SSH Basics and Key Authentication

How does SSH key authentication work? How do you set it up?

Linuxbeginner

// interview question

How does SSH key authentication work? How do you set it up?

Sample answer

SSH key authentication uses a public-private key pair. Generate keys with ssh-keygen, copy the public key (~/.ssh/id_rsa.pub) to the server's ~/.ssh/authorized_keys file using ssh-copy-id. The private key stays on your machine and should never be shared. Key auth is more secure than passwords and enables passwordless login.

Why this matters

SSH is the primary way to access remote servers securely. Understanding key-based authentication is essential for DevOps work, as it's used for server administration, Git operations, CI/CD pipelines, and automated deployments.

Code examples

Generate and deploy SSH keys

bash

# Generate new SSH key pair
ssh-keygen -t ed25519 -C 'your@email.com'

# Copy public key to server
ssh-copy-id user@server.example.com

# Or manually copy the key
cat ~/.ssh/id_ed25519.pub | ssh user@server 'cat >> ~/.ssh/authorized_keys'

# Connect to server
ssh user@server.example.com

SSH config file for easier access

bash

# Create/edit ~/.ssh/config
Host prod-server
  HostName 192.168.1.100
  User deploy
  IdentityFile ~/.ssh/prod_key
  Port 22

# Now connect with just:
ssh prod-server

# Run command on remote server
ssh prod-server 'df -h'

Common mistakes to avoid

Setting wrong permissions on .ssh directory (should be 700) and authorized_keys (should be 600)
Sharing or committing private keys to version control
Not using a passphrase on private keys

Likely follow-ups

What's the difference between RSA, ECDSA, and Ed25519 keys?
How do you use SSH agent to manage multiple keys?
How would you set up SSH tunneling/port forwarding?

Answer out loud first, then check yourself against the model answer.

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