Network Working Group                                           T. Lemon
Internet-Draft                                             Nominum, Inc.
Intended status: Standards Track                          April 13, 2013
Expires: October 15, 2013

    Customizing DHCP Configuration on the Basis of Network Topology


   DHCP servers have evolved over the years to provide significant
   functionality beyond that which is described in the DHCP base
   specifications.  One aspect of this functionality is support for
   context-specific configuration information.  This memo describes some
   such features and makes recommendations as to how they can be used.

Status of This Memo

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   This Internet-Draft will expire on October 15, 2013.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Locality  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Simple Subnetted Network  . . . . . . . . . . . . . . . . . .   5
   4.  Regional Configuration Example  . . . . . . . . . . . . . . .   6
   5.  Dynamic Lookup  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The DHCPv4 [RFC2131] and DHCPv6 [RFC3315] protocol specifications
   describe how addresses can be allocated to clients based on network
   topology information provided by the DHCP relay infrastructure.
   Address allocation decisions are integral to the allocation of
   addresses and prefixes in DHCP.

   The DHCP protocol also describes mechanisms for provisioning devices
   with additional configuration information; for example, DNS [RFC1034]
   server addresses, default DNS search domains, and similar

   Although it was the intent of the authors of these specifications
   that DHCP servers would provision devices with configuration
   information appropriate to each device's location on the network,
   this practice was never documented, much less described in detail.

   Existing DHCP server implementations do in fact provide such
   capabilities; the goal of this document is to describe those
   capabilities for the benefit both of operators and of protocol
   designers who may wish to use DHCP as a means for configuring their
   own servies, but may not be aware of the capabilities provided by
   modern DHCP servers.

2.  Locality

   Figure 1 illustrates a simple hierarchy of network links with Link D
   serving as a backbone to which the DHCP server is attached.

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           Link A                   Link B
        |===+===========|    |===========+======|
            |                            |
            |                            |
        +---+---+                    +---+---+
        | relay |                    | relay |
        |   A   |                    |   B   |
        +---+---+                    +---+---+
            |                            |
            |       Link C               |
                  +----+---+        +--------+
                  | router |        |  DHCP  |
                  |    A   |        | Server |
                  +----+---+        +----+---+
                       |                 |
                       |                 |
                       |   Link D        |
                  | router |
                  |    B   |
            |       Link E               |
            |                            |
        +---+---+                    +---+---+
        | relay |                    | relay |
        |   C   |                    |   D   |
        +---+---+                    +---+---+
            |                            |
            |                            |
        |===+===========|    |===========+======|
           Link F                   Link G

   Figure 1

   This diagram allows us to represent a variety of different network
   configurations and illustrate how existing DHCP servers can provide
   configuration information customized to the particular location from
   which a client is making its request.

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   It's important to understand the background of how DHCP works when
   considering this diagram.  DHCP clients are assumed not to have
   routable IP addresses when they are attempting to obtain
   configuration information.

   The reason for making this assumption is that one of the functions of
   DHCP is to bootstrap the DHCP client's IP address configuration; if
   the client does not yet have an IP address configuration, it cannot
   route packets to an off-link DHCP server, and so some kind of relay
   mechanism is required.

   The details of how this works are different between DHCPv4 and
   DHCPv6, but the essence is the same: whether or not the client
   actually has an IP configuration, it generally communicates with the
   DHCP server by sending its requests to a DHCP relay agent on the
   local link; this relay agent, which has a routable IP address, then
   forwards the DHCP requests to the DHCP server.  In some cases in
   DHCPv4, when a DHCP client has a routable IPv4 adddress.

   In either case, the DHCP server is able to obtain an IP address that
   it knows is on-link for the link to which the DHCP client is
   connected: either the DHCPv4 client's routable IPv4 address, or the
   relay agent's IP address on the link to which the client is

   DHCPv6 also has support for more finely grained link identification,
   using Lightweight DHCPv6 Relay Agents [RFC6221] (LDRA).  In this
   case, in addition to receiving an IPv6 address that is on-link for
   the link to which the client is connected, the DHCPv6 server also
   receives an Interface Identifier option from the relay agent that can
   be used to more precisely identify the client's location on the

   What this means in practice is that the DHCP server in all cases has
   sufficient information to pinpoint, at the very least, the layer 3
   link to which the client is connected, and in some cases which layer
   2 link the client is connected to, when the layer 3 link is
   aggregated out of multiple layer 2 links.

   In all cases, then, the DHCP server will have a link-identifying IP
   address, and in some cases it may also have a link-specific
   identifier.  It should be noted that there is no guarantee that the
   link-specific identifier will be unique outside the scope of the
   link-identifying IP address.

   It is also possible for link-specific identifiers to be nested, so
   that the actual identifier that identifies the link is an aggregate
   of two or more link-specific identifiers sent by a set of LDRAs in a

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   chain; in general this functions exactly as if a single identifier
   were received from a single LDRA, so we do not treat it specially in
   the discussion below, but sites that use chained LDRA configurations
   will need to be aware of this when configuring their DHCP servers.

   Routable IP address: an IP address with a scope of use wider than the
   local link.

   So let's examine the implications of this in terms of how a DHCP
   server can deliver targeted supplemental configuration information to
   DHCP clients.

3.  Simple Subnetted Network

   Consider Figure 1 in the context of a simple subnetted network.  In
   this network, there are four leaf subnets: links A, B, F and G, on
   which DHCP clients will be configured.  In a simple network like
   this, there may be no need for link-specific configuration in DHCPv6,
   since local routing information is delivered through router

   However, in IPv4, it is very typical to configure the default route
   using DHCP; in this case, the default route will be different on each
   link.  In order to accomplish this, the DHCP server will need a link-
   specific configuration for the default route.

   To illustrate, we will use an example from a hypothetical DHCP server
   that uses a simple JSON notation for configuration.  Although we know
   of no DHCP server that uses this specific syntax, every commercial
   DHCP server provides similar functionality.

     {"": {"options": {"routers": [""]}
              "on-link": ["a"]}}
      "": {"options": {"routers": [""}}
              "on-link": ["b"]}
      "": {"options": {"routers": [""}}
              "on-link": ["f"]}
      "": {"options": {"routers": [""}}
              "on-link": ["g"]}}

   Figure 2

   In figure 2, we see a configuration example for this scenario: a set
   of prefixes, each of which has a set of options and a list of links
   for which it is on-link.  We have defined one option for each prefix:

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   a routers option.  This option contains a list of values; each list
   only has one value, and that value is the IP address of the router
   specific to the prefix.

   When the DHCP server receives a request, it searches the list of
   prefixes for one that encloses the link-identifying IP address
   provided by the client or relay agent.  The DHCP server then examines
   the options list associated with that prefix and returns those
   options to the client.

   So for example a client connected to link A in the example would have
   a link-identifying IP address within the prefix, so the
   DHCP server would match it to that prefix.  Based on the
   configuration, the DHCP server would then return a routers option
   containing a single IP address:  A client on link F would
   have a link-identifying address in the prefix, and would
   receive a routers option containing the IP address

4.  Regional Configuration Example

   In this example, link C is a regional backbone for an ISP.  Link E is
   also a regional backbone for that ISP.  Relays A, B, C and D are PE
   routers, and Links A, B, F and G are actually link aggregators with
   individual layer 2 circuits to each customer-\u002Dfor example, the
   relays might be DSLAMs or cable head-end systems.  At each customer
   site we assume there is a single CPE device attached to the link.

   We further assume that links A, B, F and G are each addressed by a
   single prefix, although it would be equally valid for each CPE device
   to be numbered on a separate prefix.

   In a real-world deployment, there would likely be many more than two
   PE routers connected to each regional backbone; we have kept the
   number small for simplicity.

   In this example, the goal is to configure all the devices within a
   region with server addresses local to that region, so that service
   traffic does not have to be routed between regions unnecessarily.

     {"2001:DB8:0:0::/40":   {"on-link": ["A"]}}
      "2001:DB8:100:0::/40": {"on-link": ["B"]}
      "2001:DB8:200:0::/40": {"on-link": ["F"]}
      "2001:DB8:300:0::/40": {"on-link": ["G"]}}

     {"A": {"region": "omashu"},
      "B": {"region": "omashu"},

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      "F": {"region": "gaoling"},
      "G": {"region": "gaoling"}}}

     {"omashu": {"options": {"sip-servers": [""],
                 "dns-servers": ["",
      "gaoling": {"options": {"sip-servers": [""],
                  "dns-servers": ["",

   Figure 3

   In this example, when a request comes in to the DHCP server with a
   link-identifying IP address in the 2001:DB8:0:0::/40 prefix, it is
   identified as being on link A.  The DHCP server then looks on the
   list of links to see what region the client is in.  Link A is
   identified as being in omashu.  The DHCP server then looks up omashu
   in the set of regions, and discovers a list of region-specific

   The DHCP server then resolves the domain names listed in the options
   and sends a sip-server option containing the IP addresses that the
   resolver returned for, and a dns-server option
   containing the IP addresses returned by the resolver for and

   Similarly, if the DHCP server receives a request from a DHCP client
   where the link-identifying IP address is contained by the prefix
   2001:DB8:300:0::/40, then the DHCP server identifies the client as
   being connected to link G.  The DHCP server then identifies link G as
   being in the gaoling region, and returns the sip-servers and dns-
   servers options specific to that region.

   As with the previous example, the exact configuration syntax and
   structure shown above does not precisely match what existing DHCP
   servers do, but the behavior illustrated in this example can be
   accomplished with all existing commercial DHCP servers.

5.  Dynamic Lookup

   In the Regional example, the configuration listed several domain
   names as values for the sip-servers and dns-servers options.  The
   wire format of both of these options contains one or more IPv6
   addresses-\u002Dthere is no way to return a domain name to the

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   This was understood to be an issue when the original DHCP protocol
   was defined, and historical implementations even from the very early
   days would accept domain names and resolve them.  Some early DHCP
   implementations, particularly those based on earlier BOOTP
   implementations, had very limited capacity for reconfiguration.

   However, all modern commercial DHCP servers handle name resolution by
   querying the resolver each time a DHCP packet comes in.  This means
   that if DHCP servers and DNS servers are managed by different
   administrative entities, there is no need for the administrators of
   the DHCP servers and DNS servers to communicate when changes are
   made.  When changes are made to the DNS server, these changes are
   immediately and automatically adopted by the DHCP server.  Similarly,
   when DHCP server configurations change, DNS server administrators
   need not be aware of this.

6.  Acknowledgments

   Thanks to Dave Thaler for suggesting that even though "everybody
   knows" how DHCP servers are deployed in the real world, it might be
   worthwhile to have an IETF document that explains what everybody
   knows, because in reality not everybody is an expert in how DHCP
   servers are administered.

7.  Security Considerations

   This document explaine existing practice with respect to the use of
   Dynamic Host Configuration Protocol [RFC2131] and Dynamic Host
   Configuration Protocol Version 6 [RFC3315].  The security
   considerations for these protocols are described in their
   specifications and in related documents that extend these protocols.
   This document introduces no new functionality, and hence no new
   security considerations.

8.  IANA Considerations

   The IANA is hereby absolved of any requirement to take any action in
   relation to this document.

9.  References

9.1.  Normative References

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

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   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.

9.2.  Informative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC6221]  Miles, D., Ooghe, S., Dec, W., Krishnan, S., and A.
              Kavanagh, "Lightweight DHCPv6 Relay Agent", RFC 6221, May

Author's Address

   Ted Lemon
   Nominum, Inc.
   2000 Seaport Blvd
   Redwood City, CA  94063

   Phone: +1-650-381-6000

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