How to Troubleshoot Common SSH Connection Issues on Ubuntu Servers

Table of Contents

1. Initial Checks
2. Diagnose Network & Firewall
3. Verify SSH Daemon Status
4. Audit SSH Configuration
5. Analyze System Logs
6. Check Security Tools (fail2ban, etc.)
7. Test with SSH Verbose Mode
8. Pro Tips & Best Practices
9. Summary & Next Steps

1. Initial Checks

• Ping your server:

  ping your.server.ip

• Try connecting from multiple networks (office, mobile, VPN) to rule out local client/firewall issues.
• Using a custom SSH port? Run:

  ssh -p your_port user@your.server.ip

2. Diagnose Network & Firewall

Check if the SSH port is open:

nc -zv your.server.ip 22

If “refused,” check on server:

sudo ufw status
sudo iptables -L

Cloud Servers: Make sure firewall/security group allows inbound TCP on port 22 from your IP.

3. Verify SSH Daemon Status

• With console/KVM access: is sshd running?

  sudo systemctl status ssh

• If not, restart:

  sudo systemctl restart ssh

4. Audit SSH Configuration

• Edit SSH config:

  sudo nano /etc/ssh/sshd_config

• Check for:
  Port 22 (or your custom), PermitRootLogin no, PubkeyAuthentication yes, PasswordAuthentication no (if key-only), admin user in AllowUsers
• Validate config:

  sudo sshd -t

• Set correct permissions:

  chmod 600 ~/.ssh/authorized_keys
  chown youruser:youruser ~/.ssh/authorized_keys

5. Analyze System Logs

sudo tail -F /var/log/auth.log

Look for lines mentioning authentication failed or connection refused.

6. Check Security Tools (fail2ban, etc.)

• fail2ban status:

  sudo fail2ban-client status sshd

  Unban your IP or temporarily disable fail2ban for testing.

• Denyhosts: Check /etc/hosts.deny and /etc/hosts.allow for accidental blocks.

7. Test with SSH Verbose Mode

ssh -vvv user@your.server.ip

This outputs where the connection fails (auth, handshake, etc).

8. Pro Tips & Best Practices

• Always keep a second SSH session open while modifying configs—don’t close all before confirming success!
• Backup your config:

  sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.bak

• Restrict SSH in your firewall (allow only trusted IPs).
• Implement 2FA (Google Authenticator or similar) for all remote access.
• Regularly review server and fail2ban logs for attack attempts.

9. Summary & Next Steps

The structured troubleshooting approach:

• Connectivity check
• Port & firewall check
• SSH service/config audit
• Log analysis
• Check for accidental bans

will solve 99% of SSH connection failures on Ubuntu and most Linux servers.

Use my contact page—I’ll help you decode it!