Книга: Iptables Tutorial 1.2.2

Limit match

Limit match

The limit match extension must be loaded explicitly with the -m limit option. This match can, for example, be used to advantage to give limited logging of specific rules etc. For example, you could use this to match all packets that do not exceed a given value, and after this value has been exceeded, limit logging of the event in question. Think of a time limit: You could limit how many times a certain rule may be matched in a certain time frame, for example to lessen the effects of DoS syn flood attacks. This is its main usage, but there are more usages, of course. The limit match may also be inverted by adding a ! flag in front of the limit match. It would then be expressed as -m limit ! --limit 5/s.This means that all packets will be matched after they have broken the limit.

To further explain the limit match, it is basically a token bucket filter. Consider having a leaky bucket where the bucket leaks X packets per time-unit. X is defined depending on how many matching packets we get, so if we get 3 packets, the bucket leaks 3 packets per that time-unit. The --limit option tells us how many packets to refill the bucket with per time-unit, while the --limit-burst option tells us how big the bucket is in the first place. So, setting --limit 3/minute --limit-burst 5, and then receiving 5 matches will empty the bucket. After 20 seconds, the bucket is refilled with another token, and so on until the --limit-burst is reached again or until they get used.

Consider the example below for further explanation of how this may look.

We set a rule with -m limit --limit 5/second --limit-burst 10/second. The limit-burst token bucket is set to 10 initially. Each packet that matches the rule uses a token.

We get packet that matches, 1-2-3-4-5-6-7-8-9-10, all within a 1/1000 of a second.

The token bucket is now empty. Once the token bucket is empty, the packets that qualify for the rule otherwise no longer match the rule and proceed to the next rule if any, or hit the chain policy.

For each 1/5 s without a matching packet, the token count goes up by 1, upto a maximum of 10. 1 second after receiving the 10 packets, we will once again have 5 tokens left.

And of course, the bucket will be emptied by 1 token for each packet it receives.

Table 10-20. Limit match options

Match --limit
Kernel 2.3, 2.4, 2.5 and 2.6
Example iptables -A INPUT -m limit --limit 3/hour
Explanation This sets the maximum average match rate for the limit match. You specify it with a number and an optional time unit. The following time units are currently recognized: /second /minute /hour /day. The default value here is 3 per hour, or 3/hour. This tells the limit match how many times to allow the match to occur per time unit (e.g. per minute).
Match --limit-burst
Kernel 2.3, 2.4, 2.5 and 2.6
Example iptables -A INPUT -m limit --limit-burst 5
Explanation This is the setting for the burst limit of the limit match. It tells iptables the maximum number of tokens available in the bucket when we start, or when the bucket is full. This number gets decremented by one for every packet that arrives, down to the lowest possible value, 1. The bucket will be refilled by the limit value every time unit, as specified by the --limit option. The default --limit-burst value is 5. For a simple way of checking out how this works, you can use the example Limit-match.txt one-rule-script. Using this script, you can see for yourself how the limit rule works, by simply sending ping packets at different intervals and in different burst numbers. All echo replies will be blocked when the burst value has been exceeded, and then be refilled by the limit value every second.

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