NAT Working Group Bernard Aboba INTERNET-DRAFT Microsoft Category: Informational 26 May 2000 NAT and IPSEC 1. 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 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. 2. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. 3. Abstract Perhaps the most common use of IPSEC is in providing virtual private networking capabilities. One very popular use of VPNs is to provide tele-commuter access to the corporate Intranet. With NATs being increasingly deployed in home gateways, NAT-IPSEC incompatibilities have become a major barrier to deployment of IPSEC in one of its principal uses. This draft discusses the incompatibilities between NAT and IPSEC and suggests how IPSEC might be made more NAT friendly. 4. 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 [2]. Aboba Informational [Page 1] INTERNET-DRAFT NAT and IPSEC 26 May 2000 5. Introduction Perhaps the most common use of IPSEC [6] is in providing virtual private networking capabilities. One very popular use of VPNs is to provide tele-commuter access to the corporate Intranet. With NATs being increasingly deployed in home gateways, NAT-IPSEC incompatibilities have become a major barrier to deployment of IPSEC in one of its principal uses. This draft discusses the incompatibilities between NAT and IPSEC and suggests how IPSEC might be made more NAT friendly. 6. NAT/IPSEC incompatibilities The known incompatibilities between NAT and IPSEC are as follows: a) IPSEC AH [3] will not go through the NAT, because the AH header incorporates the IP source and destination fields in the authentication hash. b) IPSEC ESP [4] does not incorporate the IP source and destination fields in its authentication hash. However, there is an implicit dependency on source and destination addresses within TCP/UDP/SCTP checksums which cover the "pseudo-header." Therefore IPSEC ESP will only go through the NAT if TCP/UDP/SCTP protocols are not involved (as in IPSEC tunnel mode or IPSEC/GRE), UDP checksums are turned off (TCP checksums are required), or if TCP/UDP/SCTP checksums are ignored by the receiving party. c) Where IP addresses are used as identifiers in IKE MM [7] or QM, IKE will only go through the NAT if the parties do not check or use IP addresses in IKE MM identifiers (several current implementations don't do this) AND if in addition they don't check or use IP addresses in IKE QM identifiers (most implementations DO use addresses and check them). d) Because of IKE re-keying behavior, it is necessary for implementations to float their IKE source port in order to enable NATs to de-multiplex incoming re-keys which may not use the same cookies as the earlier traffic. Otherwise it is possible for the re-key to be sent to the wrong SA by the NAT. e) In order to enable an IPSEC implementation to send traffic down the correct IPSEC SA, it is necessary for those SAs to be differentiated in some way. In practice this implies negotiation of non-overlapping SPD entries. For example, if two clients behind a NAT were to negotiate the same SPD entries, then there would be no way to decide which SA Aboba Informational [Page 2] INTERNET-DRAFT NAT and IPSEC 26 May 2000 to use to protect a given packet. f) Since ESP traffic is encrypted and thus opaque to the NAT, the NAT must use elements of the IP and IPSEC header to demultiplex incoming IPSEC traffic. The combination of the source IP address, security protocol (AH/ESP) and IPSEC SPI is typically used for this purpose. As noted in [6]: "The receiver-orientation of the Security Association implies that, in the case of unicast traffic, the destination system will normally select the SPI value. By having the destination select the SPI value, there is no potential for manually configured Security Associations to conflict with automatically configured (e.g., via a key management protocol) Security Associations or for Security Associations from multiple sources to conflict with each other." This implies that if the source is located behind a NAT, but the destination is not, then the combination of the destination address, security protocol and SPI will be unique. However, if the destination is located behind a NAT, then it is possible (though unlikely) that the same SPI value may be chosen by two or more destinations behind the NAT. In this case the NAT could send the IPSEC packets to the wrong destination. g) Since the payload is integrity protected, any IP addresses enclosed within the payload will not be translatable by the NAT. There are many protocols that utilize embedded IP addresses, including FTP, IRC, SNMP, LDAP, H.323 SIP, and many games. 7. Recommendations It is recommended that the following actions be taken to improve the NAT-friendliness of IPSEC: a) Since IPSEC ESP null provides much the same security services as IPSEC AH, but without explicitly covering the IP header in its authentication hash, it is recommended that IPSEC ESP null be used instead of AH. b) Since transport mode IPSEC traffic is integrity protected and authenticated using strong cryptography, there is little to gained by having the receiver check TCP/UDP/SCTP checksums on traffic protected by IPSEC transport mode SAs. It is therefore recommended that checksum verification be made optional in this case. Aboba Informational [Page 3] INTERNET-DRAFT NAT and IPSEC 26 May 2000 c) Since proper de-multiplexing of IKE re-keys is dependent on initiators floating their IKE source ports, it is recommended that IKE implementations float their source ports. d) It is recommended that IP addresses not be used as identifiers in IKE MM or QM, where this can be avoided. Where user authentication is desired, an ID type of ID_USER_FQDN can be used, as described in [5]. Where machine authentication is desired, an ID type of ID_FQDN can be used. In practice, use of IP addresses as identifiers in IKE provides little security value, since assuming that the integrity of the IKE packets is verified, it can be assumed that the correspondent has possession of the negotiated keys. Note that restricting identifiers to ID_USER_FQDN or ID_FQDN would prevent use of subnet or address range identifiers, which may be required for gateway to gateway communications. Thus this approach is not universally applicable. e) In tele-commuter scenarios, it is expected that both IPSEC transport mode (for L2TP/IPSEC as well as other UDP and TCP) and IPSEC tunnel mode will be commonly used. In these cases, the SPD entries typically only need to protect traffic between the two endpoints. In such circumstances, ID_USER_FQDN or ID_FQDN identifiers should be used within the SPD negotiation in IKE QM. Since restricting identifiers to ID_USER_FQDN or ID_FQDN would prevent use of subnet or address range identifiers, this approach may not be applicable in gateway to gateway communications. f) If the above techniques are not feasible, alternative approaches should be considered. One currently popular technique is to encapsulate IP/IPSEC within a TCP or UDP payload, and then remove the outermost IP and transport header at the receiver, thus reconstructing the original packet without modification. While this method does introduce extra overhead, it does not require any modifications to IPSEC/IKE or checksum verification procedures. 8. Security considerations It is not believed that the changes described above will impact IPSEC security adversely. There is no security value to TCP/UDP/SCTP checksums, so not checking them does not decrease security. Similarly, use of IPSEC ESP null instead of AH does not introduce any security vulnerabilities. Use of ID_FQDN or ID_USER_FQDN identifiers in IKE QM does raise the issue as to what traffic will be accepted in the IPSEC SA. Since packets will be integrity protected, it is possible to verify that the source is in posession of the negotiated keys. Thus for transport mode SAs, it does not appear strictly necessary to filter by address, only to verify Aboba Informational [Page 4] INTERNET-DRAFT NAT and IPSEC 26 May 2000 packet integrity. However, for tunnel mode SAs, if subnet or IP address range identifiers are not used, it is reasonable to assume that only traffic from a single IP address is permitted inside the tunnel. A reasonable assumption would be that this IP address corresponds to the source address used when setting up the IKE QM SA. 9. Acknowledgments Thanks to Steve Bellovin of AT&T Research, William Dixon of Microsoft, and Daniel Senie for useful discussions of this problem space. 10. References [1] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G., and Palter, B., "Layer Two Tunneling Protocol L2TP", RFC 2661, August 1999. [2] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [3] Kent,S., Atkinson, R., "IP Authentication Header", RFC 2402, November 1998. [4] Kent,S., Atkinson, R., "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. [5] Piper, D., "The Internet IP Security Domain of Interpretation of ISAKMP", RFC 2407, November 1998. [6] Atkinson, R., Kent, S., "Security Architecture for the Internet Protocol", RFC 2401, November 1998. [7] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)", RFC 2409, November 1998. 11. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 Phone: 425-936-6605 EMail: bernarda@microsoft.com Aboba Informational [Page 5] INTERNET-DRAFT NAT and IPSEC 26 May 2000 12. 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. 13. 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 implementation 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." Aboba Informational [Page 6] INTERNET-DRAFT NAT and IPSEC 26 May 2000 14. Expiration Date This memo is filed as , and expires January 1, 2001. Aboba Informational [Page 7]