_unbound-hook is invoked by the unbound DNS server to inform pluto of a new hostname, IP address and public key, so that pluto can opportunistically initiate an IPsec connection. Please see the libreswan wiki for more information about Opportunistic IPsec.
ipsec(8), ipsec_pluto(8).
Man page written for the libreswan project <m[blue]https:/libreswan.org/m[][1]> by Paul Wouters.
Paul Wouters