I have a SmoothWall Express box that is currently configured with a Green and Purple interface. Both interfaces are in the same /24 subnet (which seems odd to me). The green interface (address of .254) has a DHCP server that is pushing addresses from .1 to .100 and the purple interface (.253) is pushing addresses from .101 to .120. Every machine here is trusted, and as such is connected to the green interface via a wired connection or wireless APs. Nothing is connected at all (port is physically empty, traffic graphs show no activity) to the purple interface. However, every machine here is pulling addresses from the purple interface. So the question boils down to, how do I remove/stop my machines from pulling from the purple dhcp interface? Also, shouldn't the purple interface (if we were using it for guest Wifi or something) be on a different subnet (i.e. 192.168.100.0/24 instead of 192.168.1.0/24 with all the trusted machines)?
Keeping untrusted devices (i.e. guest wifi) separate from a trusted network is advised and putting them in a different sub-net would achieve this.
From what I understand, your devices are being assigned addresses from the DHCP server running on your purple interface (.253) and you'd prefer them to take addresses from the green interface (.254).
All your devices are connected to the green interface via a physical cable.
Now, I'm not an expert on Smoothwall, I've been tinkering with it for a few months now myself but I'm willing to try and help.
I'm not sure why they're taking addresses assigned by the purple interface. It sounds like they're actually just connected to the purple interface but, assuming this is not the case, can you try switching the addresses?
The purple interface would take on .254 and the green interface would take on .253
If the devices are just searching for a DHCP server and they take on the first one they come accross (.253 which is purple) then this might be the cause?
simplest way would be to login to SW box (https://IP.Add.Re.ss:441) on port 441 and turn DHCP OFF, on your 'purple' nic. If you are using Red, Green and Purple, I think you've set up the SW router to have your AP's connected to the separate nic 'purple'. Again, easiest, if you're not using it is to 'turn it off'.
Again, login > Networking (tab) > Interfaces > 'Purple' interface > (pull down)
Services (tab) > dhcp > select 'Purple' interface (pulldown) > 'disable' (uncheck enable)