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Network Working Group                                           R. Droms
Internet-Draft                                             Cisco Systems
Expires: August 30, 2002                                       T. Narten
                                                         IBM Corporation
                                                                B. Aboba
                                                                Mar 2002

              Using DHCPv6 for DNS Configuration in Hosts

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
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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

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   This Internet-Draft will expire on August 30, 2002.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   An IPv6 device can configure its addresses and locate neighboring
   routers through stateless address autoconfiguration (RFC2462) and
   router discovery (RFC2461).  Most IPv6 devices will require
   information about DNS services to make use of the basic IPv6
   connectivity.  The current version of DHCPv6 already supports the
   features needed to provide a simple DNS configuration mechanism.  DNS
   configuration requires only a small subset of the DHCPv6 protocol and
   can be provided through relatively simple client and server

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1. Introduction

   An IPv6 device configures its IPv6 addresses through stateless
   address autoconfiguration [3] and/or through DHCP [5].  In addition,
   Neighbor Discovery [2] provides an IPv6 device with a set of
   neighboring routers it can use.  With addresses and a list of
   neighboring routers, a node has IP-level connectivity to the

   In many cases, packet-level connectivity is not enough.  Devices
   typically also need to be configured to be able to use DNS [1].
   Needed DNS configuration information can include the address of one
   or more DNS servers, the DNS name of the device itself, a list of DNS
   domain names to append to queries to make them fully qualified, etc.
   Specifically, Section 2 of the DNS Discovery team report [4], defines
   the information for DNS name resolution to be:

   o  One or more addresses of DNS servers.  If a list is obtained, a
      client need only rediscover DNS servers if all addresses in the
      list are unreachable.  However, if a list is obtained from a
      single point, such as one of the DNS servers, then a requirement
      exists that the list of servers be up-to-date and easily

   o  Domain name

   o  Search path.  It is currently common practice, for the search path
      to be computed by a device based on its domain name obtained.
      However, a DHCPv6 option [5] is being proposed in the DHC WG, and
      so search path configuration is likely to be a requirement in

   This document describes how to use DHCPv6 messages to obtain DNS
   configuration information without necessarily requiring a "stateful"
   DHCPv6 server to perform address assignment.

2. Terminology

   DHCP - Dynamic Host Configuration Protocol; unless otherwise
      qualified, "DHCP" refers to DHCP for IPv6

   DHCP option - A component of a DHCP message that carries
      configuration information for a DHCP client

   Configuration information - Information used by the host to configure
      its IP stack, DNS name resolver, etc.

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   DNS configuration information - Configuration information used
      specifically to configure a DNS name resolver, including the
      addresses of DNS servers, the host's domain name and a search list
      for name resolution

3. Summary of DHCP operation

   DHCPv6 is conceptually similar to DHCP for IPv4 [7].  DHCPv6 can
   provide configuration that includes address assignment, where the
   DHCP server selects an address for each DHCP client and maintains
   state about the assignment of that address to the client.

   DHCPv6 also provides configuration parameters to clients that do not
   need to have an address assigned.  This mode of operation is expected
   to be much more widely used than in IPv4 networks, because hosts can
   use stateless autoconfiguration to select IPv6 addresses rather than
   depending on a DHCP server or manual configuration.

   A host that has selected IPv6 addresses through stateless
   autoconfiguration uses a single message exchange with a DHCP server
   to obtain configuration information.  As in DHCPv4, the host sends a
   message to a well-known multicast address to contact a DHCP server.
   The server may return the same configuration information to every
   client, or it may be configured with policies to return customized
   information to groups of hosts or even individual hosts.  Each
   message exchange is independent of other message exchanges so that
   the server need not retain any dynamic state about each host.

   The DHCP specification allows for the coexistence of clients using
   DHCP for stateful address assignment and clients using DHCP for
   obtaining configuration information.  DHCP clients use different
   messages for configuration and for stateful address assignment, and a
   server that receives a stateful configuration request does not
   respond if it is not configured for stateful configuration.  Thus,
   even if DHCP messages are multicast to all DHCP servers, only those
   servers performing stateful address assignment will respond to
   requests for address assignment through DHCP.

   Because a DHCP server may be designed to respond only to Information-
   Request messages, it is possible to implement a DHCP server that is
   much less complex than a server that provides stateful address
   assignment.  A DHCP server that provides only DNS configuration
   information is easier to set up during deployment and requires fewer
   computational resources.  As discussed in Section 5.1, a DHCP server
   for DNS configuration information can be designed to be almost
   completely self-configuring.  Similarly, a DHCP client that uses only
   Information-Request messages to obtain other configuration

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   information would be much simpler than a client that uses DHCP for
   address assignment.

4. Client Behavior for DNS Configuration through DHCP

   When a host boots, it starts by generating a link-local address and
   then soliciting a Router Advertisement.  Use of DHCP by IPv6 hosts is
   controlled through two flags in Router Advertisements:

   M - if TRUE, the host invokes stateful autoconfiguration (DHCP) to
      configure additional addresses.  (It may have already configured
      some address through stateless address autoconfiguration.)

   O - if TRUE, the host obtains other configuration (e.g, non-address)
      information through DHCP

   If the 'M' bit is set TRUE, the host obtains its address through
   stateful address assignment using DHCP.  The host will also obtain
   other configuration through the same message exchange with the DHCP
   service.  Thus, if the 'M' bit is TRUE, the host will always obtain
   all of its configuration through DHCP.

   If the 'M' bit is set FALSE, the host uses stateless address
   autoconfiguration exclusively for address assignment.  If the 'O' bit
   is set TRUE in this case, the host will obtain its DNS configuration
   (and, perhaps, other configuration information) through DHCP.  In
   this case a host will perform the following steps:

   1.  Send a DHCP Information-Request message to a well-known multicast
       address requesting DNS configuration information.

   2.  Wait for the corresponding DHCP Reply message.

   3.  Extract the DNS configuration information from the Reply message
       and use it to configure local host behavior with regards to DNS

   If the host is using DHCP for DNS configuration, the DHCP
   specification allows the host to send DHCP Information-Request
   messages at times other than just once at boot time.  For example, a
   host may poll the DHCP service periodically to obtain a more up-to-
   date list of DNS servers.  Alternatively, if no servers in the host's
   current list of DNS servers is responding, the host may choose to
   send a DHCP Information-Request message to refresh its list of
   servers to find one that is available.  The point here is that DHCP
   already provides mechanisms to obtain DNS configuration, the
   mechanism involves a single request/response message, and can be
   invoked as needed to refresh a client's configuration information.

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5. DHCP service for DNS configuration

   To meet the requirements of a host using DHCP for DNS configuration,
   the DHCP service must return a Reply message in response to an
   Information-Request message received from the client.  The Reply
   message will contain just the options supplying DNS configuration

   The capability to carry DNS configuration information already exists
   in the DHCP specification.  As described in the previous section, a
   host uses the Information-Request message to request configuration
   information without stateful address assignment.  The DHCP
   specification includes three options for carrying DNS configuration

   Domain Name Server option: provides a list of Domain Name System that
      a client name resolver can use to access DNS services

   Domain Name option:        informs the DHCP client of its domain name

   Domain Search option:      provides a list of domain names a client
      can use to resolve DNS names

   The following sections describe several scenarios in which the
   requirements for DHCP service in DNS configuration can be met through
   different configurations of DHCP servers.

5.1 Minimal DHCP servers for DNS configuration

   One obvious way to provide DHCP service is to place a DHCP server on
   every router.  Except in the case of a network composed of a single
   link, every link has at least one router that is already providing
   router advertisement service in addition to forwarding packets.

   Adding DHCP service for DNS configuration to a router does not add
   unreasonable complexity in terms of implementation or performance,
   especially for a reduced functionality server that only supplies DNS
   configuration information.  Responding to an Information-Request
   message requires only the formation and transmission of a Reply
   message.  The contents of every Reply message is the same and
   contains just the DNS configuration information.

   Note that the market in IPv4 has already demonstrated the feasibility
   of this approach.  It is common now for off-the-shelf small routers
   to provide NAT & DHCP services, requiring little or no configuration
   by end users.  There is no reason to believe that this would be
   different in IPv6 with DHCPv6, especially considering that the IPv4
   implementations are fully functional DHCP servers doing stateful

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   address assignment.

   One question is how would the router determine what DNS configuration
   information it should advertise through DHCP.  The answer is it can
   determine this in any of several different ways.  In the strictly
   zeroconf manner, this information might come from an upstream ISP.
   But that ISP can provide the information to the router through DHCP,
   or whatever mechanism it uses to provide configuration information to
   the router.  (After all, the router needs, for example, a public IP
   address on the ISP side.)  Thus, there is no special problem here
   that wouldn't also exist in any protocol the provides DNS
   configuration, and there are a number of ways this could be done
   without requiring end-user configuration.

5.2 DHCP service provided by DNS servers

   Another deployment scenario is to provide DHCP server in a DNS
   server.  In this scenario, the DNS server is extended with the
   capability to receive Information-Request messages and respond with
   Reply messages.  DHCP would be used as the format to carry the
   configuration information obtained directly from the DNS server.

   In this scenario, a host uses the DHCP Information-Request message to
   poll for available DNS servers, and builds a list of DNS servers by
   merging the DNS server addresses in the responses.  As described in
   Section 4, a client can use the Information-Request message to manage
   its list of available DNS servers dynamically.

   Providing DHCP service with DNS servers requires that DHCP
   Information-Request messages be forwarded across multiple links from
   the host to the server.  The DHCPv6 specification defines a DHCP
   Relay Agent that receives DHCP messages sent to a link-local
   multicast address and forwards those messages to DHCP servers.  Relay
   Agents usually reside in routers and are therefore readily available
   on every link.

   DHCP Relay Agents must be deployed and managed so that they are
   configured with the addresses of DHCP servers.  To avoid the overhead
   of managing DHCP Relay Agents on every link, a host that has used
   stateless autoconfiguration to determine addresses with site or
   global scope can use the site local "All DHCP Servers" multicast
   address to send a DHCP Inform message to DHCP servers without using a
   local DHCP Relay Agent.

5.3 DHCP servers co-located with DNS servers

   A similar deployment scenario is to co-locate DHCP service with a DNS
   server.  The DHCP service returns information about its associated

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   DNS server, and only responds to messages from hosts if the DNS
   server is available.

5.4 DHCP servers providing both address assignment and configuration

   A site that is providing address assignment to some DHCP clients can
   use the same DHCP server to provide configuration information to
   hosts that use statless address autoconfiguration.  The DHCP
   specification allows a server to be configured with policies that
   allow it to provide address assignment to some clients while
   providing DNS configuration information to other clients.  Thus, the
   site need not have two different sets of DHCP servers for the two
   types of DHCP service.

6. Coexistence of DHCP servers

   A site may have a mix of hosts using DHCP in three different ways:
   address assignment; DNS configuration; configuration information
   including DNS configuration and other parameters.  The DHCP service
   Infrastructure may be mixed, with DNS-only servers in some routers or
   DNS servers as well as DHCP servers providing address assignment and
   other parameters.  Further, because of DHCP relay agent configuration
   or use of multicast, a message from any client may be delivered to
   any server.  From the point of view of the host, these servers co-
   exist as follows:

   Address assignment:     Hosts requesting address assignment use DHCP
      Solicit, Request, Renew, and Rebind messages.  The DHCP
      specification requires that servers that do not assign addresses -
      in particular, DHCP servers doing just DNS configuration - ignore
      these messages.  Routers that have both a relay agent and a DHCP
      DNS configuration server forward address assignment messages to
      full DHCP servers.  These servers then respond, so clients asking
      for address assignment receive responses only from configuring

   Full configuration:     Hosts asking for configuration information
      specify a list of the information of interest.  Any server with
      corresponding information may respond.  Servers providing only DNS
      configuration won't have all of the requested information and may
      choose not to respond.  If a server providing only DNS
      configuration does respond, the host has the option to ignore the
      reply and choose another response from a DHCP server that supplies
      more complete information.

   DNS-only configuration: A host requesting only DNS configuration will
      identify those DNS configuration options in the Information-

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      Request message it sends.  Any server configured to respond to
      this host will do so.  If the server sends back additional
      information, clients may choose to ignore the extra information.

7. Open Issues

   There are three open issues for the DHCPv6 specification raised by
   this document:

   Site-scoped multicast:  The most recent DHCPv6 specification only
      allows a client to send an Information-Request message to the
      link-scoped "All_DHCP_Agents" multicsat address or to a server's
      unicast address.  To allow for discovery of DHCP servers without
      the use of relay agents, a client could use the site-scoped
      "All_DHCP_Servers" multicast address.

   DNS and DHCP on one computer: A DNS server providing DNS
      configuration through DHCP and a DHCP server will experience a
      conflict in the use of the well-known DHCP port numbers.

   Authentication:               DHCP includes a framework for
      authentication of DHCP messages.  Use of authentication with DNS
      configuration will be important because of the potentical spoofing
      attack that can be mounted through the DNS search path.  It may be
      possible to improve on DHCP authentication through the use of

8. Conclusion and Recommendations

   DHCP can be used for DNS configuration of hosts.  The stateless
   version of DHCP, in which a host can obtain DNS configuration from a
   server with a two message exchange, imposes minimal implementation
   overhead.  The configuration of a DHCP server providing DNS
   configuration information can be automated to minimize deployment and
   management overhead.

   Using an existing protocol has several advantages over designing and
   deploying a special-purpose protocol for DNS configuration of hosts.
   Using DHCP will minimize deployment complexity, allowing a site to
   reuse a single protocol infrastructure for host configuration that
   will likely be deployed at most sites.  Sites with both protocols
   deployed will have to carefully coordinate the administration of the
   two protocols to avoid giving conflicting information to hosts.  If a
   second protocol is available, sites will have to decide which to
   deploy and how to upgrade to DHCP if necessary.  Specifying a new
   protocol for DNS configuration will require analysis of interactions

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   between the new protocol and DHCP, as well as careful specification
   of client behavior in the case both sources of DNS configuration
   information are available.

   Almost all of the functions required for using DHCP for DNS
   configuration are already specified in the latest version of the DHCP
   document [5].  The open issues are discussed in Section 7 of this

   Because DHCP can meet the requirements for DNS configuration of
   hosts,, and because the deployment and management of DHCP for this
   purpose can be accomplished with minimum overhead, we recommend that
   DHCP be adopted as standard mechanism for DNS configuration.  This
   recommendation should be documented, along with recommended
   implementations and deployment strategies, in an Applicability
   Statement or BCP document.


   [1]  Mockapetris, P., "Domain names - concepts and facilities", STD
        13, RFC 1034, November 1987.

   [2]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
        IP Version 6 (IPv6)", RFC 2461, December 1998.

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

   [4]  Aboba, B., Bound, J., Deering, S., Guttman, E., HAGINO, J.,
        Hinden, R., JINMEI, T., Onoe, A., Soliman, H. and D. Thaler,
        "Analysis of DNS Server Discovery Mechanisms for IPv6", draft-
        ietf-ipngwg-dns-discovery-analysis (work in progress), July

   [5]  Bound, J., Carney, M., Perkins, C. and R. Droms (ed.), "Dynamic
        Host Configuration Protocol for IPv6 (DHCPv6)", draft-ietf-dhc-
        dhcpv6 (work in progress), February 2002.

   [6]  Hagino, J. and D. Thaler, "IPv6 Stateless DNS Discovery", draft-
        ietf-ipngwg-dns-discovery (work in progress), July 2001.

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

   [8]  Bound, J., Carney, M., Perkins, C., Lemon, T., Volz, B. and R.
        Droms, "The DNS Configuration options for DHCPv6", draft-ietf-
        dhc-dhcpv6-opt-dnsconfig (work in progress), Jan 2002.

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Authors' Addresses

   Ralph Droms
   Cisco Systems
   250 Apollo Drive
   Chelmsford, MA  01824

   Phone: +1 978 497 4733
   EMail: rdroms@cisco.com

   Thomas Narten
   IBM Corporation
   P.O. Box 12195
   Research Triangle Park, NC  27709-2195

   Phone: +1 919 254 7798
   EMail: narten@us.ibm.com

   Bernard Aboba
   One Microsoft Way
   Redmond, WA  98052

   EMail: aboba@internaut.com

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