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Mike B
Asked: 2013-03-07 14:09:03 +0800 CST

What exactly do --limit 1/s and --limit-burst mean in iptables rules?

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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?

centos

3 Answers

  1. Best Answer

    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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  2. --limit: Specifies the rate at what tokens get refilled into the bucket. 4/hour means 4 tokens per hour (1 token every 15 minutes).

    --limit-burst: Specifies the maximum amount of tokens that can be filled in the bucket. (This is also the amount of tokens the bucket starts out with).

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  3. I read both answers, but the actual man itables-extensions made me grok it:

    limit

    This module matches at a limited rate using a token bucket filter. A rule using this extension will match until this limit is reached. It can be used in combination with the LOG target to give limited logging, for example.

    xt_limit has no negation support - you will have to use -m hashlimit ! --hashlimit rate in this case whilst omitting --hashlimit-mode.

    • --limit rate[/second|/minute|/hour|/day] -- Maximum average matching rate: specified as a number, with an optional /second, /minute, /hour, or /day suffix; the default is 3/hour.

    • --limit-burst number -- Maximum initial number of packets to match: this number gets recharged by one every time the limit specified above is not reached, up to this number; the default is 5.

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