Sshguard

is an intrusion prevention system that parses server logs, determines malicious activity, and uses the system firewall to block the IP addresses of malicious connections. sshguard is written in C so it does not tax an interpreter.

How it works
sshguard is a simple daemon that continuously tracks one or more log files. It parses the log events that daemons send out in case of failed login attempts and then blocks any further attempts from those connections by updating the system's firewall.

Unlike what the name implies, sshguard does not only parse SSH logs. It also supports many mail systems as well as a few FTP ones. A full listing of supported services can be found on the sshguard.net website.

Emerge
Install :

Also make sure that is installed and used as the system firewall. At the time of writing, sshguard does not yet support.

More information about using and configuring IPtables can also be found on the IPtables article.

Preparing the firewall
When sshguard blocks any malicious users (by blocking their IP addresses), it will use the chain.

Prepare the chain, and make sure it is also triggered when new incoming connections are detected:

Watching logfiles
The basic idea behind sshguard is that the administrator passes on the log file(s) to watch as options to the application - there is no native sshguard configuration file.

On Gentoo, the options can be best configured in the file:

Make sure that the log files are accessible for the runtime user that sshguard uses.

OpenRC
Have sshguard be started by default by adding it to the default runlevel, and then start it:

Blacklisting hosts
With the blacklisting option after a number of abuses the IP address of the attacker will be blocked permanently. The blacklist will be loaded at each startup and extended with new entries during operation. inserts a new address after it exceeded a threshold of abuses.

Blacklisted addresses are never scheduled to be released (allowed) again.

The  command line option enables blacklisting and requires a filename to use for permanent storage of the blacklist. An optional threshold is configurable within the same option. This threshold is then first, after which the filename is mentioned separated by.

To enable blacklisting, create an appropriate directory and file:

Add the blacklist file to the configuration and alter the SSHGUARD_OPTS variable:

Restart the daemon to have the changes take effect:

File '/var/log/auth.log' vanished while adding!
When starting up, sshguard reports the following error:

Such an error (the file path itself can be different) occurs when the target file is not available on the system. Make sure that it is created, or update the sshguard configuration to not add it for monitoring.

On a syslog-ng system with OpenRC, the following addition to can suffice:

Reload the configuration for the changes to take effect:

External resources
The sshguard documentation provides all the information needed to further tune the application.