I logged into a newly provisioned VM recently to see a message that there had been 86,876 failed login attempts from a domain in another country.

 

That got me to looking for best practices on securing Linux against attacks, and looking for help from RackSpace (one of the better Cloud hosting providers). 

As an FYI, you can use the lastb command to find the failed logins, and with a little piping you can figure out the top offenders:

In my case this showed the following IP addresses as probing my SSH:

You can dig or nslookup to try and figure out where these hosts are, but mostly they are just bots that don’t follow the rules in the first place.

The RackSpace support guys pointed me to a few good documents:

• http://www.rackspace.com/knowledge_center/article/rackspace-cloud-essentials-basic-cloud-server-security
• http://www.rackspace.com/knowledge_center/article/linux-cloud-server-security-best-practices
• http://www.rackspace.com/knowledge_center/article/fail2ban

I decided to install fail2ban as the first step in this process, since what it does is block IP addresses temporarily when they are doing things that look like attacks. To install (assuming you have the right yum repos set up), you just run the command:

Once it is installed, you just enable the service, and the default configuration protects you from a number of attacks (including the SSH brute force login hack I was seeing).

It’s a good idea to update the jail.conf parameters that send email, so that you’ll get notified on these sorts of failures. That way you know when somebody gets locked out, and can see if it is an actual attack or just a forgotten password.

Testing is pretty simple: just fail logging in a few times in a row, and you should get a “connection refused” on the fourth try. You can try again in a few minutes, since fail2ban only adds the rule to the firewall temporarily (I believe it is 5 failed logins will block for 5 minutes by default).

Next I needed to add a rule that is not included with fail2ban directly: I wanted to do the same thing for my WordPress site (I actually had some lame hacker sneak into a WP installation a year or so ago, and had to rebuild the whole server to get rid of the nefarious code).

Whipping out my trusty Google search, I found a few different ways to watch the WP logins, and settled on using a simple regex.

First I created a filter in the /etc/fail2ban/filter.d folder (naming it apache-wp-login.conf):

Next I created a jail.local file in the /etc/fail2ban folder:

The above rule tells fail2ban to watch the system access logs for repeated logins, and will block the IP after 3 successive failures within 5 minutes. So if somebody tries to login (which for my sites is only used for content authors and admins) and brute force the site, they will get blocked for 5 minutes.