Ssh

SSH Skill

This skill provides capabilities for establishing and managing SSH connections to remote machines. Windows PowerShell equivalents for SSH config creation, key paths, ssh-agent, and permissions are in references/windows.md.

Capabilities

• Establish SSH connections using password or key-based authentication
• Generate and manage SSH key pairs
• Configure SSH for easier connections
• Execute commands on remote machines
• Transfer files between local and remote machines
• Manage SSH configurations and known hosts

Authentication Methods

Password Authentication

ssh username@hostname

When prompted, you should ask the user for their password or a private key.

Key-Based Authentication

Generate a new SSH key pair:

ssh-keygen -t ed25519 -f ~/.ssh/key_name -C "comment" -N ""

Copy the public key to the remote server:

ssh-copy-id -i ~/.ssh/key_name.pub username@hostname

Connect using the private key:

ssh -i ~/.ssh/key_name username@hostname

SSH Configuration

Create or edit the SSH config file for easier connections:

mkdir -p ~/.ssh
cat > ~/.ssh/config << 'EOF'
Host alias
    HostName hostname_or_ip
    User username
    IdentityFile ~/.ssh/key_name
    Port 22
    ServerAliveInterval 60
EOF
chmod 600 ~/.ssh/config

Then connect using the alias:

ssh alias

Common SSH Options

• -p PORT: Connect to a specific port
• -X: Enable X11 forwarding
• -L local_port:remote_host:remote_port: Set up local port forwarding
• -R remote_port:local_host:local_port: Set up remote port forwarding
• -N: Do not execute a remote command (useful for port forwarding)
• -f: Run in background
• -v: Verbose mode (add more v's for increased verbosity)

File Transfer with SCP

Copy a file to the remote server:

scp /path/to/local/file username@hostname:/path/to/remote/directory/

Copy a file from the remote server:

scp username@hostname:/path/to/remote/file /path/to/local/directory/

Copy a directory recursively:

scp -r /path/to/local/directory username@hostname:/path/to/remote/directory/

SSH Agent

Start the SSH agent:

eval "$(ssh-agent -s)"

Add a key to the agent:

ssh-add ~/.ssh/key_name

Troubleshooting

• Check SSH service status on remote: systemctl status sshd
• Verify SSH port is open: nc -zv hostname 22
• Debug connection issues: ssh -vvv username@hostname
• Check permissions: SSH private keys should have 600 permissions (chmod 600 ~/.ssh/key_name)
• Verify known_hosts: If host key changed, remove the old entry with ssh-keygen -R hostname

Secure SSH Key Management

Local Storage with Proper Permissions

The most basic approach is to ensure proper file permissions:

# Set correct permissions for private keys
chmod 600 ~/.ssh/id_ed25519
# Set correct permissions for public keys
chmod 644 ~/.ssh/id_ed25519.pub
# Set correct permissions for SSH directory
chmod 700 ~/.ssh