You are most likely using Netfilter, aka IPTables, if you are running a recent Linux distro. Most firewalls supplied with hosting control panels or simple setup scripts fail to take advantage of some of the more advanced features available. A feature that can used to combat SSH brute force attacks is a module that remembers recent connections. By tracking recent connections SSH’s port, you can begin to block IP addresses based on the rate at which they connect to SSH. By usingIPTables to rate-limit connections, you can mitigate SSH brute force attacks without the mess of third party software or having to deal with ever growing ban lists.

The rules are relatively simple.

This rule will block an IP if it attempts more than 3 connections per minute. Notice that the state is set to NEW. This means only new connections not established ones are impacted. Established connections are the result of a successful SSH authentication, so users who authenticate properly will not be blocked. The Debian Administration site has more details on how to rate-limit connections using IPTables.

If you need to see what’s being done, you may want to log these drops. You can do so by setting up a log rule and then using these rules instead.

Notice that I’ve changed the rule from DROP to LOGDROP. This way your drops will get logged and you can see the results in your logs:

Effectiveness

I always try to get a sense of effectiveness of any tool or configuration we deploy. I find many “security” tools that are popular amongst the web hosting crowd provide little to no value. In many cases, appropriate configuration of your server or web application could achieve similar results without the hassle of maintaining a third party product.

Are the IPTables rules effective? In short yes.

During a recent attack on a server, the SSH service remained fully accessible with no service interruption.

Previously such aggressive attack would have caused service interruptions. So on the service side, this approach works. When I dug into the logs, I found three failed user attempts against SSH prior to the rate-limiting kicked in. The attack then sent 67 more attempts before it gave up.

Pros and Cons

The benefit of this approach is you don’t need any added software. IPtables is likely sitting on your server already if not already in use. There are no cumbersome ban lists to clear out or fix. The rate of false positives should be low as legitimate user should not trip the limits. If you have some automation tools tripping the limits, you could white list these by allowing them before the rate-limiting rule or raise the limits.

One of the drawbacks is that this approach does not lock accounts. A slow, distributed attack could fall under the radar. If it was a directed attack against a specific user account, the attacker could churn away for days or weeks without detection. For that, you would need something that can lock user accounts after failures. PAM includes a module called pam_tally that does just this. If you fail too many times, an account is locked.

Stay Tuned

In addition to these IPTables settings, there are some things you can do withinSSH itself to harden SSH from attacks.