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I just did a test that told me my gateway (debian squeeze) responds when being pinged from outside.

It's a box with two network cards, eth0 is for my internal network (192.168.1.0/24) and eth1 goes to "the internet" (as ppp0).

Making the gateway invisible when being pinged randomally would increase the safety.

What needs to be configured in order to make ppp0 (eth1) invisible, while eth0 still remains open?

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What do you want do do that for? Your system is certainly very much visible by all other network activity it participates in. – vonbrand Feb 25 at 0:50
Realistically, it won't increase safety - ICMP is typically not the most useful tool attackers use, as it is often blocked at firewalls; instead they scan for specific ports and specific banners/headers being returned, as this identifies vulnerable services which the firewall is allowing through. – Rory Alsop Feb 25 at 9:13

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You could achieve this by adding the following iptables rule that effectively drops the incoming ECHO requests on any ppp device:

iptables -A INPUT -p icmp --icmp-type 8 -i ppp+ -j DROP

This rule should be added before any rule that allows the ICMP traffic. I would not recommend you to disable all ICMP traffic as it may break the connectivity.

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Wouldn't the wildcard for the interface have to be a + instead of a *, i.e. ppp+? -- Your sample line works for me if I use +, could it be it depends on the distribution? – zebonaut Mar 4 at 8:46
@zebonaut yes, sorry. this is typo. I fixed the answer – Serge Mar 4 at 13:42

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