I ran into a weird problem where I couldn’t access the OPNsense web UI using an OPT1 interface. The fix is easy, but it can be very confusing and it’s all caused by an unintuitive default.


If you can’t access the web UI via an OPT1 interface, try disabling the “reply-to” feature: https://docs.opnsense.org/manual/interfaces.html

I’ve been playing around with OPNsense inside a VM as I prepare to replace my current pfSense router which is no longer fast enough. I have a quad network card which is passed through at the hardware level and, in addition, I have the virtual network interface provided by the hypervisor, xcp-ng.

I thought it would be a nice idea to set the router up via the xcp-ng interface. That would mean I could configure it entirely before swapping out my old router. In particular I’d enable a DHCP server and set the IP address of the LAN interface to the same as my current router without having to segregate it from my network.

So I set two of the physical interface to LAN and WAN, as you do, and enabled the xcp-ng interface as the “OPT1” interface. OPT1 was set to be configured via DHCP so it would just be a regular device on my network. I then added a firewall rule enabling access to the web UI via OPT1.

But it didn’t work.

I searched around a bit online and it seems loads of people have the same, or similar, problems with accessing the web UI over OPT1. What was frustrating is there didn’t appear to be any answers. Most people suggested adding firewall rules, but I had already done this; the traffic wasn’t getting blocked.

The problem

It turns out that any interface with an upstream gateway is considered a WAN-type interface by OPNsense (and pfSense). This is the case when configuring via DHCP. The upstream gateway is my network’s router. This is the first thing I hadn’t fully understood.

The second thing is that both OPNsense and pfSense set a rather strange rule on any WAN-type interface. This is apparently a very old default and unlikely to change, but it can also be useful in some situations. Essentially, WAN-type interfaces reply to any traffic, no matter where it comes from, via the upstream gateway. This is called “reply-to”.

I’m still a little unsure about what exactly was happening. At one point while I was troubleshooting I had two interfaces plugged into my network, both configured via DHCP. I saw packets entering the OPT1 interface then leaving to my network’s router via the other interface! Whatever happened, though, the replies were not making it back to my PC.

The solution

The solution is to disable reply-to somehow. This can be done globally or with firewall rules. Now that I understand the problem, I went for the latter. It’s really quite simple in the end. Make a rule on OPT1 with the following settings:

  • Action: Pass
  • Direction: in
  • TCP/IP: IPv4+IPv6
  • Protocol: TCP
  • Source: any
  • Destination: This Firewall
  • Ports: HTTP
  • Reply-to: Disable (the most important; under advanced)

This is similar to the anti-lockout rule that would be added to the LAN interface, but OPNsense doesn’t add it to OPT1.

Hope this helps someone out of a similar situation! Happy routing!