That method is subject to interface filtering, which isn't ideal for
DHCP traffic that probably uses an internal interface on which the IKE
daemon might be disabled. In that case `giaddr` is set to an incorrect
public IP, which in turn might prevent the plugin from receiving the
DHCP server's unicast response, in particular if the DHCP socket
is bound to the internal interface.
This new approach connects the client socket and thereby determines the
source address to reach the DHCP server.
Closesstrongswan/strongswan#1573
Tobias Brunner
3c8887326a