SSH Skill

This skill provides capabilities for establishing and managing SSH connections to remote machines.

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