SSH Hardening Skill

Secure SSH access to VPS servers by implementing industry-standard hardening practices.

What This Skill Does

This skill helps AI agents harden SSH configuration on VPS servers. SSH is the primary entry point for server management, making it a critical attack vector. Proper SSH hardening prevents unauthorized access, brute-force attacks, and credential theft.

Key capabilities:

• Create non-root users with sudo privileges
• Generate and configure SSH key authentication
• Disable password-based authentication
• Disable root login over SSH
• Configure security settings in sshd_config
• Test configuration before applying

When to Use

Use this skill when you need to:

• Set up a new VPS server with secure SSH access
• Replace password authentication with SSH keys
• Disable root login for security compliance
• Harden an existing server against brute-force attacks
• Fix security audit findings related to SSH
• Implement principle of least privilege

Critical understanding: Root can do anything. One typo, one compromised session, and your entire system is gone. Passwords can be guessed. SSH keys can't be brute-forced in any practical timeframe.

Prerequisites

• Root or existing sudo user access to the server
• SSH access to the server (keep current session open!)
• SSH client on local machine (ssh, ssh-keygen, ssh-copy-id)
• Terminal access to local machine

SSH Hardening Steps

Step 1: Create Non-Root User

CRITICAL: Complete this step and test before disabling root login!

Create a new user for daily operations:

# Create user (replace 'deployer' with desired username)
sudo adduser deployer

Enter a strong password when prompted.

Add user to sudo group:

sudo usermod -aG sudo deployer

Test sudo access before proceeding:

# Switch to new user
su - deployer

# Test sudo
sudo whoami
# Should output: root

# Exit back to original user
exit

Step 2: Generate SSH Key Pair (Local Machine)

On your local machine (not the server), generate an SSH key:

ssh-keygen -t ed25519 -C "[email protected]"

Key type explained:

• ed25519 - Modern, secure, fast (recommended)
• Alternative: rsa -b 4096 for older systems

When prompted:

• Press Enter to save to default location (~/.ssh/id_ed25519)
• Enter a strong passphrase (recommended) or leave empty

Step 3: Copy SSH Key to Server

From your local machine, copy the public key to the server:

ssh-copy-id deployer@your-server-ip

Enter the user's password when prompted.

Manual alternative (if ssh-copy-id is unavailable):

# On local machine, display public key
cat ~/.ssh/id_ed25519.pub

# On server, as the new user
mkdir -p ~/.ssh
chmod 700 ~/.ssh
nano ~/.ssh/authorized_keys
# Paste the public key, save and exit
chmod 600 ~/.ssh/authorized_keys

Step 4: Test SSH Key Authentication

CRITICAL: Test in a NEW terminal window, keep existing session open!

ssh deployer@your-server-ip

You should connect without entering a password (or only your SSH key passphrase).

If connection fails, DO NOT proceed to Step 5! Debug the issue first.

Step 5: Harden SSH Configuration

WARNING: Make these changes carefully. Test in a new terminal before closing existing sessions!

Edit SSH daemon configuration:

sudo nano /etc/ssh/sshd_config

Update or add these settings:

# Disable root login
PermitRootLogin no

# Disable password authentication
PasswordAuthentication no

# Disable empty passwords
PermitEmptyPasswords no

# Limit authentication attempts
MaxAuthTries 3

# Allow only specific users (optional but recommended)
AllowUsers deployer

# Use only SSH protocol 2
Protocol 2

# Disable X11 forwarding (unless needed)
X11Forwarding no

# Set login grace time
LoginGraceTime 60

# Disable host-based authentication
HostbasedAuthentication no

Optional advanced settings:

# Change default port (security through obscurity, optional)
# Port 2222

# Disable agent forwarding (unless needed)
# AllowAgentForwarding no

# Disable TCP forwarding (unless needed)
# AllowTcpForwarding no

# Set idle timeout
# ClientAliveInterval 300
# ClientAliveCountMax 2

Step 6: Test Configuration

Test the configuration file for syntax errors:

sudo sshd -t

No output means the configuration is valid.

Step 7: Restart SSH Service

CRITICAL: Test in a new terminal BEFORE restarting!

# Test connection in NEW terminal first
ssh deployer@your-server-ip

# If successful, restart SSH (in original terminal)
sudo systemctl restart sshd

Verification:

sudo systemctl status sshd

Step 8: Verify Root Login is Disabled

Try to connect as root (should fail):

ssh root@your-server-ip

Expected result: Permission denied (publickey)

Configuration Reference

AllowUsers vs AllowGroups

Restrict SSH access to specific users or groups:

# Option 1: Specific users
AllowUsers deployer admin

# Option 2: Users in specific group
AllowGroups sshusers

Port Changes

Changing the default SSH port (22) reduces noise from automated scanners:

Port 2222

Remember to:

1. Update firewall rules before restarting SSH
2. Use the new port when connecting: ssh -p 2222 user@host

Key-Based Authentication Only

Ensure these settings work together:

PubkeyAuthentication yes
PasswordAuthentication no
ChallengeResponseAuthentication no
UsePAM yes

Security Best Practices

1. Always test before closing sessions - Keep one session open until verified
2. Use SSH key passphrases - Adds another layer of security
3. Limit user access - Use AllowUsers or AllowGroups
4. Monitor authentication logs - Check /var/log/auth.log regularly
5. Use fail2ban - Add automated banning for repeated failed attempts
6. Regular key rotation - Periodically generate new keys
7. Disable unused features - X11Forwarding, TCP forwarding, etc.

Troubleshooting

Locked Out of Server

Prevention is key:

• Always keep one session open when making changes
• Test in a new terminal before closing existing sessions
• Have console access via hosting provider's control panel

Recovery:

• Use hosting provider's console/VNC access
• Revert changes to /etc/ssh/sshd_config
• Restart sshd service

SSH Key Not Working

# Check file permissions on server
ls -la ~/.ssh/
# Should show:
# drwx------ .ssh/
# -rw------- authorized_keys

# Fix permissions if needed
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

# Check SSH logs
sudo tail -f /var/log/auth.log

Connection Refused After Changes

# Check SSH service status
sudo systemctl status sshd

# View recent errors
sudo journalctl -u sshd -n 50

# Test configuration
sudo sshd -t

Common Mistakes to Avoid

• ❌ Closing all SSH sessions before testing new configuration
• ❌ Disabling password auth before setting up SSH keys
• ❌ Not testing sudo access for new user
• ❌ Typos in sshd_config causing service failure
• ❌ Forgetting to restart sshd after changes
• ❌ Not updating firewall when changing SSH port

Additional Resources

See [references/sshd-config.md](references/sshd-config.md) for complete sshd_config reference.

See [scripts/setup-ssh-hardening.sh](scripts/setup-ssh-hardening.sh) for automated setup script.

Related Skills

• firewall-configuration - Restrict SSH port access
• fail2ban-setup - Auto-ban brute-force attempts
• auto-updates - Keep SSH patched against vulnerabilities