TKEY Exchange (GSS-TSIG hook)

     ,-------------.          ,----------.
     |Kea D2 server|          |DNS server|
     `------+------'          `----+-----'
            |     TKEY request     |
            |---------------------->
            |                      |
            |TKEY response (signed)|
            |<----------------------
     ,------+------.          ,----+-----.
     |Kea D2 server|          |DNS server|
     `-------------'          `----------'