Knoppix uses the NetworkManager. You can get access to it with right-mouseclick on the icon in the right side of the panel nearby the clock. Try "Edit connections" and check all the values; perhaps you'll find the error.

Otherwise start PCManFM (in the left side of the panel) => Menu "Tools" => "Open current folder as root"
Within the new window of PCManFM go to '/etc/NetworkManager/system-connections' and tell us the name and content of the file you'll find there.