Dynamic Host Congiguration T. Chown
Internet-Draft University of Southampton
Expires: January 17, 2005 S. Venaas
UNINETT
C. Strauf
Technical University of Clausthal
July 19, 2004
DHCP: IPv4 and IPv6 Dual-Stack Issues
draft-ietf-dhc-dual-stack-01
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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 [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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the most important aspect of which is how to handle potential
problems in clients processing configuration information received
from DHCPv4 and DHCPv6 servers.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Configuration scenarios . . . . . . . . . . . . . . . . . . . 3
3. Dual-stack issues . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Handling multiple responses . . . . . . . . . . . . . . . 4
3.2 Different administrative management . . . . . . . . . . . 4
3.3 Multiple interfaces . . . . . . . . . . . . . . . . . . . 5
3.4 DNS load balancing . . . . . . . . . . . . . . . . . . . . 5
3.5 DNS search path issues . . . . . . . . . . . . . . . . . . 5
3.6 Protocol startup sequence . . . . . . . . . . . . . . . . 5
3.7 DHCP option variations . . . . . . . . . . . . . . . . . . 5
3.8 Security issues . . . . . . . . . . . . . . . . . . . . . 6
4. Potential solutions . . . . . . . . . . . . . . . . . . . . . 6
4.1 Separate DHCP servers . . . . . . . . . . . . . . . . . . 6
4.2 Single DHCPv6 server . . . . . . . . . . . . . . . . . . . 7
4.3 Administrative and other areas . . . . . . . . . . . . . . 8
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
8. Informative References . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . 12
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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 [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, 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
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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 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.
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 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 Different 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
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wireless environment. This poses problems for consistency of data
offered by either DHCP version.
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? DHCPv6 introduces the idea of a DHCP
Unique Identifier (DUID) which does not yet exist for DHCP; some
effort is being made to retrofit the concept to DHCPv4 [6].
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.
3.4 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.
3.5 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.
3.6 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?
3.7 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 different.
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There may be some sites that would 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 mix of IPv4 and IPv6 addresses.
3.8 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.
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.
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. The information served
from the DHCP servers could be generated from a single database
instance for consistency.
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
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query. 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, e.g. 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 provided 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 (e.g. 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, e.g. "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 values returned from either DHCP are consistent (a principle
which extends if other methods are used, e.g. NIS or 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 predicatability comes
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
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to configure interfaces with addresses, and get other configuration,
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 should 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, e.g. going away from using MAC addresses for per-host
address assignment but instead using DUIDs/IAIDs, etc (although there
is ongoing work to provide 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 receiving IPv4
configuration settings that it does not want or be able to
meaningfully handle.
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 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
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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.
It can certainly be argued that until a site is completely
dual-stack, an IPv4 DHCP service will always be required (e.g. while
there are still legacy printers, IP webcams or devices which 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, there 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.
This work has overlap with multihoming and multi-interface
configuration issues. It is also interwoven with the Detecting
Network Attachment area, e.g. where a node may move from an
IPv4-only network to a dual-stack network, or vice versa. Both
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aspects may be best abstracted for discussion in the IETF multi6 and
dna WGs for discussion.
The authors also noted that the original working title of the draft
was not as appropriate as it might be; we have thus renamed it "DHCP:
IPv4 and IPv6 Dual-Stack Issues". We are open to further renaming if
comments warrant it.
There is not a full consensus in the DHC WG on solutions for the DHCP
dual-stack configuration issue at present. This text is intended to
provoke discussion towards a consensus, and it may then document that
consensus and the reasons behind it for future reference.
6. 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 draft:
Bernie Volz, AK Vijayabhaskar, Ted Lemon, Ralph Droms, Robert Elz,
Changming Liu, Margaret Wasserman and Greg Daley. The draft may not
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 draft.
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., "Node-Specific Client Identifiers for DHCPv4",
draft-ietf-dhc-3315id-for-v4-02 (work in progress), February
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2004.
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
UNINETT
Trondheim NO 7465
Norway
EMail: venaas@uninett.no
Christian Strauf
Technical University of Clausthal
Erzstr. 51
Clausthal-Zellerfeld D-38678
Germany
EMail: strauf@rz.tu-clausthal.de
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