PIX IPSEC VPN Failing - Authentication Signature

I have a PIX to PIX VPN (both running v6.3) that sets up a connection when pinged from either side of the VPN, but fails to work, always showing recieve errors with "sh crypto ipsec sa".

The output of "debug crypto ipsec" and "debug crypto isakmp" running on the destination PIX produces the following

IPSEC(cipher_ipsec_request): decap failed for pix-scotland -> pix-outside
IPSEC(sw_ah_decap): authentication signature does not match

Any ideas where to start hunting?

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drev001Connect With a Mentor Commented:
In this setup, I'd configure the NAT device to forward ALL incoming traffic to the PIX's Outside interface. Do you have any control over the NAT device?
Sounds like the pre-shared keys don't match...
stefanf001Author Commented:

Unfourtantely not. I was just using "cisco1234" for testing - pasted straight from notepad into HyperTerminal.

Daft thought... One of the PIXs is inside a managed office, where the outside of the firewall is in a Private Address space (10.20.x.x)
There is a further firewall between this PIX and the Internet, with the other PIX set to connect to the Public Address (static) of the firewall.

Pings from one PIX to the other always show the correct Public Address, am I correct in assume IPSEC also follows this - so the pre-shared key is associated with the correct peer?

Many thanks in advance

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So, you have
PIX1 ---Internet ---PIX2--private 10.20.x.x--PIX3
   PIX2 maps a public IP to PIX3 private outside address..
   You are attempting VPN between PIX1 and PIX3?
  Does PIX2 provide access for ISAKMP and ESP, forwarded to PIX3 from PIX1 public address?
Yes, the key is always associated with a peer - using public addresses on each end.
The issue here may be in PIX2 if you don't control it...
stefanf001Author Commented:
That is correct, although I'm not sure whether PIX2 is a PIX ... Have passed you question on to our ISP and will post when I know. Cheers!

stefanf001Author Commented:
I found out the PIX2 is just a straight NAT device (not sure what though) and the ISP reckons it is not possible to use IPSEC over NAT.

What about NAT-T, or are there any other options?

Current relevant PIX1 config is the following (PIX3 is similar)

access-list cd9_vpn permit ip
nat (inside) 0 access-list cd9_vpn

isakmp enable outside
isakmp key cisco1234 address pix-wandsworth netmask
isakmp policy 9 authentication pre-share
isakmp policy 9 hash md5
isakmp policy 9 encrypt des

crypto ipsec transform-set xtra-ts esp-3des esp-sha-hmac
crypto map xtra-map 1 ipsec-isakmp
crypto map xtra-map 1 match address cd9_vpn
crypto map xtra-map 1 set peer pix-wandsworth
crypto map xtra-map 1 set transform-set xtra-ts
crypto map xtra-map interface outside

sysopt connection permit-ipsec
lrmooreConnect With a Mentor Commented:
Agree with drev001.
Whatever this "nat device" is, it needs to have a 1-1 static nat for all traffic, no filters, no port redirection, from a public IP to the PIX3 outside interface.
Nat-Transparency is crucial to the NAT device only if using PAT..
It's a question whether the obstacle here is NAT, or just the ISP not being willing to open some ports on his routers for traffic (I've encountered that before).

I can't remember I've ever experienced any problem with PIX VPN via NAT in a situation as the one described (and we're talking 6.3 here) .....
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