IPSEC Working Group Baiju Patel INTERNET-DRAFT Intel Category: Standards Track Bernard Aboba <draft-ietf-ipsec-dhcp-05.txt> Microsoft Expires: November 1, 2000 Scott Kelly RedCreek Communications Vipul Gupta Sun Microsystems, Inc. DHCP Configuration of IPSEC Tunnel Mode This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. This document is an Internet-Draft. Internet-Drafts are working docu- ments of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 1. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. 2. Abstract 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. The Dynamic Host Configuration Protocol (DHCP) provides for such remote host configuration. This draft explores the requirements for host configuration in IPSEC tunnel mode, and describes how the DHCP protocol may be leveraged for configuration in this case. Patel, Aboba, Kelly & Gupta Standards Track [Page 1]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 3. 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. The Dynamic Host Configuration Protocol (DHCP) provides for such remote host configuration. This draft explores the requirements for host configuration in IPSEC tunnel mode, and describes how the DHCP protocol may be leveraged for configuration in this case. 3.1. 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. DHCP server A DHCP server or "server" is an Internet host that returns configuration parameters to DHCP clients. 3.2. 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]. 3.3. Configuration requirements for IPSEC tunnel mode The configuration requirements of a host with an IPSEC tunnel mode interface are described in [15]. These include the need: 1. to obtain an IP address and other configuration parameters appropriate to the class of host 2. to reconfigure when required 3. to authenticate where required 4. to support address pool management 5. to support failover 6. to integrate with existing IP address management facilities such as DHCP 7. to maintain security and simplicity in the IKE implementation. Leveraging DHCP for the configuration of IPSEC tunnel mode satisfies these requirements. Since DHCP already provides for rich configuration capabilities, it is possible to utilize these facilities in configuring Patel, Aboba, Kelly & Gupta Standards Track [Page 2]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 IPSEC tunnel mode interfaces. Where DHCP authentication is required, this can be supported on an IPSEC tunnel mode interface as it would be on any other interface. When leveraging DHCP, it is possible to reuse existing address pool assignment facilities so that compatibility and integration with existing addressing implementations and IP address management software is assured. In addition, DHCP supports the concept of configuration leases, and there is a proposal for handling forced reconfiguration [14]. Since when leveraging DHCP, configuration and addressing state is kept on the DHCP server, not within the IKE implementation, it is easier to support failover. Leveraging DHCP also makes it easier to maintain security in the IKE implementation. In contrast, alternatives to DHCP, such as IKECFG, described in [13], do not meet the requirements. While IKECFG can provide for IP address assignment as well as configuration of a few additional parameters, the rich configuration facilities of DHCP are not supported. Past experience with PPP IPCP leads to the conclusion that eventually it will either be necessary to duplicate much of the functionality of the DHCP protocol, or support for DHCPINFORM will be required. While IKECFG can support mutual authentication of the IPSEC tunnel endpoints, it is difficult to integrate IKECFG with DHCP authentication. This is because the IPSEC tunnel server will not typically have access to the client credentials necessary to sign the DHCP authentication option on the client's behalf. Furthermore, IKECFG does not currently support the functionality necessary for the IPSEC tunnel mode server to issue an authenticated DHCP request on the client's behalf. Similarly, IKECFG does not provide a mechanism for the client to indicate a preference for a particular address pool. This makes it difficult for the ISPEC tunnel mode server to ensure that the client receives an IP address assignment from the appropriate address pool, such as via the User Class option, described in [16]. 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 IPSEC tunnel mode servers themselves. Since IKECFG is not integrated with existing IP address management facilities, it is difficult to integrate this with policy management services that may be dependent on the user to IP address binding. Finally, 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 Patel, Aboba, Kelly & Gupta Standards Track [Page 3]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 duplicate the configuration, authentication and failover facilities of DHCP. This duplication will greatly increase the scope of work, eventually compromising the security of IKE. As a result of this requirements evaluation, it is apparent that leveraging DHCP for configuration of IPSEC tunnel mode is the superior alternative. As a result, this document describes how DHCP may be leveraged to provide for configuration of IPSEC tunnel mode clients. No modifications to DHCP are required in order to accomplish this. 4. Scenario overview IPSEC [2], [9]-[12] 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 VPN server, 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: A remote host on the Internet will connect to the VPN server and then establish an IPSEC tunnel to it. The remote host then interacts via the IPSEC tunnel with an agent 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 VPN server as shown below. corporate net +------------------+ | | externally | +--------+ | !~~~~~~~~~~! |+-------+ visible | | | | ! rmt host ! ||virtual| host | |security| |---! virtual ! || host | |--------|gateway/| | ! presence ! || |<================>| DHCP |----| !~~~~~~~~~~! |+-------+ |--------| Relay | | +------------------+ ^ +--------+ | +--------+ | |---| DHCP | IPsec tunnel | | server | with encapsulated | +--------+ traffic inside Patel, Aboba, Kelly & Gupta Standards Track [Page 4]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 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 [4], DHCP, 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 VPN server. This can be accomplished via DNS, using either A, KX, or SRV records. Since the DHCP server will typically not reside on the same machine as the VPN server, it is necessary for the VPN server to act as a DHCP relay, as well as an IPSEC security gateway between the Internet and the intranet. 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. 4.1. Configuration walkthrough The configuration of the intranet interface of the IPSEC tunnel mode host is accomplished in the following steps: 1) The remote host establishes an IKE security association with the VPN server in a main mode or aggressive mode exchange. This IKE SA then serves to secure additional quick mode IPSEC SAs. 2) The remote host establishes a DHCP SA with the VPN server in a quick mode exchange. The DHCP SA is an IPSEC tunnel mode SA established to protect initial DHCP traffic between the VPN server and the remote host. 3) DHCP messages are sent back and forth between the remote host and the DHCP server, using the VPN server as a DHCP relay. This traffic is protected between the remote host and the VPN server using the DHCP SA established in step 2. After the DHCP conversation completes, the remote hosts's intranet interface obtains an IP address as well as other configuration parameters. 4) The remote host MAY request deletion of the DHCP SA since future DHCP messages will be carried over a new VPN 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]. 5) The remote host establishes a tunnel mode SA to the VPN server Patel, Aboba, Kelly & Gupta Standards Track [Page 5]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 in a quick mode exchange. 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 DHCP messages) between the remote host and the intranet are now tunneled over this VPN SA. Since the security parameters used for different SAs are based on the unique requirements of the remote host and the VPN server, they are not described in this document. The mechanisms described here work best when the VPN is implemented using a virtual interface. 5. Detailed description This section provides details relating to the messages exchanged during the setup and teardown of the DHCP SAs. 5.1. Generation of the DHCPDISCOVER message 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 a value TBD, 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 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 VPN server IP 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" Patel, Aboba, Kelly & Gupta Standards Track [Page 6]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 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 chaddr field of the DHCPDISCOVER should include a unique identifier. The client must use the same chaddr field in all subsequent messages within the same DHCP exchange. This permits the use of DHCP Relay load balancing as described in [8]. The contents of the hlen and chaddr fields SHOULD be the hlen and chaddr values for the LAN interface supplying the network connectivity used for this tunnel. Note that in order to more easily allow the DHCP server to differentiate VPN from non-VPN requests, the htype is set to a value TBD, signifying a virtual IPSEC tunnel mode interface, rather than to the LAN interface value. If there is no LAN interface, the chaddr SHOULD be the IP address of the interface supplying the network connectivity, concatenated with 2 bytes of random data. Once again, the htype is set to a value TBD signifying a virtual IPSEC tunnel mode interface. Note that in this case the chaddr will not be unique between reboots, so that the client-identifier option MUST be included, and SHOULD be set to something that is unique and persistent across reboots, such as the machine FQDN concatenated with the interface name and a number. In order to deliver the DHCPDISCOVER packet from the intranet interface to the VPN server, an IKE Phase 1 SA is established between the Internet interface and the VPN server. 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 tunnel mode server 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 client to server: Any to Any, destination: UDP port 67 >From server to client: 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 DHCP message (DHCPDISCOVER or DHCPREQUEST) is then tunneled to the VPN server using the tunnel mode SA. Since the VPN server is acting as a DHCP relay, it will forward the message to one or more intranet DHCP servers, and will store the xid and the chaddr in a table Patel, Aboba, Kelly & Gupta Standards Track [Page 7]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 so as to be able to route the corresponding DHCPOFFER message(s) back to the remote host. 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 VPN server using the DHCP SA. The DHCP Server than replies with a DHCPACK or DHCPNAK message, which is forwarded down the DHCP SA by the VPN server. The remote host Internet interface then forwards the DHCPACK or DHCPNAK message to the intranet interface after IPSEC processing. To learn the 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. At this point, the intranet interface is configured and the internet interface can establish a new IPSEC tunnel mode SA to the VPN server. The IDci of the quick mode exchange used to establish the new IPSEC tunnel mode SA should be the address of the intranet interface as obtained via DHCP. 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 messages subsequently sent by the DHCP server will be forwarded by the VPN server/DHCP relay using the VPN SA, including DHCPOFFER, DHCPACK, and DHCPNAK messages. It SHOULD be possible to configure the client to forward all internet- bound traffic through the tunnel. While this adds overhead to roundtrips between the client 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. 5.2. DHCP considerations The DHCP server may wish to assign VPN clients different configurations or address pool assignments based on a number of variables. These may include the chaddr; the client-identifier option; the DHCP relay option [17]; the vendor-class-identifier option; the vendor-specific information option; the user class option [16]; or the host name option. Conditional configuration of clients 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 Patel, Aboba, Kelly & Gupta Standards Track [Page 8]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 used to determine quality of service; allocation of special address ranges for VPN clients; special treatment for various hardware types, etc. As noted in the security section, these mechanisms, while useful, do not constitute a security mechanism, since they can be evaded by a client choosing its own IP address. Note that in the case of a dialup client, the chaddr will typically change between reboots so that the client SHOULD include the client identifier option in its DHCPDISCOVER message. The client identifier option MUST be unique; thus a unique identifier such as the client FQDN concatenated with the interface name and a number can be used, in the form of an ASCII null terminated string. 6. References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [2] Atkinson, R., Kent, S., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [3] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [4] McGregor, G., "The PPP Internet Protocol Control Protocol (IPCP)", RFC 1332, May 1992. [5] Alexander, S., Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997. [6] Droms, R., Arbaugh, W., "Authentication for DHCP Messages", Internet draft (work in progress), draft-ietf-dhc- authentication-12.txt, October 1999. [7] Cobb, S., "PPP Internet Protocol Control Protocol Extensions for Name Server Addresses", RFC 1877, December 1995. [8] Droms, R., Kinnear, K., Stapp, M., Volz, B., Gonczi, S., Rabil, G., Dooley, M., Kapur, A., "DHCP Failover Protocol", Internet draft (work in progress), draft-ietf-dhc-failover-05.txt, October 1999. [9] Kent,S., Atkinson, R., "IP Authentication Header", RFC 2402, November 1998. [10] Kent,S., Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. Patel, Aboba, Kelly & Gupta Standards Track [Page 9]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 [11] Piper, D., "The Internet IP Security Domain of Interpretation of ISAKMP", RFC 2407, November 1998. [12] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. [13] Pereira, R., Anand, S., Patel, B., "The ISAKMP Configuration Method", Internet draft (work in progress), draft-ietf-ipsec- isakmp-mode-cfg-05.txt, August 1999. [14] De Schrijver, P., T'Joens, Y., Hublet, C., "Dynamic host configuration : DHCP reconfigure extension", Internet draft (work in progress), draft-ietf-dhc-pv4-reconfigure-00.txt, March 2000. [15] Aboba, B., "IPSEC Remote Access Evaluation Criteria", Internet draft (work in progress), draft-aboba-ipsra-req-00.txt, November 1999. [16] Stump, G., Droms, R., Gu, Y., Vyaghrapuri, R., Demirtjis, A., Beser, B., Privat, J., "The User Class Option for DHCP", Internet draft (work in progress), draft-ietf-dhc-userclass-05.txt, February 2000. [17] Patrick, M., "DHCP Relay Agent Information Option", Internet draft (work in progress), draft-ietf-dhc-agent-options-09.txt, March 2000. 7. Security Considerations This protocol is secured using IPSEC, and as a result the DHCP packets flowing between the client 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. However, even when authenticated DHCP is used, DHCP is not an access control mechanism. Thus address assignment MUST NOT be depended upon for security, since doing so would enable a client to set its own IP address and thus evade the security measures. Instead the security gateway will need to use other techniques such as instantiating packet filters or quick mode selectors on a per-tunnel basis. Patel, Aboba, Kelly & Gupta Standards Track [Page 10]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 8. IANA Considerations This draft requires that an htype value be allocated for use with IPSEC tunnel mode. No new number spaces are created for IANA administration. 9. Acknowledgements This draft has been enriched by comments from John Richardson and Prakash Iyer of Intel, Gurdeep Pall and Peter Ford of Microsoft. 10. Authors' Addresses Baiju V. Patel Intel Corp, JF3-206 2511 NE 25th Ave Hillsboro, OR 97124 Phone: +1 (503) 264-2422 EMail: baiju.v.patel@intel.com Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: +1 (425) 936-6605 EMail: bernarda@microsoft.com Scott Kelly RedCreek Communications 3900 Newpark Mall Road Newark, CA 94560 Phone: +1 (510) 745-3969 Email: skelly@redcreek.com Vipul Gupta Sun Microsystems, Inc. 901 San Antonio Rd. Palo Alto, CA 94303 Phone: +1 (650) 786 3614 Fax: +1 (650) 786 6445 EMail: vipul.gupta@eng.sun.com Patel, Aboba, Kelly & Gupta Standards Track [Page 11]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 11. 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. 12. Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implmentation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE." Patel, Aboba, Kelly & Gupta Standards Track [Page 12]
INTERNET-DRAFT DHCP Configuration of IPSEC Tunnel Mode 8 April 2000 13. Expiration Date This memo is filed as <draft-ietf-ipsec-dhcp-05.txt>, and expires November 1, 2000. Patel, Aboba, Kelly & Gupta Standards Track [Page 13]
