be apparent which setting should be used if, for   example, an NTP server option is received on multiple interfaces,   potentially over different protocols.3.4.  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, if the knowledge of   the load balancing is not available to both servers.3.5.  DNS Search Path Issues   The DNS search path may vary for administrative reasons.  For   example, a site under the domain example.com may choose to place an   early IPv6 deployment under the subdomain ipv6.example.com, until it   is confident of offering a full dual-stack service under its mainChown, et al.                Informational                      [Page 5]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   domain.  The subtlety here is that the DNS search path then affects   the choice of protocol used, such as IPv6 for nodes in   ipv6.example.com.3.6.  Protocol Startup Sequence   In the dual-stack environment, one needs to consider what happens if,   for example, the IPv6 interface (transport) is started after DHCPv4   was used to configure the client.  Should the client then simply   discard the current IPv4 information, or merge it with a subsequent   IPv6 response?  It may also be possible that one protocol is shut   down or started while the system is running.  There are similarities   here to issues when DHCP renewals have information that may appear   that previously was not available (or no longer carry information   that has been removed).3.7.  DHCP Option Variations   Some options in DHCP are not available in DHCPv6 and vice versa.   Some IP-version limitations naturally apply; for example, only IPv6   addresses can be in an IPv6 NTP option.  The DHCP and DHCPv6 option   numbers may be different.   Some sites may choose to use IPv4-mapped addresses in DHCPv6-based   options.  The merits and drawbacks of such an approach need   discussion.   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 a mix of IPv4 and IPv6 addresses.3.8.  Security Issues   This document does not introduce any new security issues per se.  A   detailed analysis of DHCP and DHCPv6 security is out of scope for   this document.   While there is a specification for authentication 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.Chown, et al.                Informational                      [Page 6]

RFC 4477                DHCP: Dual-Stack Issues                 May 20064.  Potential Solutions   Here we discuss the two broad solution strategies proposed within the   IETF dhc WG.  The first is to run separate DHCP and DHCPv6 servers   (with the client merging information received from both where   necessary, or perhaps choosing to query a particular version first).   The second is to run only a DHCPv6 server and relay IPv4   configuration information within (new) IPv4 configuration options.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.  The information served   from the DHCP servers could be generated from a single database   instance for consistency.  One might have a single server instance   supporting both DHCPv4 and DHCPv6 protocols.   In this approach, some best practice guidance is required for how   multiple responses are handled or merged.  Administrators have the   onus to maintain consistency (for example, 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, then those name servers should provide the same response to a   given query.  In cases where sites may be operating a 'two-faced   DNS', this will still hold true if the node is on the same   topological point on the network from an IPv4 or IPv6 perspective.   The order of DNS servers in a node's configuration is not important,   unless DNS load balancing is required.   In other cases, inconsistencies may be an issue; for example, where   lists of values are returned, an algorithm is needed for list merger   (e.g., "alternate, DHCPv6 first").  Or there may be incompatible   configuration values where, for example, DHCPv6 supplies domain names   (such the SMTP or POP servers) whereas DHCPv4 provides only IPv4   addresses.   In the case of separate servers, there are some options, like DNS   search path, that aren't used in a specific IP protocol context.   The multiple server approach will have some simplifications.  The   DHCPv4 and DHCPv6 servers may provide the same value for a particular   parameter, in which case there is no conflict.  In some cases, the   value may be different, but the effect should be the same (such as anChown, et al.                Informational                      [Page 7]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   NTP server).  The crux of the issue is to identify where differences   may occur and where these differences will have an impact on node   behaviour.   One possible solution is to have per-host preferences, or an ordered   list of preferences, for example, "use manually configured", "prefer   DHCPv4", or "prefer DHCPv6", assuming the host can act based upon   which protocol is used.  It is then up to the site administrator to   ensure that values returned from either DHCP are consistent (a   principle that extends if other methods are used, such as NIS or   Service Location Protocol (SLP)).4.2.  Single DHCPv6 Server   There is an argument for not having to configure and operate both   DHCP and DHCPv6 servers in a dual-stack site environment.  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 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.   A general argument is that which DHCP protocol is used (whether it's   over IPv4 or IPv6) shouldn't affect what kind of addresses you can   get configured with it, and that simplicity and predictability come   from using a single server over a single transport.  IPv4-capable   hosts will likely remain for at least 10 years, probably much longer;   do we want dual-stack hosts (which will become the norm) to do both   DHCPv4 and DHCPv6 forever while dual-stack?  If you need both servers   to configure interfaces with addresses, and get other configurations,   then you rely on two separate protocols to work (servers and relays,   etc.) in order for the host to behave correctly.   This approach may require the listing of a mix of IPv4 and IPv6   addresses for an option.  This could then be considered when new IPv6   options are introduced.  There could be just two options needed, one   new option for the address delegation, and one for doing   encapsulation.   Also, there are a number of paradigms in DHCPv6 that we miss in   DHCPv4.  An example is movement away from using MAC addresses for   per-host address assignment and instead using DHCP Unique Identifier   (DUIDs) or Identity Association Identifiers (IAIDs).  As stated inSection 9 of RFC3315, DHCPv6 servers use DUIDs to identify clients   for the selection of configuration parameters and in the associationChown, et al.                Informational                      [Page 8]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   of IAs with clients.  DHCPv6 clients use DUIDs to identify a server   in messages where a server needs to be identified.  However, in this   particular example, the new DHCPv6 functionality has recently been   retrofitted to IPv4 via a specification for DUIDs for DHCPv4 [6].   However, there are a number of potential problems with this approach:   o  IPv4-only nodes would not have any DHCP service available to them;      such an approach is only possible in a fully dual-stack      environment.   o  The client node may then be IPv6-only and receive IPv4      configuration settings that it does not want or be able to handle      meaningfully.   o  The DHCPv4 servers need to be configured anyway to support IPv4-      only hosts, so there is still duplication of information.   o  What happens if there are DHCPv6 servers that don't return IPv4      information?  Does this mean the client can't run IPv4 (since it      won't do DHCPv4)?   o  If IPv4 information is served from a DHCPv6 server as well as an      IPv4 DHCP server, IPv4 address space will need to be allocated to      both servers, fragmenting the potentially precious IPv4 global      address resource for the site.4.3.  Optimising for Failure with Lists of Addresses   There is a generic issue with any option that includes a list of   addresses of servers (such as DNS server addresses).  The list is   offered to cater for resilience, such as whether the listed server   itself fails or connectivity to the server fails.  If the client does   not know the cause of failure, its optimal strategy is to try a   different server, via a different protocol.  The problem today is   that the IPv4 list is returned via DHCPv4, and the IPv6 list via   DHCPv6; the client really has no way to "try a different server",   since that information is lost by the protocol, even though it may be   known by the server.   Just putting merging heuristics in the client cannot provide the best   behaviour, since information is lost.  By comparison, if IPv4-mapped   addresses were included in the DHCPv6 option along with IPv6   addresses, the DHCP server can give an intelligent order that takes   into account which addresses are of the same DNS/whatever server.   IPv6-only clients have to know to discard the IPv4-mapped addresses   in this solution, and it's much easier to solve this in the combined-   DHCP-server case than in the two-server case.Chown, et al.                Informational                      [Page 9]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   One can argue that this is only an optimisation, and in many cases   the list has only two elements, so the "next" choice is forced.   However, this particular issue highlights the subtleties of merging   responses from separate servers.4.4.  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, such as separate   /etc/resolv.conf files for DNS settings on UNIX systems.  However,   this seems a complex solution.  The problem should be better solved   by other, more generalised methods.   It may be important at times to be able to distinguish DHCP client   and server identities.  DHCPv6 introduces the idea of a DHCP Unique   Identifier (DUID).  The DUID concept has also been retrofitted to   DHCPv4 [6], and thus it may form the basis of part of the solution   space for the problem at hand.   Some differences in DHCP and DHCPv6 may not be reconciled, but may   not need to be, such as different ways to assign addresses by DUID in   DHCPv6, or the lack of a comparable option in both DHCP versions.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, with DHCPv6   deployment growing, there is an operational requirement to determine   best practice for DHCP server provision in dual-stack environments,   which may or may not imply additional protocol requirements.  The   principal choice is whether separate DHCP and DHCPv6 services should   be maintained by a site, or whether DHCPv6 should be extended to   carry IPv4 configuration settings for dual-stack nodes.   It can certainly be argued that until a site is completely dual-   stack, an IPv4 DHCP service will always be required (for example,   while there are still legacy printers, IP webcams, or other devices   that still configure via DHCPv4), and a single IPv6 transport DHCP   server offering configuration information for both protocols will   then not be sufficient.  In that case, IPv4 DHCP is required, and   thus thereChown, et al.                Informational                     [Page 10]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006   is a good rationale for focusing effort on how to combine the   information received from separate IPv4 DHCP and (stateless) DHCPv6   servers.   In theory, it should be relatively straightforward to write a   configuration manager that would accept a single configuration   specification from the service manager and distribute the correct   (and consistent) configurations to the DHCPv4 and DHCPv6 servers   (whether on the same host or not).  In this case, maintaining   coordinated configurations in two servers is an interface issue, not   a protocol issue.  The question then is whether the client has all   the information it needs to make reasonable choices.  We are aware of   one implementation of separate DHCPv4 and DHCPv6 clients that is   using a preference option for assisting client-side merging of the   received information.   Another issue for discussion is whether a combined DHCP service only   available over IPv6 transport is a desirable longer-term goal for   networks containing only dual-stack or IPv6-only nodes (or IPv4-only   nodes where DHCPv4 is not needed).  The transition to the long-term   position may easily take more than 10 years.   Upon reflection on the above observations, the dhc WG reached a   strong consensus to adopt the two-server approach (separate DHCP and   DHCPv6 servers), rather than have a combined single server returning   IPv4 information over IPv6.  The two servers may be co-located on a   single node and may have consistent configuration information   generated from a single asset database.   It should be noted that deployment experience of DHCPv6 is still in   its infancy; thus, a full understanding of the issues may only   develop over time, but we feel we have reached the best consensus   given the current status.  Future work is now required to determine   best practice for merging information from multiple servers,   including merger of lists of addresses where options carry such   information.   As a footnote, we note that this work has overlap with multihoming   and multi-interface configuration issues.  It is also interwoven with   the Detecting Network Attachment area, for example, where a node may   move from an IPv4-only network to a dual-stack network, or vice   versa.  Both aspects may be best abstracted for discussion and   progression in the respective IETF multi6, shim6, and dna WGs, in   parallel with the two-server progression in the dhc WG.Chown, et al.                Informational                     [Page 11]

RFC 4477                DHCP: Dual-Stack Issues                 May 20066.  Security Considerations   There are no security considerations in this problem statement per   se, as it does not propose a new protocol.7.  Acknowledgements   The authors thank the following people for input to this document:   Bernie Volz, AK Vijayabhaskar, Ted Lemon, Ralph Droms, Robert Elz,   Changming Liu, Margaret Wasserman, Dave Thaler, Mark Hollinger, and   Greg Daley.  The document may not necessarily fully reflect the views   of each of these individuals.   The authors would also like to thank colleagues on the 6NET project   for contributions to this document.8.  Informative References   [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., "Stateless Dynamic Host Configuration Protocol (DHCP)        Service for IPv6",RFC 3736, April 2004.   [6]  Lemon, T. and B. Sommerfeld, "Node-specific Client Identifiers        for Dynamic Host Configuration Protocol Version Four (DHCPv4)",RFC 4361, February 2006.Chown, et al.                Informational                     [Page 12]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006Authors' 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   UNINETT   Trondheim  NO 7465   Norway   EMail: venaas@uninett.no   Christian Strauf   Clausthal University of Technology   Erzstr. 51   Clausthal-Zellerfeld  D-38678   Germany   EMail: strauf@rz.tu-clausthal.deChown, et al.                Informational                     [Page 13]

RFC 4477                DHCP: Dual-Stack Issues                 May 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Chown, et al.                Informational                     [Page 14]