linux-hardening
Apply CIS benchmarks and secure Linux servers. Configure SSH, manage users, implement firewall rules, and enable security features. Use when hardening Linux systems for production or meeting security compliance requirements.
What this skill does
# Linux Hardening Secure Linux servers following CIS benchmarks and security best practices. ## When to Use This Skill Use this skill when: - Hardening production servers - Meeting compliance requirements - Implementing security baselines - Configuring secure SSH access ## SSH Hardening ```bash # /etc/ssh/sshd_config PermitRootLogin no PasswordAuthentication no PubkeyAuthentication yes MaxAuthTries 3 ClientAliveInterval 300 ClientAliveCountMax 2 AllowUsers deploy admin Protocol 2 ``` ## User Security ```bash # Password policy sudo apt install libpam-pwquality # /etc/security/pwquality.conf minlen = 14 dcredit = -1 ucredit = -1 ocredit = -1 lcredit = -1 # Lock inactive accounts useradd -D -f 30 # Audit sudo usage echo "Defaults logfile=/var/log/sudo.log" >> /etc/sudoers ``` ## Firewall Configuration ```bash # UFW setup ufw default deny incoming ufw default allow outgoing ufw allow ssh ufw allow 443/tcp ufw enable # Or iptables iptables -P INPUT DROP iptables -P FORWARD DROP iptables -A INPUT -i lo -j ACCEPT iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p tcp --dport 22 -j ACCEPT ``` ## Kernel Hardening ```bash # /etc/sysctl.d/99-security.conf net.ipv4.conf.all.send_redirects = 0 net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.all.accept_source_route = 0 net.ipv4.icmp_echo_ignore_broadcasts = 1 kernel.randomize_va_space = 2 fs.suid_dumpable = 0 # Apply sysctl -p ``` ## File Permissions ```bash # Critical files chmod 600 /etc/shadow chmod 644 /etc/passwd chmod 700 /root chmod 600 /etc/ssh/sshd_config # Find world-writable files find / -type f -perm -0002 -ls # Find SUID files find / -perm -4000 -type f -ls ``` ## Audit Configuration ```bash # Install auditd apt install auditd # /etc/audit/rules.d/audit.rules -w /etc/passwd -p wa -k identity -w /etc/shadow -p wa -k identity -w /etc/sudoers -p wa -k actions -a always,exit -F arch=b64 -S execve -k exec ``` ## Best Practices - Disable unused services - Keep system updated - Use fail2ban for intrusion prevention - Enable SELinux/AppArmor - Regular security audits - Monitor log files - Implement least privilege ## Related Skills - [cis-benchmarks](../cis-benchmarks/) - Compliance scanning - [firewall-config](../../network/firewall-config/) - Firewall rules
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