In message <LCEKLACNFGLMOPOGNBNMMEBBCEAA.tim@eng.bellsouth.net>, "Tim Irwin" wr

I looked at this a while back... I am dusting off the cobwebs of my mind, so
no flames please. I believe that the NATing device must modify the SPI
values. The sending device sends out an ESP packet with src addy of, say, to the NAT router. The router must look at the TCP port to
determine that it's IPSEC in order to figure out that it's a special case
and NAT it. It then must modify the SPI value (which is partially made up
of the src IP address) as it leaves because the NAT dst device will use the
info in the SPI value in the formulation of it's reply.

If this is wrong, please correct me... I'm interested in knowing as well.

That doesn't work -- the SPI is protected by ESP's authentication check
(section 2 of RFC 2406) or by AH (section 2 of RFC 2402).

    --Steve Bellovin, http://www.research.att.com/~smb