Why doesn't the VPN connection work through my ICS host?

Question

I can set up Internet Connection Sharing on my Windows-7 laptop without problems. (I connect to the DSL modem via wireless)

The client laptop (Windows XP) is directly connected via LAN cable to the Windows-7 laptop and the client correctly gets an IP, Standardgateway and nameserver from the Windows-7 ICS host.

However, when I start my VPN client, I get the error message:

Unable to resolve the IP address of the remote server. Verify the host name in the Destination field is correct.

I can ping the Windows-7 host from the client PC that should run the VPN application.

What's wrong?

Answer 1

Note: I'm asking and self-answering this after the fact, but the reason I didn't find anything on google seems to be that I searched in the wrong order with the wrong keywords, so this may be of help to people with the same problem.

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It turns out that you should take error messages by their word. "... unable to resolve IP address of remote server ...". This was a hint that name resolution didn't work.

It turns out that, for whatever reason, the nameserver on the Win-7 ICS host didn't work for the client(s). It was/is routing traffic allright, but as long as the client PCs have the nameserver they get via DHCP -- namely the IP of the ICS-host=standardgateway), name resolution doesn't work. (I can ping <internet-ip-address> but I cannot ping www.google.com.)

The fix is, once you have the connection running, to go to the IPv4 settings of the (LAN) connection ob the client PC and change the nameserver from "auto" to a fixed nameserver. (I used google's 8.8.8.8 and 8.8.4.4.)

Once name resolution worked, I could also connect to the VPN network.