One document matched: draft-aboba-nat-ipsec-00.txt

                             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
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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].



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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



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   to use to protect a given packet.

f) Since ESP traffic is encrypted and thus opaque to the NAT,
   the NAT ust use elements of the IP and IPSEC header to
   demultiplex incoming IPSEC traffic. The combination of the
   source IP address and IPSEC SPI is typically used for this
   purpose. If the initiator is behind a NAT then since the
   responder chooses the SPI, the combination of source address
   and SPI will be unique. However, if the responder is behind
   a NAT then it is possible (though unlikely) that the same
   SPI value will be chosen by two or more responders. In this
   case the NAT could send the IPSEC packets to the wrong
   destination.

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.

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. Where user authentication is done, a network
   access identifier (user@realm) can be used instead. In the
   case of machine authentication, an FQDN can be used. In practice
   use of IP address identifiers  in IKE MM 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 correct keys.

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



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   between the two endpoints. In such circumstances, use of
   an FQDN (machine identifier) or NAI (user identifier) should
   be permitted within the SPD negotiation in IKE QM.


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.

9.  Acknowledgments

Thanks to William Dixon of Microsoft for many 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



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Phone: 425-936-6605
EMail: bernarda@microsoft.com

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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
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IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED



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WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."


14.  Expiration Date

This memo is filed as <draft-aboba-ipsec-nat-00.txt>, and expires
January 1, 2001.












































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