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I'm using CentOS 5.x trying to wrap my mind around the following iptables rule on one of my servers:

-A RH-Firewall-1-INPUT -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT

On another server I have:

-A RH-Firewall-1-INPUT -p icmp --icmp-type echo-request -m limit --limit 1/s --limit-burst 3 -j ACCEPT

I understand that both of these rules are designed to allow (and throttle) incoming ping requests but what is the limit-burst option about? And are these allowances on a per host basis? Or do they apply to any/all incoming ICMP connections at all?

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up vote 9 down vote accepted

The math is fully explained in the netfilter docs, but it's reasonable to say that the limit-burst argument specifies the number of matches that are allow through before the limit of 1 per second "kicks in". These two rules both apply only to ICMP echo request packets (incoming PING requests). These are not per-host limits and apply to anything the rule matches (which, in this case, would be all ICMP echo requests).

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So for the second rule, it will allow up to 3 icmp echo request packets (no matter how quickly they're received) and then only allow 1 icmp packet per second after that (regardless of which host the request is coming from)? At what point does iptables "reset" and allow a burst of up to three packets again? – Mike B Mar 6 '13 at 23:05
It's a token bucket. The bucket starts w/ 3 tokens in it, and a token is removed for each ICMP echo-request that is received. For each 1 second of no ICMP echo requests 1 token is added to the bucket until the bucket reaches 3 tokens again, where it stays until it receives more ICMP echo requests. – Evan Anderson Mar 6 '13 at 23:42
Awesome. That's exactly what I needed to know. Thank you sir. – Mike B Mar 7 '13 at 0:17

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