I have spent the better part of the day trying to diagnose an IPSec connectivity problem between a Cisco PIX (version 6.3.5) and a WatchGuard Firebox of some kind (I have no visibility of it). The problem turns out to be rather subtle, so I thought I would share it here.
The configuration on the PIX is fairly standard (IP addresses have been changed to protect the innocent !):-
crypto ipsec transform-set Esp3DesMD5 esp-3des esp-md5-hmac
:
:
access-list VPNToFirebox permit 192.168.1.0 255.255.255.0 192.168.2.0 255.255.255.0
:
:
crypto map VPN 60 ipsec-isakmp
crypto map VPN 60 match address VPNToFirebox
crypto map VPN 60 set pfs
crypto map VPN 60 set peer 2.2.2.2
crypto map VPN 60 set transform-set Esp3DesMD5
crypto map VPN 60 set security-association lifetime seconds 86400 kilobytes 8192
:
:
isakmp key ******** address 2.2.2.2 netmask 255.255.255.255 no-xauth no-config-mode
The ISAKMP (Phase 1) SA establishes just fine, but the IPSEC (Phase 2) SA never comes up. Watching the debug information on the PIX, here's what happens:-
ISAKMP : Checking IPSec proposal 1
ISAKMP: transform 1, ESP_3DES
ISAKMP: attributes in transform:
ISAKMP: SA life type in seconds
ISAKMP: SA life duration (VPI) of 0x0 0x1 0x51 0x80
ISAKMP: SA life type in kilobytes
ISAKMP: SA life duration (basic) of 8192
ISAKMP: group is 2
ISAKMP: encaps is 61433
ISAKMP: authenticator is HMAC-MD5
ISAKMP (0): atts not acceptable. Next payload is 0
ISAKMP (0): SA not acceptable!
ISAKMP (0): sending NOTIFY message 14 protocol 0
return status is IKMP_ERR_NO_RETRANS
The key is the "encaps is 61433" line. What this should say is "encaps is 61443" which is the (old, pre-RFC3947) encapsulation ID for ESP via NAT Traversal. As it is, the PIX has no idea what "61433" is supposed to be and the SA negotiation fails.
Here's what the debug output looks like when talking to another PIX (which sends the correct ID):-
ISAKMP (0): processing SA payload. message ID = 177759204
ISAKMP : Checking IPSec proposal 1
ISAKMP: transform 1, ESP_3DES
ISAKMP: attributes in transform:
ISAKMP: encaps is 61443
ISAKMP: SA life type in seconds
ISAKMP: SA life duration (VPI) of 0x0 0x1 0x51 0x80
ISAKMP: SA life type in kilobytes
ISAKMP: SA life duration (basic) of 8192
ISAKMP: authenticator is HMAC-MD5
ISAKMP: group is 1
ISAKMP (0): atts are acceptable.IPSEC(validate_proposal_request): proposal part #1,Here the encapsulation ID is correct and the tunnel comes up.
The problem only arises when NAT is detected on the path between the Firebox and the PIX...otherwise there is no need to encapsulate the ESP inside UDP.
References
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