1. OS-Level Hardening
🔹 Minimal Installation – Install only necessary packages. Avoid GUI on servers unless required.
🔹 Update Regularly – Apply security patches promptly using apt update && apt upgrade (Debian) or yum update (RHEL).
🔹 Disable Unused Services – Use systemctl disable --now <service> for unnecessary daemons.
🔹 Enable SELinux or AppArmor – Use setenforce 1 (SELinux) or aa-enforce (AppArmor) for MAC enforcement.
🔹 Filesystem Hardening – Mount /tmp, /var/tmp, and /home with noexec, nosuid, and nodev.
2. User & Authentication Security
🔹 Enforce Strong Passwords – Use PAM (pam_pwquality.so) and MFA for user authentication.
🔹 Disable Root Login – Set PermitRootLogin no in /etc/ssh/sshd_config.
🔹 Limit User Privileges – Implement sudo with least privilege and restrict shell access (chsh -s /sbin/nologin).
🔹 Use SSH Key Authentication – Replace password-based logins with key-based authentication (ssh-keygen -t ed25519).
3. Network & Firewall Hardening
🔹 Enable Firewall – Use ufw, firewalld, or iptables to restrict traffic. Example:
bashufw allow ssh
ufw allow 443/tcp
ufw enable
🔹 Disable Unused Ports – Check open ports using netstat -tulnp or ss -tulnp.
🔹 Enable Fail2Ban – Prevent brute-force attacks on SSH and web services.
🔹 Use VPN or Jump Hosts – Restrict direct server access by implementing WireGuard/OpenVPN and a bastion host.
🔹 Configure TCP Hardening – Add to /etc/sysctl.conf:
bashnet.ipv4.tcp_syncookies = 1
net.ipv4.conf.all.rp_filter = 1
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.conf.all.accept_source_route = 0
4. Application & Service Security
🔹 Use Web Application Firewall (WAF) – Deploy ModSecurity or Cloudflare for web-facing apps.
🔹 Run Services in Containers/Sandbox – Use Docker with minimal privileges (--no-new-privileges flag).
🔹 Enforce HTTPS Everywhere – Use Let’s Encrypt or a commercial CA for SSL/TLS.
🔹 Log & Monitor Everything – Use auditd, syslog, and SIEM solutions like ELK or Splunk.
5. Storage & Data Security
🔹 Encrypt Data at Rest – Use LUKS for full disk encryption or eCryptfs for home directories.
🔹 Secure Backups – Encrypt backups (gpg, openssl enc) and store them in secure locations.
🔹 Apply File Integrity Monitoring (FIM) – Use AIDE or Tripwire to detect unauthorized changes.
🔹 Use Immutable Logs – Mount logs with chattr +i to prevent tampering.
6. Advanced Security Techniques
🔹 Enable Kernel Hardening – Implement Grsecurity or Kernel Lockdown Mode.
🔹 Use Mandatory Access Controls (MAC) – SELinux/AppArmor profiles for service isolation.
🔹 Deploy IDS/IPS – Use Suricata or Snort for intrusion detection/prevention.
🔹 Implement SIEM/SOC Monitoring – Automate threat detection using OSSEC, Wazuh, or commercial solutions.
🔹 Automate Compliance Audits – Use OpenSCAP or Lynis for continuous compliance checking.