Tool cheatsheet

SSH (OpenSSH) 10 Cheatsheet

Connect to servers, manage keys, write ~/.ssh/config, forward ports, jump through bastion hosts and harden sshd.

Connecting

Connect to a server — Open a shell on a remote host.
Connect on another port with a specific key — Use a non-default port and an identity file.
Run a command on the server — Run one command and print its output locally.
Run an interactive program remotely — Allocate a terminal so programs like top and sudo work.
Debug a failing connection — Print detailed logs of the connection and authentication attempts. Add more v's for more detail.
Connect through a bastion host — Reach a private server by hopping through a jump host.
Copy a file over SSH — Copy a local file to a server.
Copy with resumable transfers — Use rsync over SSH to resume interrupted transfers.

Create a key pair — Generate an Ed25519 key. Set a passphrase when asked.
Install your public key on a server — Add your public key to the server's authorized_keys so you can log in without a password.
Print your public key — Show the public key to paste into GitHub or a server.
Start the agent — Start ssh-agent in the current shell.
Add a key to the agent — Load a key so you enter its passphrase once. On macOS, add --apple-use-keychain to remember it.
List loaded keys — Show the keys in the agent.
Show a key's fingerprint — Print the fingerprint of a key, to compare with what a service displays.
Remove an old host key — Remove a host from known_hosts after its key legitimately changed, such as after reinstalling.
Fix permissions that SSH rejects — SSH ignores keys and config files that other users can read.

Define a host alias — Connect with ssh web instead of the long form.
Configure a jump host — Always reach a private server through the bastion.
Keep idle connections alive — Send a keepalive every minute so idle sessions don't drop.
Reuse connections for speed — Share one connection between sessions, which makes repeated ssh and scp commands fast.
Use a specific key for GitHub — Use a dedicated key for one service.

Reach a remote service locally — Make a remote database available at localhost:5433, through the server.
Expose a local service on the server — Make your local web server available on the server's port 8080.
Create a SOCKS proxy — Run a SOCKS5 proxy on localhost:1080 that sends traffic through the server.
Run a tunnel in the background — Forward a port without opening a shell. -N means no command, -f means go to the background.
Stop a background tunnel — Find and end the tunnel process.

Test the server configuration — Check /etc/ssh/sshd_config for errors before restarting. A mistake can lock you out.
Disable password logins — Allow keys only and block root logins. Confirm key login works first, in a second session.
Apply sshd changes — Reload the SSH server after editing its configuration. Existing sessions stay open.
See login attempts — Follow SSH logins and failures on systemd systems.
Limit who can log in — Allow only listed users to connect.
Change the SSH port — Move sshd to another port. Open the new port in the firewall before restarting.

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