Configure IPv6 using dhcp6c for servers from online.net:

pkg install dhcp6

Transform the DUID into a binary file (needed for dhcp6c):

echo <DUID> | awk '{ gsub(":"," "); printf "0: 0a 00 %s\n", $0 }' | xxd -r > /var/db/dhcp6c_duid

Add this in /usr/local/etc/dhcp6c.conf:

id-assoc pd {
        prefix-interface igb0 {
        };
};

id-assoc na {
};

interface igb0 {
    send ia-pd 0;
    send ia-na 0;
};

Add this to /etc/rc.conf:

ifconfig_igb0_ipv6="inet6 -ifdisabled accept_rtadv up"
dhcp6c_enable="YES"
dhcp6c_interfaces="igb0"
rtsold_enable="YES"

source: https://angristan.xyz/configure-ipv6-freebsd-online-net-dedibox/