Securing Alpine Linux

Securing Alpine Linux using Security Technical Implementation Guides (STIGs) involves several steps. STIGs are a series of security requirements and configurations that help to secure systems. While there might not be a specific STIG for Alpine Linux, you can follow general Linux hardening guidelines and apply the principles from other Linux STIGs. Here’s a step-by-step process:

1. 1. 1. Step 1: Update and Upgrade System

1. **Update package lists:**

  ```sh
  sudo apk update
  ```

2. **Upgrade installed packages:**

  ```sh
  sudo apk upgrade
  ```

1. 1. 1. Step 2: Install Necessary Security Tools

1. **Install `audit` package:**

  ```sh
  sudo apk add audit
  ```

2. **Install other necessary security packages:**

  ```sh
  sudo apk add sudo logrotate bash-completion openssh-server
  ```

1. 1. 1. Step 3: User and Access Management

1. **Disable root login over SSH:**

  Edit `/etc/ssh/sshd_config`:
  ```sh
  sudo vi /etc/ssh/sshd_config
  ```
  Set the following parameter:
  ```sh
  PermitRootLogin no
  ```

2. **Ensure password complexity:**

  Edit `/etc/security/pwquality.conf`:
  ```sh
  sudo vi /etc/security/pwquality.conf
  ```
  Add or update the following lines:
  ```sh
  minlen = 14
  dcredit = -1
  ucredit = -1
  ocredit = -1
  lcredit = -1
  ```

3. **Lock unused system accounts:**

  ```sh
  for user in `awk -F: '($3 < 1000) {print $1}' /etc/passwd`; do
      if [ $user != "root" ]; then
          sudo passwd -l $user
          sudo chage -E 0 $user
      fi
  done
  ```

1. 1. 1. Step 4: File System and Directory Permissions

1. **Set appropriate permissions on important directories:**

  ```sh
  sudo chmod 700 /root
  sudo chmod 600 /boot/grub/grub.cfg
  sudo chmod 600 /etc/ssh/sshd_config
  ```

2. **Configure mount options:**

  Edit `/etc/fstab`:
  ```sh
  sudo vi /etc/fstab
  ```
  Add `nosuid`, `nodev`, and `noexec` options to non-root partitions:
  ```sh
  /dev/sda1 /home ext4 defaults,nosuid,nodev,noexec 0 2
  ```

1. 1. 1. Step 5: Network Security

1. **Disable unnecessary services:**

  ```sh
  sudo rc-update del <service_name>
  sudo rc-service <service_name> stop
  ```

2. **Configure firewall (iptables):**

  ```sh
  sudo apk add iptables
  sudo rc-service iptables start
  sudo rc-update add iptables
  ```

  Create a basic firewall ruleset:
  ```sh
  sudo vi /etc/iptables/rules.v4
  ```
  Example rules:
  ```sh
  *filter
  :INPUT DROP [0:0]
  :FORWARD DROP [0:0]
  :OUTPUT ACCEPT [0:0]
  -A INPUT -i lo -j ACCEPT
  -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
  -A INPUT -p tcp --dport 22 -j ACCEPT
  COMMIT
  ```

1. 1. 1. Step 6: Logging and Auditing

1. **Configure system logging:**

  Edit `/etc/rsyslog.conf` to ensure all log files are being captured:
  ```sh
  sudo vi /etc/rsyslog.conf
  ```
  Example configuration:
  ```sh
  *.info;mail.none;authpriv.none;cron.none /var/log/messages
  authpriv.* /var/log/secure
  mail.* -/var/log/maillog
  cron.* /var/log/cron
  ```

2. **Set up audit rules:**

  Edit `/etc/audit/rules.d/audit.rules`:
  ```sh
  sudo vi /etc/audit/rules.d/audit.rules
  ```
  Example rules:
  ```sh
  -w /etc/passwd -p wa -k passwd_changes
  -w /etc/shadow -p wa -k shadow_changes
  -w /etc/group -p wa -k group_changes
  ```

1. 1. 1. Step 7: Apply Kernel and Service Hardening

1. **Disable unused filesystems:**

  Edit `/etc/modprobe.d/disable-filesystems.conf`:
  ```sh
  sudo vi /etc/modprobe.d/disable-filesystems.conf
  ```
  Add the following lines:
  ```sh
  install cramfs /bin/true
  install freevxfs /bin/true
  install jffs2 /bin/true
  install hfs /bin/true
  install hfsplus /bin/true
  install squashfs /bin/true
  install udf /bin/true
  install vfat /bin/true
  ```

2. **Configure kernel parameters:**

  Edit `/etc/sysctl.conf`:
  ```sh
  sudo vi /etc/sysctl.conf
  ```
  Add or update the following parameters:
  ```sh
  net.ipv4.ip_forward = 0
  net.ipv4.conf.all.accept_source_route = 0
  net.ipv4.conf.all.accept_redirects = 0
  net.ipv4.conf.all.secure_redirects = 0
  net.ipv4.conf.all.log_martians = 1
  net.ipv4.conf.default.log_martians = 1
  net.ipv4.icmp_echo_ignore_broadcasts = 1
  net.ipv4.icmp_ignore_bogus_error_responses = 1
  net.ipv4.tcp_syncookies = 1
  net.ipv4.conf.all.send_redirects = 0
  net.ipv4.conf.default.send_redirects = 0
  ```

1. 1. 1. Step 8: Regular Maintenance

1. **Set up regular updates:**

  Create a cron job for regular updates:
  ```sh
  sudo crontab -e
  ```
  Add the following line to update daily at 2 AM:
  ```sh
  0 2 * * * apk update && apk upgrade
  ```

2. **Review and monitor logs regularly:**

  Ensure logs are rotated and reviewed frequently:
  ```sh
  sudo logrotate /etc/logrotate.conf
  ```

1. 1. 1. Conclusion

This process provides a foundation for securing an Alpine Linux system. Regular reviews and updates, along with compliance with the latest security guidelines, are essential to maintaining a secure environment.