What is Fail2Ban technology and how to use it to protect a public VPN or proxy server from brute-force attacks and port scanning?

Publicly available virtual private network and proxy servers hosted on rented VPS are constantly subjected to automated scanning by bots looking for vulnerabilities and password guessing attempts for unauthorized access. To automatically protect the infrastructure from such threats, the Fail2Ban system log monitoring service is used.

The program works by analyzing text log files of the operating system and running services for suspicious activity. If a series of failed authentication attempts or an abnormal number of requests in a short period of time is recorded from a specific IP address, Fail2Ban dynamically adds a rule to the system firewall, temporarily or permanently blocking access from that address.

To protect an OpenVPN- or IPsec-based VPN server, the administrator needs to configure a corresponding filter in the Fail2Ban configuration file, specifying the path to the service log file and a regular expression to search for authorization error strings. The maximum number of failed attempts and the IP blocking time are also set.

Regular monitoring of system status and proper configuration of firewall rules can significantly reduce the load on the server from parasitic traffic and prevent the compromise of user credentials, which is a critical aspect of administering any network services.