Securing OpenSSH on CentOS and Red Hat

This guide provides comprehensive steps to secure your OpenSSH server on CentOS and Red Hat systems, focusing on best practices such as public key-only authentication for privileged accounts, firewall restrictions, and deploying a chroot environment.

Setting Up Public Key-Only Authentication

Using public key-only authentication enhances the security of SSH connections by requiring a cryptographic key pair, making it harder for attackers to gain access. To configure this:

• Generate a Key Pair: On the client machine, use the command ssh-keygen -t rsa -b 4096 to generate a new key pair.
• Copy Public Key: Then, copy the public key to your CentOS or Red Hat server with ssh-copy-id user@yourserver.com.
• Configure SSH: On the server, edit the /etc/ssh/sshd_config file to disable password authentication by setting PasswordAuthentication no and ChallengeResponseAuthentication no.
• Restart SSH: Apply changes by restarting the SSH service with sudo systemctl restart sshd.

Configuring the Firewall

Limiting access to the SSH server via firewall settings is crucial to prevent unauthorized access:

• Modify Firewalld Settings: Use sudo firewall-cmd --permanent --add-service=ssh to allow SSH connections.
• Restrict IP Addresses: If you want to allow SSH access from specific IP addresses, use sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="xx.xx.xx.xx" service name="ssh" accept' for each IP.
• Reload Firewalld: Reload the settings to apply changes with sudo firewall-cmd --reload.

Deploying a Chroot Environment

Chroot confines the root directory for specific users to a particular directory, enhancing the security by isolating the filesystem:

• Install the Jailkit: First, install Jailkit using sudo yum install jailkit.
• Set Up the Jail: Configure the jail environment with sudo jk_init -v /chroot ssh.
• Adding Users: Add users to the chroot environment with sudo jk_jailuser -m -j /chroot username.

Maintaining Security

Regular updates and monitoring are essential to maintain the security of your SSH server:

• Update Regularly: Keep your system up-to-date with sudo yum update.
• Monitor Logs: Regularly check the SSH logs for unauthorized attempts by examining /var/log/secure.