So we have had issues over the past week with Internet connection being intermittent through our hardware proxy.
We have solved the problem which was that someone had connected to the wireless network with their phone but statically set their IP to the same ip as the proxy.
My question is what is the best way of preventing this from happening again?
If your router does VLANs, I would put your servers/proxies on a different subnet than your clients. That way they can't take the IP of any of your core devices because they're on a different subnet. If you can't do that, you could do something that verifies the MAC address is the Sophos MAC before allowing any connection.