You buy a nice new firewall appliance. You install OpnSense on it, set all the WAN and LAN stuff up to match your existing firewall, and you drop it into place. WTF, no internet…?
First of all, if you’re using a cable ISP, remember that most cable modems are MAC address locked, and will refuse to talk to a new MAC address if they’ve already seen a different one connected. So, remember to FULLY power-cycle your cable modem. Buttons won’t cut it, in many cases—you gotta unplug the power cable out of that sucker, give it a count of five to think about its sins, then plug it back in and let it re-sync.
If you still don’t have any internets after power-cycling and your modem showing everything sync’ed and online, you may be falling afoul of a weirdness in OpnSense’s default gateway configs. By default, it will mark a gateway as “down” if it doesn’t return pings… but many ISP gateway addresses (not the WAN address your router gets, the one just upstream of it) don’t return pings. So, OpnSense reports it as down and refuses to even try slinging packets through it.
To fix this, go to System–>Gateways–>Single and select your WANGW gateway for editing. Now scroll down, find “Disable Gateway monitoring” and give that sucker a checkmark. Once you click “Save”, you should now see your gateway green and online, and packets should start flowing.