SSH Troubleshooting Guide: Common Issues and Solutions

SSH issues can be frustrating when trying to connect to remote servers. Let’s explore common SSH problems and their solutions to help you get back on track quickly.

Common SSH Issues

1. Connection Refused

ssh: connect to host example.com port 22: Connection refused

Solutions:

Verify the SSH service is running:

sudo systemctl status ssh  # For Ubuntu/Debian
sudo service sshd status  # For RHEL/CentOS

Check firewall settings:

sudo ufw status  # Ubuntu
sudo firewall-cmd --list-all  # CentOS

Confirm the correct port:
```
ssh -p PORT_NUMBER user@hostname
```

2. Permission Denied

Permission denied (publickey,password)

Solutions:

Check key permissions:

chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/id_rsa.pub

Verify key is added to agent:

ssh-add ~/.ssh/id_rsa
ssh-add -l  # List added keys

Check authorized_keys file:

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

3. Host Key Verification Failed

WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

Solutions:

Remove old key (if you trust the change):
```
ssh-keygen -R hostname
```

Update known_hosts manually:

ssh-keyscan -H hostname >> ~/.ssh/known_hosts

4. SSH Agent Issues

Solutions:

Start SSH agent:
```
eval $(ssh-agent)
```

Add key permanently (add to ~/.bashrc):

if [ -z "$SSH_AUTH_SOCK" ]; then
    eval $(ssh-agent -s)
    ssh-add
fi

Best Practices

Key Management

Use strong key encryption:
```
ssh-keygen -t ed25519 -a 100
```

Backup your keys:

cp -r ~/.ssh/id_* /secure/backup/location/

Security Configuration

Disable password authentication:

# In /etc/ssh/sshd_config
PasswordAuthentication no

Use specific user permissions:

# In /etc/ssh/sshd_config
AllowUsers user1 user2

Performance Optimization

Enable connection multiplexing:

# In ~/.ssh/config
Host *
  ControlMaster auto
  ControlPath ~/.ssh/control:%h:%p:%r
  ControlPersist 1h

Use compression for slow connections:
```
ssh -C user@hostname
```

Modern Alternatives

While SSH is powerful, modern cloud platforms offer simpler solutions. For example, Thunder Compute eliminates SSH complexity entirely, letting you directly connect to cloud instances with tnr connect.

No key management required
Automatic security updates
Zero-configuration setup

Debugging Tools

Verbose logging:
```
ssh -vvv user@hostname
```
Test connectivity:
```
nc -zv hostname 22
```

Check SSH daemon logs:

sudo tail -f /var/log/auth.log  # Ubuntu
sudo tail -f /var/log/secure    # CentOS

Quick Reference

Common Error Codes

Exit 255: Generic error
Exit 1: Invalid command
Exit 126: Command not executable
Exit 127: Command not found

Essential SSH Commands

ssh-keygen          # Generate new key
ssh-copy-id         # Copy key to server
ssh-add             # Add key to agent
ssh-keyscan         # Scan host keys

Next Steps

After mastering SSH troubleshooting:

Set up SSH config files
Implement key rotation
Configure bastion hosts
Explore modern alternatives

For a hassle-free experience without SSH complexity, check out Thunder Compute where you can access your instances directly through your browser!