15.10. How do I setup multiple IP addresses on the WAN interface?

Although the m0n0wall webGUI only allows setting up a single IP address on the WAN interface, you can still have m0n0wall accept packets destined to secondary IP addresses. It is not necessary to tell m0n0wall to use these IP addresses on the WAN interface (however in some cases proxy ARP has to be used - see below), but you have to tell it what to do with packets that are sent to them. There are two possibilities:

15.10.1. Proxy ARP

If any of the following applies to your setup, you should be fine without proxy ARP:

  • the additional IP addresses that you're trying to use are part of a subnet that is routed to you by your ISP (i.e. your ISP has a static route for that subnet with your m0n0wall's WAN IP address as the gateway)

  • you're using PPPoE or PPTP on WAN

Using proxy ARP under these conditions will not achieve anything. If however you use static IP addresses or DHCP on WAN and don't have a routed subnet, adding proxy ARP entries for the additional addresses/ranges/subnets in the webGUI will make sure that m0n0wall responds to ARP queries for these addresses on the WAN interface.

Adding Proxy ARP when it is not required usually will not hurt anything, so when in doubt, add it!


Do not add Proxy ARP entries for IP addresses that are not assigned to you! Most DHCP servers will attempt to do an ARP query before assigning an IP address to a client, and if you enable Proxy ARP on IP's that are not yours, they will appear to be in use to the DHCP server. We have heard of instances where people enabled Proxy ARP for their entire WAN subnet, and got disconnected because they were "taking up all the DHCP addresses." Technically you aren't taking all the leases, you're just answering ARP on all of them which is just as bad. This is typically only an issue when your WAN is an Ethernet network, but don't ever do it.

Note that it is never necessary (and strongly discouraged) to use IP aliasing on the WAN interface (by means of ifconfig commands).