SSH connection basics: password, keys, and parameters

Basic SSH syntax: ssh [options] [user@]hostname

Password authentication

ssh user@hostname
# Prompts for password

Key-based authentication

Use a specific private key:

ssh -i ~/.ssh/id_rsa user@hostname

With custom key location:

ssh -i /path/to/private/key user@hostname

Common key paths:

• ~/.ssh/id_rsa — RSA key
• ~/.ssh/id_ed25519 — Ed25519 key (recommended)
• ~/.ssh/id_ecdsa — ECDSA key

Useful parameters

Option	Purpose
-p port	Custom SSH port (default 22)
-i keyfile	Specify private key file
-v	Verbose output (debug connection issues)
-vv or -vvv	More verbose
-X	Enable X11 forwarding
-L local:remote	Local port forwarding
-R remote:local	Remote port forwarding
-N	Don’t open shell (useful with port forwarding)
-f	Run in background
-t	Force pseudo-terminal allocation

Examples

Connect on custom port with specific key:

ssh -i ~/.ssh/id_ed25519 -p 2222 user@hostname

Run remote command and exit (no shell):

ssh user@hostname "ls -la /tmp"

Port forwarding (local):

ssh -L 8080:localhost:3000 user@hostname -N -f
# Maps localhost:8080 → remote:3000

Verbose debugging:

ssh -vvv user@hostname

SSH key best practices

• Use Ed25519 keys (ssh-keygen -t ed25519)
• Protect with passphrase
• Store in ~/.ssh/ with restricted permissions (chmod 600)
• Use SSH agent for convenience