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rfc:rfc3456

Network Working Group B. Patel Request for Comments: 3456 Intel Corp Category: Standards Track B. Aboba

                                                             Microsoft
                                                              S. Kelly
                                                             Airespace
                                                              V. Gupta
                                                Sun Microsystems, Inc.
                                                          January 2003
            Dynamic Host Configuration Protocol (DHCPv4)
                 Configuration of IPsec Tunnel Mode

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This memo explores the requirements for host configuration in IPsec
 tunnel mode, and describes how the Dynamic Host Configuration
 Protocol (DHCPv4) may be leveraged for configuration.  In many remote
 access scenarios, a mechanism for making the remote host appear to be
 present on the local corporate network is quite useful.  This may be
 accomplished by assigning the host a "virtual" address from the
 corporate network, and then tunneling traffic via IPsec from the
 host's ISP-assigned address to the corporate security gateway.  In
 IPv4, DHCP provides for such remote host configuration.

Patel, et. al. Standards Track [Page 1] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

Table of Contents

 1. Introduction...................................................  2
   1.1 Terminology.................................................  2
   1.2 Requirements Language.......................................  3
 2. IPsec tunnel mode configuration requirements...................  3
   2.1 DHCP configuration evaluation...............................  3
   2.2 Summary.....................................................  4
 3. Scenario overview..............................................  4
   3.1 Configuration walk-through..................................  5
 4. Detailed description...........................................  6
   4.1 DHCPDISCOVER message processing.............................  6
   4.2 DHCP Relay behavior.........................................  9
   4.3 DHCPREQUEST message processing.............................. 10
   4.4 DHCPACK message processing.................................. 10
   4.5 Configuration policy........................................ 11
 5. Security Considerations........................................ 11
 6. IANA Considerations............................................ 12
 7. Intellectual Property Statement................................ 12
 8. References..................................................... 13
   8.1 Normative References........................................ 13
   8.2 Informative References...................................... 13
 9. Acknowledgments................................................ 14
 Appendix - IKECFG evaluation...................................... 15
 Authors' Addresses................................................ 17
 Full Copyright Statement ......................................... 18

1. Introduction

 In many remote access scenarios, a mechanism for making the remote
 host appear to be present on the local corporate network is quite
 useful.  This may be accomplished by assigning the host a "virtual"
 address from the corporate network, and then tunneling traffic via
 IPsec from the host's ISP-assigned address to the corporate security
 gateway.  In IPv4, Dynamic Host Configuration Protocol (DHCP) [3]
 provides for such remote host configuration. This document explores
 the requirements for host configuration in IPsec tunnel mode, and
 describes how DHCPv4 may be leveraged for configuration.

1.1. Terminology

 This document uses the following terms:
 DHCP client
       A DHCP client or "client" is an Internet host using DHCP to
       obtain configuration parameters such as a network address.

Patel, et. al. Standards Track [Page 2] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 DHCP server
       A DHCP server or "server" is an Internet host that returns
       configuration parameters to DHCP clients.

1.2. Requirements language

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in [1].

2. IPsec tunnel mode configuration requirements

 As described in [21], the configuration requirements of a host with
 an IPsec tunnel mode interface include the need to obtain an IPv4
 address and other configuration parameters appropriate to the class
 of host.  In addition to meeting the basic requirements [21], the
 following additional capabilities may be desirable:
    a. integration with existing IPv4 address management facilities
    b. support for address pool management
    c. reconfiguration when required
    d. support for fail-over
    e. maintaining security and simplicity in the IKE implementation.
    f. authentication where required

2.1. DHCP configuration evaluation

 Leveraging DHCP for configuration of IPsec tunnel mode meets the
 basic requirements described in [21].  It also provides the
 additional capabilities described above.
 Basic configuration
       In IPv4, leveraging DHCPv4 [3] for the configuration of IPsec
       tunnel mode satisfies the basic requirements described in [21].
       Since the required configuration parameters described in [21]
       are a subset of those already supported in DHCPv4 options [4],
       no new DHCPv4 options are required, and no modifications to
       DHCPv4 [3] are required.
 Address management integration
       Since DHCPv4 is widely deployed for address management today,
       reuse of DHCPv4 for IPsec tunnel mode address management
       enables compatibility and integration with existing addressing
       implementations and IPv4 address management software.

Patel, et. al. Standards Track [Page 3] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 Address pool management
       As described in [18], DHCPv4 implementations support
       conditional behavior so that the address and configuration
       parameters assigned can be dependent on parameters included in
       the DHCPDISCOVER.  This makes it possible for the security
       gateway to ensure that the remote host receives an IP address
       assignment from the appropriate address pool, such as via the
       User Class option, described in [16].
 Reconfiguration
       DHCP supports the concept of configuration leases, and there is
       a proposal for handling forced reconfiguration [14].
 Fail-over support
       When leveraging DHCPv4, configuration and addressing state is
       kept on the DHCP server, not within the IKE implementation.  As
       a result, the loss of a tunnel server does not result in the
       loss of configuration and addressing state, thus making it
       easier to support fail-over [12].
 Security and simplicity
       Leveraging DHCPv4 also makes it easier to maintain security in
       the IKE implementation since no IKE modifications are required
       to support configuration.
 Authentication
       Where DHCPv4 authentication [5] is required, this can be
       supported on an IPsec tunnel mode interface as it would be on
       any other interface.

2.2. Summary

 As described, DHCPv4 [3] meets the IPsec tunnel mode configuration
 requirements [21], as well as providing additional capabilities.  As
 described in the Appendix, IKECFG [13] does not meet the basic
 requirements, nor does it provide the additional capabilities.  As a
 result, DHCPv4 is the superior alternative for IPsec tunnel mode
 configuration.

3. Scenario overview

 IPsec [2], [6]-[9] is a protocol suite defined to secure
 communication at the network layer between communicating peers.
 Among many applications enabled by IPsec, a useful application is to
 connect a remote host to a corporate intranet via a security gateway,
 using IPsec tunnel mode.  This host is then configured in such a
 manner so as to provide it with a virtual presence on the internal
 network.  This is accomplished in the following manner:

Patel, et. al. Standards Track [Page 4] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 A remote host on the Internet will connect to the security gateway
 and then establish an IPsec tunnel to it.  The remote host then
 interacts via the IPsec tunnel with a DHCPv4 server which provides
 the remote host with an address from the corporate network address
 space.  The remote host subsequently uses this as the source address
 for all interactions with corporate resources.  Note that this
 implies that the corporate security gateway continues to recognize
 the host's original, routable IP address as the tunnel endpoint.  The
 virtual identity assumed by the remote host when using the assigned
 address appears to the corporate network as though it were situated
 behind a security gateway bearing the original routable IP address.
 All the traffic between the remote host and the intranet will be
 carried over the IPsec tunnel via the security gateway as shown
 below:
                                        corporate net
  +------------------+                      |
  |    externally    |        +--------+    |   !~~~~~~~~~~!
  |+-------+ visible |        |        |    |   ! rmt host !
  ||virtual| host    |        |security|    |---! virtual  !
  || host  |         |--------|gateway/|    |   ! presence !
  ||       |<================>|  DHCP  |----|   !~~~~~~~~~~!
  |+-------+         |--------| Relay  |    |
  +------------------+   ^    +--------+    |   +--------+
                         |                  |---| DHCPv4 |
                       IPsec tunnel         |   | server |
                       with encapsulated    |   +--------+
                       traffic inside
 This scenario assumes that the remote host already has Internet
 connectivity and the host Internet interface is appropriately
 configured.  The mechanisms for configuration of the remote host's
 address for the Internet interface are well defined; i.e., PPP IP
 control protocol (IPCP), described in [10], DHCPv4, described in [3],
 and static addressing.  The mechanisms for auto-configuration of the
 intranet are also standardized.  It is also assumed that the remote
 host has knowledge of the location of the security gateway.  This can
 be accomplished via DNS, using either A, KX [23], or SRV [24]
 records.
 A typical configuration of the remote host in this application would
 use two addresses: 1) an interface to connect to the Internet
 (Internet interface), and 2) a virtual interface to connect to the
 intranet (intranet interface).  The IP address of the Internet and
 intranet interfaces are used in the outer and inner headers of the
 IPsec tunnel mode packet, respectively.

Patel, et. al. Standards Track [Page 5] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

3.1. Configuration walk-through

 The configuration of the intranet interface of the IPsec tunnel mode
 host is accomplished in the following steps:
 a. The remote host establishes an IKE security association with the
    security gateway in a main mode or aggressive mode exchange.  This
    IKE SA then serves to secure additional quick mode IPsec SAs.
 b. The remote host establishes a DHCP SA with the IPsec tunnel mode
    server in a quick mode exchange.  The DHCP SA is an IPsec tunnel
    mode SA established to protect initial DHCPv4 traffic between the
    security gateway and the remote host.  The DHCP SA MUST only be
    used for DHCP traffic.  The details of how this SA is set up are
    described in Section 4.1.
 c. DHCP messages are sent back and forth between the remote host and
    the DHCPv4 server.  The traffic is protected between the remote
    host and the security gateway using the DHCP SA established in
    step b.  After the DHCP conversation completes, the remote host's
    intranet interface obtains an IP address as well as other
    configuration parameters.
 d. The remote host MAY request deletion of the DHCP SA since future
    DHCP messages will be carried over a new IPsec tunnel.
    Alternatively, the remote host and the security gateway MAY
    continue to use the same SA for all subsequent traffic by adding
    temporary SPD selectors in the same manner as is provided for name
    ID types in [2].
 e. If a new IPsec tunnel is required, the remote host establishes a
    tunnel mode SA to the security gateway in a quick mode exchange.
    In this case, the new address assigned via DHCPv4 SHOULD be used
    in the quick mode ID.
 At the end of the last step, the remote host is ready to communicate
 with the intranet using an IPsec tunnel.  All the IP traffic
 (including future DHCPv4 messages) between the remote host and the
 intranet are now tunneled over this IPsec tunnel mode SA.
 Since the security parameters used for different SAs are based on the
 unique requirements of the remote host and the security gateway, they
 are not described in this document.  The mechanisms described here
 work best when the VPN is implemented using a virtual interface.

Patel, et. al. Standards Track [Page 6] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

4. Detailed description

 This section provides details relating to the messages exchanged
 during the setup and teardown of the DHCP SAs.

4.1. DHCPDISCOVER message processing

 The events begin with the remote host intranet interface generating a
 DHCPDISCOVER message.  Details are described below:
 FIELD      OCTETS       DESCRIPTION
 op            1  Message op code / message type.
                  1 = BOOTREQUEST, 2 = BOOTREPLY
 htype         1  Hardware address type.  Set to value 31.
                  signifying an IPsec tunnel mode virtual interface.
 hlen          1  Hardware address length
 hops          1  Client sets to zero, optionally used by relay agents
                  when booting via a relay agent.
 xid           4  Transaction ID, a random number chosen by the
                  client, used by the client and server to associate
                  messages and responses between a client and a
                  server.
 secs          2  Filled in by client, seconds elapsed since client
                  began address acquisition or renewal process.
 flags         2  Flags.  Broadcast bit MUST be set to zero.
 ciaddr        4  Client IP address; only filled in if client is in
                  BOUND, RENEW or REBINDING state.
 yiaddr        4  'your' (client) IP address.
 siaddr        4  IP address of next server to use in bootstrap;
                  returned in DHCPOFFER, DHCPACK by server.
 giaddr        4  Security gateway interface IPv4 address, used in
                  booting via a relay agent.
 chaddr       16  Client hardware address.  Should be unique.
 sname        64  Optional server host name, null terminated string.
 file        128  Boot file name, null terminated string; "generic"
                  name or null in DHCPDISCOVER, fully qualified
                  directory-path name in DHCPOFFER.
 options     var  Optional parameters field.
       Table 1:  Description of fields in the DHCP message
 The htype value is set to the value 31, signifying a virtual IPsec
 tunnel mode interface, in order to enable the DHCP server to
 differentiate VPN from non-VPN requests.  The chaddr field of the
 DHCPDISCOVER MUST include an identifier unique to the virtual subnet.
 The client MUST use the same chaddr field in all subsequent messages
 within the same DHCPv4 exchange.  In addition, the chaddr SHOULD be

Patel, et. al. Standards Track [Page 7] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 persistent between reboots so that the DHCP server will be able to
 re-assign the same address if desired.
 The hlen and chaddr fields SHOULD be determined as follows:
 a. If one or more LAN interfaces are available, the hlen and chaddr
    fields SHOULD be determined from the active LAN interface with the
    lowest interface number.  If no active LAN interface is available,
    then the parameters SHOULD be determined from the LAN interface
    with the lowest interface number.  This enables the chaddr to be
    persistent between reboots, as long as the LAN interface hardware
    is not removed.
 b. If there is no LAN interface, the chaddr field SHOULD be
    determined by concatenating x'4000', the IPv4 address of the
    interface supplying network connectivity, and an additional octet.
    The x'4000' value indicates a locally administered unicast MAC
    address, thus guaranteeing that the constructed chaddr value will
    not conflict with a globally assigned value.
    The additional octet (which MAY represent an interface number)
    SHOULD be persistent between reboots, so that the chaddr value
    will be persistent across reboots if the assigned IPv4 address
    remains consistent.
 If the above prescription is followed, then the chaddr will always be
 unique on the virtual subnet provided that the remote host only
 brings up a single tunnel to the security gateway.  Where a LAN
 interface is available, the chaddr will be globally unique.  When a
 non-LAN interface is available and a unique Internet address is
 assigned to the remote host, the chaddr will also be globally unique.
 Where a private IP address [22] is assigned to a non-LAN interface,
 it will not be globally unique.  However, in this case packets will
 not be routed back and forth between the remote host and the security
 gateway unless the external network and corporate network have a
 consistent addressing plan.  In this case the private IP address
 assigned to the remote host will be unique on the virtual subnet.
 For use in DHCPv4 configuration of IPsec tunnel mode, the client-
 identifier option MUST be included, MUST be unique within the virtual
 subnet and SHOULD be persistent across reboots.  Possibilities
 include:
 a. The htype/chaddr combination.  If assigned as described above,
    this will be unique on the virtual subnet.  It will be persistent
    across reboots for a LAN interface.  If a non-LAN interface is
    used, it may not be persistent across reboots if the assigned IP
    address changes.

Patel, et. al. Standards Track [Page 8] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 b. The machine FQDN concatenated with an interface number.  Assuming
    that the machine FQDN does not conflict with that of another
    machine, this will be unique on the virtual subnet as well as
    persistent across reboots.
 c. The user NAI concatenated with an interface number.  Assuming that
    the user is only connected to the VPN at one location, this will
    be unique on the subnet as well as persistent across reboots.
 In order to deliver the DHCPDISCOVER packet from the intranet
 interface to the security gateway, an IKE Phase 1 SA is established
 between the Internet interface and the security gateway.  A phase 2
 (quick mode) DHCP SA tunnel mode SA is then established.  The key
 lifetime for the DHCP SA SHOULD be on the order of minutes since it
 will only be temporary.  The remote host SHOULD  use an IDci payload
 of 0.0.0.0/UDP/port 68 in the quick mode exchange.  The security
 gateway will use an IDcr payload of its own Internet address/UDP/port
 67.  The DHCP SA is established as a tunnel mode SA with filters set
 as follows:
    From remote host to security gateway: Any to Any, destination: UDP
    port 67
    From security gateway to remote host: Any to Any, destination: UDP
    port 68
 Note that these filters will work not only for a client without
 configuration, but also with a client that has previously obtained a
 configuration lease, and is attempting to renew it.  In the latter
 case, the DHCP SA will initially be used to send a DHCPREQUEST rather
 than a DHCPDISCOVER message.  The initial DHCPv4 message
 (DHCPDISCOVER or DHCPREQUEST) is then tunneled to the security
 gateway using the tunnel mode SA.  Note that since the DHCPDISCOVER
 packet has a broadcast address destination, the IPsec implementations
 on both the remote host and the security gateway must be capable of
 handling this.

4.2. DHCP Relay behavior

 While other configurations are possible, typically the DHCPv4 server
 will not reside on the same machine as the security gateway, which
 will act as a DHCPv4 relay, inserting its address in the "giaddr"
 field.  In this case, the security gateway relays packets between the
 client and the DHCPv4 server, but does not request or renew addresses
 on the client's behalf.  While acting as a DHCP Relay, the security
 gateway MAY implement DHCP Relay load balancing as described in [19].

Patel, et. al. Standards Track [Page 9] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 Since DHCP Relays are stateless, the security gateway SHOULD insert
 appropriate information in the DHCP message prior to forwarding to
 one or more DHCP servers.  This enables the security gateway to route
 the corresponding DHCPOFFER message(s) back to the remote host on the
 correct IPsec tunnel, without having to keep state gleaned from the
 DISCOVER, such as a table of the xid, chaddr and tunnel.
 If the security gateway maintains a separate subnet for each IPsec
 tunnel, then this can be accomplished by inserting the appropriate
 interface address in the giaddr field.  Alternatively, the security
 gateway can utilize the DHCP Relay Agent Information Option [17].  In
 this case, the virtual port number of the tunnel is inserted in the
 Agent Circuit ID Sub-option (sub-option code 1).
 To learn the internal IP address of the client in order to route
 packets to it, the security gateway will typically snoop the yiaddr
 field within the DHCPACK and plumb a corresponding route as part of
 DHCP Relay processing.
 Where allocating a separate subnet for each tunnel is not feasible,
 and the DHCP server does not support the Relay Agent Information
 Option, stateless Relay Agent behavior will not be possible.  In such
 cases, implementations MAY devise a mapping between the xid, chaddr,
 and tunnel in order to route the DHCP server response to the
 appropriate tunnel endpoint.  Note that this is particularly
 undesirable in large VPN servers where the resulting state will be
 substantial.

4.3. DHCPREQUEST message processing

 After the Internet interface has received the DHCPOFFER message, it
 forwards this to the intranet interface after IPsec processing.  The
 intranet interface then responds by creating a DHCPREQUEST message,
 which is tunneled to security gateway using the DHCP SA.

4.4. DHCPACK message processing

 The DHCPv4 server then replies with a DHCPACK or DHCPNAK message,
 which is forwarded down the DHCP SA by the security gateway.  The
 remote host Internet interface then forwards the DHCPACK or DHCPNAK
 message to the intranet interface after IPsec processing.
 After processing of the DHCPACK, the intranet interface is configured
 and the Internet interface can establish a new IPsec tunnel mode SA
 to the security gateway.  The remote host may now delete the DHCP
 tunnel mode SA.  All future DHCP messages sent by the client,
 including DHCPREQUEST, DHCPINFORM, DHCPDECLINE, and DHCPRELEASE
 messages will use the newly established VPN SA.  Similarly, all DHCP

Patel, et. al. Standards Track [Page 10] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 messages subsequently sent by the DHCPv4 server will be forwarded by
 the security gateway (acting as a DHCP Relay) using the IPsec tunnel
 mode SA, including DHCPOFFER, DHCPACK, and DHCPNAK messages.
 It SHOULD be possible to configure the remote host to forward all
 Internet-bound traffic through the tunnel.  While this adds overhead
 to round-trips between the remote host and the Internet, it provides
 some added security in return for this, in that the corporate
 security gateway may now filter traffic as it would if the remote
 host were physically located on the corporate network.

4.5. Configuration policy

 Several mechanisms can be used to enable remote hosts to be assigned
 different configurations.  For example, clients may use the User
 Class Option [16] to request various configuration profiles.  The
 DHCPv4 server may also take a number of other variables into account,
 including the htype/chaddr; the host name option; the client-
 identifier option; the DHCP Relay Agent Information option [17]; the
 vendor-class-identifier option; the vendor-specific information
 option; or the subnet selection option [15].
 Conditional configuration of clients, described in [18], can be used
 to solve a number of problems, including assignment of options based
 on the client operating system; assignment of groups of clients to
 address ranges subsequently used to determine quality of service;
 allocation of special address ranges for remote hosts; assignment of
 static routes to clients [20], etc.  As noted in the security
 considerations, these mechanisms, while useful, do not enhance
 security since they can be evaded by a remote host choosing its own
 IP address.

5. Security Considerations

 This protocol is secured using IPsec, and as a result the DHCP
 packets flowing between the remote host and the security gateway are
 authenticated and integrity protected.
 However, since the security gateway acts as a DHCP Relay, no
 protection is afforded the DHCP packets in the portion of the path
 between the security gateway and the DHCP server, unless DHCP
 authentication is used.
 Note that authenticated DHCP cannot be used as an access control
 mechanism.  This is because a remote host can always set its own IP
 address and thus evade any  security measures based on DHCP
 authentication.

Patel, et. al. Standards Track [Page 11] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 As a result, the assigned address MUST NOT be depended upon for
 security.  Instead, the security gateway can use other techniques
 such as instantiating packet filters or quick mode selectors on a
 per-tunnel basis.
 As described in [17], a number of issues arise when forwarding DHCP
 client requests from untrusted sources.  These include DHCP
 exhaustion attacks, and spoofing of the client identifier option or
 client MAC address.  These issues can be partially addressed through
 use of the DHCP Relay Information Option [17].

6. IANA Considerations

 This document requires that an htype value be allocated for use with
 IPsec tunnel mode, as described in section 4.1.  Note that DHCP
 relies on the arp-parameters registry for definition of both the hrd
 parameter in ARP and the htype parameter in BOOTP/DHCP.  As a result,
 an assignment in the arp-parameters registry is required, even though
 IPsec-DHCP will never use that parameter for ARP purposes, since
 conceptually BOOTP/DHCP and ARP share the arp-parameters registry.
 This document does not create any new number spaces for IANA
 administration.

7. Intellectual Property Statement

 The IETF takes no position regarding the validity or scope of any
 intellectual property 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; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication 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 implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

Patel, et. al. Standards Track [Page 12] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

8. References

8.1 Normative References

 [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.
 [2]  Atkinson, R. and S. Kent, "Security Architecture for the
      Internet Protocol", RFC 2401, November 1998.
 [3]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
      March 1997.
 [4]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
      Extensions", RFC 2132, March 1997.
 [5]  Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
      RFC 3118, June 2001.
 [6]  Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
      November 1998.
 [7]  Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
      (ESP)", RFC 2406, November 1998.
 [8]  Piper, D., "The Internet IP Security Domain of Interpretation of
      ISAKMP", RFC 2407, November 1998.
 [9]  Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
      RFC 2409, November 1998.

8.2 Informative References

 [10] McGregor, G., "The PPP Internet Protocol Control Protocol
      (IPCP)", RFC 1332, May 1992.
 [11] Cobb, S., "PPP Internet Protocol Control Protocol Extensions for
      Name Server Addresses", RFC 1877, December 1995.
 [12] Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S., Rabil,
      G., Dooley, M. and A. Kapur, "DHCP Failover Protocol", Work in
      Progress.
 [13] Dukes, D. and R. Pereira, "The ISAKMP Configuration Method",
      Work in Progress.
 [14] T'Joens, Y., Hublet, C. and P. De Schrijver, "DHCP reconfigure
      extension", RFC 3203, December 2001.

Patel, et. al. Standards Track [Page 13] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 [15] Waters, G., "The IPv4 Subnet Selection Option for DHCP", RFC
      3011, November 2000.
 [16] Stump, G., Droms, R., Gu, Y., Vyaghrapuri, R., Demirtjis, A.,
      Beser, B. and J. Privat, "The User Class Option for DHCP", RFC
      3004, November 2000.
 [17] Patrick, M., "DHCP Relay Agent Information Option", RFC 3046,
      January 2001.
 [18] Droms, R., and Lemon, T., The DHCP Handbook, Macmillan,
      Indianapolis, Indiana, 1999.
 [19] Volz, B., Gonczi, S., Lemon, T. and R. Stevens, "DHC Load
      Balancing Algorithm", RFC 3074, February 2001.
 [20] Lemon, T., Cheshire, S. and B. Volz, "The Classless Static Route
      Option for Dynamic Host Configuration Protocol (DHCP)", RFC
      3442, December 2002.
 [21] Kelly, S. and S. Ramamoorthi, "Requirements for IPsec Remote
      Access Scenarios", RFC 3457, January 2003.
 [22] Rekhter, Y., Moskowitz, B., Karrenberg, D., G. de Groot, and E.
      Lear, "Address Allocation for Private Internets", BCP 5, RFC
      1918, February 1996.
 [23] Atkinson, R., "Key Exchange Delegation Record for the DNS", RFC
      2230, November 1997.
 [24] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
      specifying the location of services (DNS SRV)", RFC 2782,
      February 2000.

9. Acknowledgments

 This document has been enriched by comments from John Richardson and
 Prakash Iyer of Intel, Gurdeep Pall and Peter Ford of Microsoft.

Patel, et. al. Standards Track [Page 14] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

Appendix - IKECFG evaluation

 Alternatives to DHCPv4, such as ISAKMP CFG, described in [13], do not
 meet the basic requirements described in [21], nor do they provide
 the additional capabilities of DHCPv4.
 Basic configuration
       While ISAKMP CFG can provide for IP address assignment as well
       as configuration of a few additional parameters such as the DNS
       server and WINS server addresses, the rich configuration
       facilities of DHCPv4 are not supported.  Past experience with
       similar configuration mechanisms within PPP IPCP [11] has
       taught us that it is not viable merely to support minimal
       configuration.  Eventually, either much of the functionality
       embodied in the DHCPv4 options [4] is duplicated or support for
       DHCPINFORM [3] will be required.
 Address management integration
       Since IKECFG is not integrated with existing IP address
       management facilities, it is difficult to integrate it with
       policy management services that may be dependent on the user to
       IP address binding.
 Address pool management
       IKECFG does not provide a mechanism for the remote host to
       indicate a preference for a particular address pool.  This
       makes it difficult to support address pool management.
 Reconfiguration
       IKECFG does not support the concept of configuration leases or
       reconfiguration.
 Fail-over support
       Since IKECFG creates a separate pool of address state, it
       complicates the provisioning of network utility-class
       reliability, both in the IP address management system and in
       the security gateways themselves.
 Security and simplicity
       As past history with PPP IPCP demonstrates, once it is decided
       to provide non-integrated address management and configuration
       facilities within IKE, it will be difficult to limit the
       duplication of effort to address assignment.  Instead, it will
       be tempting to also duplicate the configuration, authentication
       and fail-over facilities of DHCPv4.  This duplication will
       greatly increase the scope of work, eventually compromising the
       security of IKE.

Patel, et. al. Standards Track [Page 15] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

 Authentication
       While IKECFG can support mutual authentication of the IPsec
       tunnel endpoints, it is difficult to integrate IKECFG with
       DHCPv4 authentication [5].  This is because the security
       gateway will not typically have access to the client
       credentials necessary to issue an DHCPv4 authentication option
       on the client's behalf.
 As a result, security gateways implementing IKECFG typically request
 allocation of an IP address on their own behalf, and then assign this
 to the client via IKECFG.  Since IKECFG does not support the concept
 of an address lease, the security gateway will need to do the renewal
 itself.  This complicates the renewal process.
 Since RFC 2131 [3] assumes that a DHCPREQUEST will not contain a
 filled in giaddr field when generated during RENEWING state, the
 DHCPACK will be sent directly to the client, which will not be
 expecting it.  As a result, it is either necessary for the security
 gateway to add special code to avoid forwarding such packets, or to
 wait until REBINDING state.  Since [3] does not specify that the
 giaddr field cannot be filled in when in the REBINDING state, the
 security gateway may put its own address in the giaddr field when in
 REBINDING state, thereby ensuring that it can receive the renewal
 response without treating it as a special case.

Patel, et. al. Standards Track [Page 16] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

Authors' Addresses

 Baiju V. Patel
 Intel Corp
 2511 NE 25th Ave
 Hillsboro, OR 97124
 Phone: +1 503 712 2303
 EMail: baiju.v.patel@intel.com
 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: +1 425 706 6605
 EMail: bernarda@microsoft.com
 Scott Kelly
 Airespace
 110 Nortech Pkwy
 San Jose CA 95134 USA
 Phone: +1 (408) 941-0500
 EMail: scott@hyperthought.com
 Vipul Gupta
 Sun Microsystems, Inc.
 MS UMTV29-235
 2600 Casey Avenue
 Mountain View, CA 94303
 Phone: +1 650 336 1681
 EMail: vipul.gupta@sun.com

Patel, et. al. Standards Track [Page 17] RFC 3456 DHCPv4 Config. of IPsec Tunnel Mode January 2003

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Acknowledgement

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Patel, et. al. Standards Track [Page 18]

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