Handling non-routable addresses from elsewhere

Even with a properly configured gateway to handle network address translation for your own network, you may find yourself in the unenviable position of having to compensate for other people's misconfigurations.

One depressingly common class of misconfigurations is the kind which lets traffic with non-routable addresses out to the Internet. Traffic from non-routeable addresses have also played a part in several DOS attack techniques, so it may be worth considering explicitly blocking traffic from non-routeable addresses from entering your network.

One possible solution is the one outlined below, which for good measure also blocks any attempt to initiate contact to non-routable addresses through the gateway's external interface:

martians = "{,,, \
    ,,, \
    , }"

block drop in quick on $ext_if from $martians to any
block drop out quick on $ext_if from any to $martians

Here, the martians macro denotes the RFC 1918 addresses and a few other ranges which are mandated by various RFCs not to be in circulation on the open Internet. Traffic to and from such addresses is quietly dropped on the gateway's external interface.

The specific details of how to implement this kind of protection will vary, among other things according to your specific network configuration. Your network design could for example dictate that you include or exclude other address ranges than these.

This completes our simple NATing firewall for a small local network.