NAT Traversal (NAT-T) » History » Version 3
NAT discovery and traversal must be enabled by setting nat_traversal=yes in the config setup section of ipsec.conf. Otherwise strongSwan's IKEv1 pluto daemon will not accept incoming IKE packets with a UDP source port different from 500.
The IKEv2 protocol includes NAT traversal in the core standard, but it's optional to implement. strongSwan implements it, and there is no configuration involved. The NAT_DETECTION_SOURCE/DESTINATION_IP notifications included in IKE_SA_INIT indicates the peers NATT capability and if a NAT situation is detected, UDP encapsulation is activated for IPsec.
strongSwan starts sending keep-alive packets if it is behind a NAT router to keep the mappings on the NAT device intact.