Building

Before building, make sure libvici.h is present, typically shipped with strongSwan.
The following command should do the trick: pkg install strongswan

Usage

Add something like this to /usr/local/etc/dhcpcd.exit-hook

case "$reason" in
BOUND6|RENEW6|REBIND6|REBOOT6|INFORM6)
    prefix=$(printenv new_dhcp6_ia_pd1_prefix1)
    /usr/local/sbin/dynamicprefixvici -p "$prefix" -n "global-ipv6"
esac