社内ネットにつないだときだけプロクシ設定をオンにする ref: http://qiita.com/yuba/items/2840f913194732a30bd4

wpad.dat

function FindProxyForURL(url,host)
{
    if ( isPlainHostName(host)
      || shExpMatch(host, "172.16.*")
      || shExpMatch(host, "172.17.*")
      || shExpMatch(host, "172.18.*")
      || shExpMatch(host, "172.19.*")
      || shExpMatch(host, "172.2?.*")
      || shExpMatch(host, "172.30.*")
      || shExpMatch(host, "172.31.*")
      || shExpMatch(host, "10.*")
      || shExpMatch(host, "192.168.*")
      || shExpMatch(host, "127.0.0.*")
      || shExpMatch(host,"*.example.co.jp"))
        return "DIRECT";
    else return "PROXY proxyserver:8080";
}