SSH rate limit per IP. New method, SSH rate con...

SSH rate limit per IP. New method, SSH rate connect limiting per IP.

Ok, we have a new method of rate limiting.

This new method is WAYYY better.


the old -m limit method limits per packet per port.. if you have someone flooding your ssh connetion, it'll actually disable the service all around, causing YOU not to be able to get on. not really the effect you wanted.

This new method, actually bases its rate limit on a per IP basis.

So if you are getting flooded from 1 ip specifically, only that 1 ip will be locked down.

Everyone else will be able to get on still. as long as they stay within the connection limit itself.


I'll be honest. *I* didnt figure this out. i found it on a url




It does work, as a few linux-noob'ers helped me test it successfully.
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The way the recent module works is fairly straightforward, you basically add IP addresses to a list, which can then be used in the future to test connection attempts against. This allows you to limit the number of connections against either a number of seconds, or connection attempts. In our example we'll do both.

An example is probably the simplest way to illustrate how it works. The following two rules will limit incoming connections to port 22 to no more than 3 attemps in a minute - an more than that will be dropped:

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent \
--set

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent \
--update --seconds 60 --hitcount 4 -j DROP

The --state flag takes a comma seperated list of connection states as an argument, by using "--state NEW" as we did we make sure that only new connections are managed by the module.

The --set parameter in the first line will make sure that the IP address of the host which initiated the connection will be added to the "recent list", where it can be tested and used again in the future i.e. in our second rule.

The second rule is where the magic actually happens. The --update flag tests whether the IP address is in the list of recent connections, in our case each new connection on port 22 will be in the list because we used the --set flag to add it in the preceeding rule.

Once that's done the --seconds flag is used to make sure that the IP address is only going to match if the last connection was within the timeframe given. The --hitcount flag works in a similar way - matching only if the given count of connection attempts is greater than or equal to the number given.

Together the second line will DROP an incoming connection if:

* The IP address which initiated the connection has previously been added to the list and
* The IP address has sent a packet in the past 60 seconds and
* The IP address has sent more than 4 packets in total.

You can adjust the numbers yourself to limit connections further, so the following example will drop incoming connections which make more than 2 connection attempts upon port 22 within ten minutes:

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent \
--set

iptables -I INPUT -p tcp --dport 22 -i eth0 -m state --state NEW -m recent \
--update --seconds 600 --hitcount 2 -j DROP

If you wish to test these rules you can script a number of connection attempts from an external host with the netcat package.

The following script attempts to connect to the IP address 192.168.1.1 5 times. The first couple of attempts you should see a welcome banner such as "SSH-2.0-OpenSSH_3.8.1p1 Debian-8.sarge.4" - after that the script will hang as it's packets are dropped and no response is sent:

#!/bin/bash

for i in `seq 1 5` ; do
echo 'exit' | nc 192.168.1.1 22 ;
done

There's a lot of documentation on the netfilter/iptables firewall, and it's available modules which you can find in the Netfilter Extension HOWTO.

This HOWTO contains documentation on many different modules, along with examples. A recommended read if you're interested in Linux firewalling.

If you wish to experiment with rules and testing it's worth remembering how to remove all active rules. The following commands will flush your iptables filewall, and remove all currently active rules:

iptables -F
iptables -X