The responder validates initiator's address in forward zone.

Road is the initiator and east is the responder.

When road initiate the connection, it send an IKE request, and
simultaneously start query for the responders IPSECKEY in reverse zone.
23.2.1.192.IN-ADDR.ARPA.    120 IN IPSECKEY 10 0 2 . AQO9bJbr3

When the east, the responder, IDi (in IKEv2 AUTH exchange), is road.

road.testing.libreswan.org. 120 IN IPSECKEY 10 0 2 . AQ

east also send adtional query for road.testing.libreswan.org. A
The address and IDi match authentication success.