This confused me recently with an ALIX with two NICS when I added a mini-PCI wireless adapter. So this may be what you are seeing.
By default, Monowall 1.3 will only show two default interfaces (LAN and WAN) in the Interface menu (assumes you are using https):
https://<monowall-ip>/interfaces_assign.php
If you have more than two network interfaces, you need to click the plus sign at the bottom right of the interfaces list to add the other interfaces (one at a time). This little icon is easy to miss. This allows you to select which interface to add. For a Net4501 with three NICs, this would be the sis2 interface as OPT1.
This is explained in the docs here for wireless, but this issue probably also applies to regular NICS:
http://doc.m0n0.ch/handbook/wireless.html#id11638673Earlier versions of monowall were available targeted for specific hardware (Soekris Net4501, Soekris Net4801, and Alix PC Engines). I suspect that these older versions assumed what the base hardware was and automatically added the OPT1 interface as an unconfigured interface in the monowall config file. The new 1.3 monowall embedded image (and later 1.3 beta images) are designed to be generic embedded images (not specific for Soekris or Alix boards). So they don't automatically know if you want more than the two interfaces for LAN and WAN (the bare minimum for a firewall) and don't bother to add the OPTx interfaces. You have to add any other interfaces manually in the WebGui and then reboot. Once you have added interfaces, then you can configure these OPTx interfaces as you please.
Probably part of the above Wireless interfaces docs now need to be repeated in the regular Interfaces section of the docs for systems with more than 2 physical interfaces installed.
It is also possible that the monowall boot up scripts for embedded devices could be modified to determine all of the network interfaces available and set the OPTx interfaces as unconfigured but available so they would show in the Interfaces menu.
You can verify what FreeBSD sees as devices during boot by using the exec.php script:
https://<monowall.ip/exec.php
This brings up a window where you can execute any shell command. In this case, the command to use:
dmesg
This is effectively the same as the following command:
cat /var/log/dmesg.boot
This prints out the FreeBSD log file created during the boot process. You would look for the sis0 (LAN by default in monowall), sis1 (WAN, by default in monowall), and sis2(potential OPT1) devices in the log. If they are there, then FreeBSD and monowall found them during the boot process and you just need to add them as usable interfaces in the monowall config file via the Monowall Interfaces menu).
-msbaker