NAT Working Group Bernard Aboba
INTERNET-DRAFT Microsoft
Category: Informational
<draft-aboba-nat-ipsec-01.txt>
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.
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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 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.
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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
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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
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14. Expiration Date
This memo is filed as <draft-aboba-ipsec-nat-00.txt>, and expires
January 1, 2001.
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