SSH hardening with Breakinguard
As proven by the logs generated by Kippo honeypot sensors have shown, attacks against SSH services are regularly seen in the wild. Even if you follow best practices for securing the service, the malicious scans will utilise resources available to your environment; CPU, bandwidth etc. In sufficient volume legitimate operation may be impacted as the server rejects failed login attempts.
This is where utilities like Breakinguard come into their own. Basically Breakinguard monitors log files for signs of malicious activity, and once a single source has triggered enough alerts blocks all connections from the source location. Other utilities (most notably fail2ban) perform the same activities, but I’m partial to Breakinguard due to it’s small size and simple configuration (and from knowing the author 😉 ).
Installation is straightforward, and for the most part automated. Once downloaded and extracted installation is handled by the configure script. On Debian based systems this will install the pre-requisite Perl modules and transfer the utilities components to the standard locations:
- breakinguard script – /usr/local/sbin/breakinguard
- config – /etc/breakinguard.conf
- init script – /etc/init.d/breakinguard
Once installed you need to edit your configuration. The breakinguard.conf file is fairly self explanatory, I normally edit:
- $alert_email -> set to the email address that you want to receive notifications of blocked attacks. On a publicly accessible system these alerts can be high volume, you may want to use a specific email account or at least setup some auto-move rules in your email client to avoid your inbox being spammed.
- $number_of_attempts -> This specifies the number of malicious log entries need to be generated by a specific IP address before the source is blocked. Due to the timing of the Breakinguard route this isn’t always an exact science, the default of 5 does a good job of avoid false positives whilst still blocking an attack in it’s infancy.
- $log_to_watch -> selects the logfile to monitor for signs of malicious activity. /var/log/auth.log is the obvious choice on a Debian based system.
- @safe_ips -> This array allows you to specify a number of trusted networks that will not be blocked, regardless of the number of times the they trigger alerts in the logs. This is useful for insuring that you don’t get locked out of your own systems in the event your keyboard ‘breaks’. On higher end systems with hardware remote management systems (iLo, DRAC, etc.) or virtual systems that provide remote access to the ‘physical’ console I leave this list to local subnet only and use the alternative options to access the server if I do lock myself out.
- $DEBUG -> set to 1 by default, this runs the utility in test mode without actually blocking malicious sources, perfect for testing configuration before going live. Once you’re good to set set $DEBUG to 0 and wait for the attacks to start. Example testing in debug mode is below:
Blocking and unblocking of malicious sources is handled via iptables. Once the $number_of_attempts limit is hit Breakinguard will run the $block_command (configurable in /etc/breakinguard.conf) which by default is ‘/sbin/iptables -I INPUT -s %s -j DROP’, with %s being replaced with the attacking IP. After a configurable timeout ($block_length), the $unblock_command removes the restriction.
You can see the IP addresses currently blocked as they are listed in /var/run/breakinguard/, alternatively listing the current iptables configuration will show sources currently being blocked, for example:
Download Breakinguard here