Troubleshooting Key Authentication

When SSH key authentication fails, systematic troubleshooting identifies the root cause quickly. Common issues include permission problems, key mismatches, and server configuration errors.

Enable verbose SSH output for debugging:

# Increasing verbosity levels
ssh -v [email protected]    # Basic verbose
ssh -vv [email protected]   # More detailed
ssh -vvv [email protected]  # Maximum detail

# Common debugging checkpoints:
# - "Offering public key" - Client sending key
# - "Server accepts key" - Key recognized
# - "Authentication succeeded" - Login successful

Server-side troubleshooting commands:

# Check SSH daemon logs
sudo journalctl -u sshd -f
sudo tail -f /var/log/auth.log  # Debian/Ubuntu
sudo tail -f /var/log/secure    # RHEL/CentOS

# Verify authorized_keys format
cat ~/.ssh/authorized_keys | ssh-keygen -lf -

# Test SSH daemon configuration
sudo sshd -t  # Test mode
sudo sshd -d  # Debug mode (stops normal daemon)

Common issues and solutions:

# Fix: Permission denied (publickey)
# Check client-side permissions
chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_*

# Check server-side permissions
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

# Fix: Host key verification failed
# Remove old host key
ssh-keygen -R server.example.com

# Fix: Agent refused operation
# Re-add key to agent
ssh-add -D  # Clear all keys
ssh-add ~/.ssh/id_ed25519  # Re-add key

SSH key authentication forms the foundation of secure server access, replacing vulnerable passwords with cryptographically strong credentials. Proper implementation requires attention to key generation parameters, secure deployment practices, and ongoing management procedures. By following these comprehensive setup guidelines, organizations establish a robust authentication framework that resists common attacks while enabling efficient operations. Regular review and updates of key management practices ensure continued security as threats evolve and infrastructure grows.## SSH Key Rotation Strategies

Regular SSH key rotation represents a critical security practice that limits the exposure window for compromised credentials and ensures compliance with security policies. Many organizations struggle with key rotation due to the complexity of updating keys across distributed systems without disrupting operations. This chapter provides comprehensive strategies for implementing automated key rotation that maintains security while minimizing operational impact.