Why Can't You Just . . . : SSH Key Setup: A Practical Guide for Linux Systems

SSH keys provide a more secure and convenient way to authenticate with remote servers compared to passwords. This guide covers the essential steps to generate, configure, and use SSH keys on Linux systems, with specific notes for Fedora users.

What You'll Need

A Linux system (examples use Fedora 42)
Terminal access
Basic command line familiarity

1. Generating Your SSH Key Pair

Modern systems should use Ed25519 keys for better security and performance:

ssh-keygen -t ed25519 -C "your.email@example.com"

For systems requiring RSA compatibility:

ssh-keygen -t rsa -b 4096 -C "your.email@example.com"

When prompted:

File location: Press Enter to use default (~/.ssh/id_ed25519)
Passphrase: Use a strong passphrase for security

Fedora Note: SELinux is enabled by default. Your keys will automatically get the correct context, but if you encounter issues:

restorecon -R ~/.ssh

2. Starting the SSH Agent

One-time startup:

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

Persistent setup (recommended):

Create a systemd user service for automatic ssh-agent management:

systemctl --user enable --now ssh-agent

Add to your ~/.bashrc or ~/.zshrc:

export SSH_AUTH_SOCK="$XDG_RUNTIME_DIR/ssh-agent.socket"

3. Adding Your Public Key to Remote Servers

Copy your public key to a remote server:

ssh-copy-id username@remote-server.com

Or manually:

cat ~/.ssh/id_ed25519.pub | ssh username@remote-server.com "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

4. SSH Configuration for Multiple Keys

Create or edit ~/.ssh/config:

# Work server
Host work-server
    HostName work.company.com
    User myusername
    IdentityFile ~/.ssh/id_work
    Port 2222

# Personal server
Host personal
    HostName personal.example.com
    User admin
    IdentityFile ~/.ssh/id_personal

Now connect simply with:

ssh work-server

5. Security Best Practices

File Permissions

Ensure correct permissions (critical for SSH to work):

chmod 700 ~/.ssh
chmod 600 ~/.ssh/id_ed25519
chmod 644 ~/.ssh/id_ed25519.pub
chmod 600 ~/.ssh/config
chmod 600 ~/.ssh/authorized_keys

Additional Security

Always use passphrases on private keys
Regularly rotate keys (annually recommended)
Use different keys for different purposes
Consider using hardware security keys for critical systems

6. Fedora-Specific Considerations

Firewall Configuration

If running SSH server on Fedora:

sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload

SELinux Context Issues

If you encounter permission denied errors:

sudo setsebool -P ssh_sysadm_login on
restorecon -R ~/.ssh

Troubleshooting Common Issues

Debug SSH connections:

ssh -v username@server.com

Test key authentication:

ssh -o PasswordAuthentication=no username@server.com

Check SSH agent:

ssh-add -l

Quick Reference

Command	Purpose
`ssh-keygen -t ed25519`	Generate new key pair
`ssh-add ~/.ssh/keyname`	Add key to agent
`ssh-copy-id user@host`	Copy public key to server
`ssh -v user@host`	Debug connection issues

Why Can't You Just . . .

6/20/25

SSH Key Setup: A Practical Guide for Linux Systems