How To: Repair a broken PC TCP stack

This probably won’t apply to 99.9% of you — what to do when you suddenly lose the ability to access your network and can find no other explanation or cause.

Symptom: Not long ago my home computer suddenly got stupid. In my case, I was using a VPN (Virtual Private Network) which creates a “virtual” network interface on your machine to “tunnel” encrypted data to and from a remote computer or server. These are used in a lot of companies that permit off-site working as ways to allow the employee secure access to the company’s systems. The VPN I am required to use takes over the routing tables to create the virtual network interfaces and tunnels.

I connected a smartphone (the Moto Q) to my computer to sync it up — the sync software is provided by MicroSoft, called ActiveSync, and one of the things it does is create another virtual network interface device on your machine, so you can use your cell phone as a network card. In other words, ActiveSync does stuff with the routing tables.
At that point, I noticed that my access to the Internet, via my wireless router, disappeared. Upon investigating (running “ipconfig” in a command window) I could see that my normal network interface had a bogus IP address to a non-existent site.
There have been suggestions that some VPN software is paranoid — that is, when it sees a second something trying to get to a DNS or DHCP server, it uses administrator privileges to destroy the network defaults: it assumes there is an attack.

Solution: TCP/IP stack repair on Windows XP with SP2.

Go to the Start menu, click “Run” and type “cmd” in the field to open a command prompt:

In this window, do the following:
1. Reset TCP/IP stack to installation defaults:
netsh int ip reset reset.log
2. Reset WINSOCK entries to installation defaults:
netsh winsock reset catalog

Reboot the machine.