Surge protection for ethernet line running between buildings?

I have a network in a building, then ran a 100ft ethernet cable from the switch in that building to a router in a small building in the back to get a wired connection back there. This is in the South East part of America, so we deal with a lot of lightning strikes. How should i protect the network from a surge from a lightning strike near or on that ethernet line (running a fiber optic cable isn't an option). I only need the protection on the main building side, the other side is using a cheap off the shelf router that i'm fine with replacing in case something happens, but the switch its plugged into on the main building side is very expensive.