The value of the object vRtrIfDHCP6SourceAddress specifies the IPv6
address that will be used by the DHCP6 relay agent as source IP
address in all messages sent to the DHCP6 servers.
If set to ::, the system will use the IPv6 address assigned on the
link on which the relay message is sent out, if such an address is
defined. As a last resort the system IPv6 address is taken.