Default Linux installations prioritize compatibility over security. For production servers exposed to the internet, "hardening"—the process of reducing the attack surface—is mandatory, not optional.
User and Access Management
The root account is the primary target for attackers. Step one in any hardening process is creating a sudo-enabled user and disabling direct root login. This forces accountability and adds a layer of protection against brute-force attacks targeting the known "root" username.
SSH keys should replace password capabilities entirely. Passwords can be guessed or brute-forced; properly generated 4096-bit RSA or Ed25519 keys are practically impregnable. Combined with Fail2Ban to ban IPs showing malicious login patterns, this virtually eliminates unauthorized access attempts.
Network Defense
A firewall should deny all incoming traffic by default, allowing only specifically required ports (typically 80, 443, and your customized SSH port). UFW (Uncomplicated Firewall) on Ubuntu/Debian or Firewalld on RHEL/CentOS simplifies this configuration.
Disable unused network services. If a server doesn't need to send email, disable Postfix. If it's not a file server, remove Samba. Every running service is a potential point of entry; if it's not needed, it shouldn't exist.
System Updates and Patches
Vulnerabilities are discovered daily. Automated security updates (unattended-upgrades on Debian/Ubuntu) ensure critical patches are applied immediately without human intervention. For kernel updates that require reboots, tools like Canonical's Livepatch can apply fixes to running kernels, minimizing downtime.
Hardening is a continuous process. Regular audits, log monitoring, and staying informed about new vulnerabilities are as important as the initial configuration.