services.strongswan-swanctl.swanctl.connections.<name>.if_id_out

XFRM interface ID set on outbound policies/SA, can be overridden by child config, see there for details.

The special value %unique allocates a unique interface ID per IKE_SA, which is inherited by all its CHILD_SAs (unless overridden there), beyond that the value %unique-dir assigns a different unique interface ID for each direction (in/out).

StrongSwan default: "0"