Hit a little bit of a roadblock on this, on both IPCop and SmoothWall theres an option to turn off ICMP on RED (internet). No such luck on IPFire for some reason.

Anyways, long story short:

1) Login to the router

2)  cd /etc/sysconfig

3) find the firewall.local file and open it in your preferred editor

4) Add the following in the start section (after the ## but before the ;;)

iptables -A CUSTOMINPUT -p icmp -i red0 –icmp-type 0 -j ACCEPT
iptables -A CUSTOMINPUT -p icmp -i red0 –icmp-type 3 -j ACCEPT
iptables -A CUSTOMINPUT -p icmp -i red0 –icmp-type 11 -j ACCEPT
iptables -A CUSTOMINPUT -p icmp -i red0 –icmp-type 8 -m limit –limit 1/second -j DROP
iptables -A CUSTOMINPUT -p icmp -i red0 -j DROP

5) Save it, reboot.

6) No more pinging on the red interface 🙂