We are having extensive (thousands per minute)
GET /wpad.dat HTTP/1.1 requests at our Linux server, causing traffic problems and slowing down access to the websites we host.
What does it stand for and how to prevent it?
It's what browsers will search for if you have 'auto-detect proxy settings' enabled.
Thousands per minute does sounds a little on the high side. How big is your network? It may be an intentional DoS attack.
If it's the server unable to keep up with the connection requests (as opposed to traffic congestion), it may help to configure your firewall to drop such requests.
If you control all machines on the network, set their browsers to not search for a proxy, either by specifying no proxy, a PAC file or a specific proxy address.
It may also be worth taking a look here.