2 NICs + Site-to-Site VPN + Http proxy = problem

From: Guillaume Tamisier (nospam_at_nospam.com)
Date: 07/29/04

Date: Thu, 29 Jul 2004 21:58:23 +0200


I have a strange problem on one of the computers of my company. It is a
Windows Server 2003 Domain Controler with 2 network cards (one public, which
does NAT, and a private one). The network requires a proxy to access the
web. This proxy is behind the public network interface. When I launch
Internet Explorer, it asks for the login/password for the proxy and I can
access the web without any problem.

Yesterday, I configured a Site-to-Site VPN connection between this computer
and another computer on the internet. When the VPN connection is connected,
IE no more uses the proxy and the computer have no access to the web !!! I
used a sniffer to understand where is the problem, and I noticed that when
the VPN is connected, IE no more forwards Http requests to the proxy, but
directly to the gateway of the public network card. If I disconnect the VPN
connection, everything works fine again.

I checked the routing table of the computer when the VPN connection is on,
and the table is good. The gateway is still the gateway of the public
network card, so it's not a route problem. I just don't understand why IE
suddenly no more forwards Http requests to the proxy.

Any idea about this problem ?

Guillaume Tamisier