OPNsense is a software firewall system based on FreeBSD developed by Deciso B.V. It originally started as a fork of pfsense but is developed independently based on upstream FreeBSD changes.
OPNsense can support DS-Lite tunnels with manual creation of the tunnel and route. The MTU should be set to 1460.
ifconfig gif0 create ifconfig gif0 inet6 tunnel <<LOCAL IPv6 ADDRESS>> <<AFTR ADDRESS>> mtu 1460 -accept_rtadv ifdisabled ifconfig gif0 inet 192.0.0.2 192.0.0.1 netmask 255.255.255.248 route add default -interface gif0