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While OPNsense has the necessary dhcp6c code path to allow your FiOS WAN interface IPv6 configuration to have both a link local (fe80:: prefix) and a global IPv6 address, it will not configure it that way by default (as of OPNsense 26.1.4 in March 2026).

You will need to also configure a custom dhcp6c client configuration file in [Interfaces->WAN->DHCPv6 Client Configuration, then select Override Configuration Mode]  Example screenshot:
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Example /etc/dhcp6c.WAN_LAN.conf below:

• ssh into opnsense and create this text file from scratch, then apply to the web gui configuration
• adjust interface names to match your system (e.g. em0 to igc0 {do not use OPNsense alias names like WAN})
• You will need to specify every interface you want a prefix assigned to in the config.  Example below include WAN, LAN, and a tagged vlan for GUEST_VLAN devices
• You can customize the sla-id per interface as desired (each interface must be unique for /64 out of PD provided /56)

Code Select Expand

interface em0 {
        send ia-pd 0;   # request prefix delegation
        request domain-name-servers;
        request domain-name;
        script "/var/etc/dhcp6c_wan_script.sh"; # we'd like some nameservers please
};
id-assoc pd 0 {
#  Support prefix 0 on LAN interface (em1)
        prefix ::/56 infinity;
        prefix-interface em1 {
                sla-id 0;
                sla-len 8;
        };
# Support prefix 1 on GUEST_VLAN interface (vlan0.20 with a parent of em1)
        prefix-interface vlan0.20 {
                sla-id 1;
                sla-len 8;
        };
# Support prefix 16 on WAN interface (em0) - requires patch to dhcp6c prefixconf.c to enable on pfSense, but works out of the box on OPNsense
        prefix-interface em0 {
                sla-id 16;
                sla-len 8;
        };
};