DNS over TCP
TCPの場合は頭に2バイトでメッセージ長が必要とのこと。RFC1035*1より。
4.2.2. TCP usage Messages sent over TCP connections use server port 53 (decimal). The message is prefixed with a two byte length field which gives the message length, excluding the two byte length field. This length field allows the low-level processing to assemble a complete message before beginning to parse it.
