When I try to ping my opnsense firewall using Linux traceroute (UDP), the firewall does not respond. However, I am able to traceroute things behind the firewall successfully, and those also show the firewall in the route. I added a WAN and LAN rule to allow anyone to reach ports 33434-33534 with UDP for that to work. I can also reach the firewall if I use traceroute with ICMP. What do I need to do to make the firewall respond to the UDP traceroute?
You've opened ports on the WAN side to get traceroute to work? That seems ill advised.
What's your traceroute command line?
You're executing it from your LAN, right?