Connection Failed

Troubleshooting guide for SSH connection failures.

Common Errors

Connection Timeout

Symptoms: Connection times out without response

Possible Causes:

• Wrong server address or port
• Network unreachable
• Firewall blocking
• SSH service not running

Solutions:

1. Verify server address and port
2. Check network connectivity
3. Test with ping or telnet
4. Check firewall rules
5. Verify SSH service status

Connection Refused

Symptoms: Connection immediately refused

Possible Causes:

• SSH service not running
• Wrong port number
• Service crashed

Solutions:

1. Check SSH service status
2. Verify port configuration
3. Restart SSH service
4. Check service logs

Host Key Verification Failed

Symptoms: Host key mismatch warning

Possible Causes:

• Server reinstalled
• IP address changed
• Security attack

Solutions:

1. Confirm server change is legitimate
2. Delete old host key
3. Re-accept new fingerprint

Authentication Errors

Permission Denied

Symptoms: Authentication failed

Possible Causes:

• Wrong username
• Wrong password
• Wrong key file
• Server configuration

Solutions:

1. Verify username
2. Check password
3. Check key file path
4. Verify server allows this user

Key Authentication Failed

Symptoms: Key authentication rejected

Possible Causes:

• Wrong key file
• Wrong key format
• Missing passphrase
• Key not authorized

Solutions:

1. Verify key file path
2. Check key format
3. Enter correct passphrase
4. Add key to authorized_keys

Network Issues

Firewall Blocking

Symptoms: Cannot reach server

Solutions:

1. Check local firewall
2. Check server firewall
3. Check network firewall
4. Open SSH port (22)

Network Unreachable

Symptoms: Network error

Solutions:

1. Check network connection
2. Verify routing
3. Check DNS resolution
4. Use correct IP address

Diagnostic Steps

Step 1: Basic Connectivity

Test basic connectivity:

ping server_address
telnet server_address 22

Step 2: SSH Service

Check SSH service:

# On server
systemctl status sshd
netstat -tlnp | grep 22

Step 3: Logs

Check logs:

# On client
cat ~/.ssh/known_hosts

# On server
tail -f /var/log/auth.log

Advanced Troubleshooting

Debug Mode

Run SSH in debug mode:

ssh -vvv user@server

Network Tools

Use network tools:

traceroute server_address
nmap -p 22 server_address

Prevention

• Keep connection info updated
• Use stable network
• Configure keep-alive
• Monitor connection status

Related Documentation

• SSH Connection - SSH features
• Connection Issues - FAQ