Let's dive a bit deeper into how Fail2Ban works to monitor log files and take action against IPs that exceed a specified number of login failures.
1. Monitoring log files:
Fail2Ban continuously monitors log files for specific services or applications. These log files contain information about various activities, including successful and unsuccessful login attempts, errors, and other events.
For our example, Fail2ban are monitoring SSH login attempts, it will check the log file specified in the configuration (e.g., /var/log/auth.log) for entries related to SSH authentication.
2. Fail2Ban filters:
Filters in Fail2Ban are used to define patterns that indicate malicious or suspicious activity in log files. Each service has its own filter, typically defined in the /etc/fail2ban/filter.d/ directory.
For SSH, the default filter is often located at /etc/fail2ban/filter.d/sshd.conf. This filter defines regular expressions that match patterns in the SSH log file indicating failed login attempts.
3. Matching patterns:
Fail2Ban scans the log files using the specified filter and looks for patterns that match failed login attempts or other predefined criteria. When a match is found, Fail2Ban increments the failure count for the corresponding IP address.
4. Banning mechanism:
The maxretry option in the Fail2Ban configuration defines the threshold for the number of failed login attempts allowed before taking action. Once an IP address exceeds this threshold, Fail2Ban takes action to block the IP.
The action taken could involve adding a rule to the system's firewall (e.g., iptables or firewalld) to reject traffic from the offending IP address. This is what is commonly referred to as "jailing" an IP.
5. Unbanning:
After a certain amount of time, specified by the bantime option in the Fail2Ban configuration, the ban is lifted, and the IP address is allowed to attempt connections again. This helps prevent a permanent block for a legitimate user who may have mistyped their password a few times.
Overall, Fail2Ban provides a flexible and configurable way to enhance the security of your system by automatically responding to and mitigating potential security threats based on patterns found in log files.