What Is SSH

The protocol that replaced plaintext

SSH (Secure Shell) is the cryptographic protocol that lets you operate a remote computer's terminal as if you were sitting in front of it — over any network, including hostile ones. It replaced telnet, which sent everything (including your password) in plaintext for any sniffer to grab. SSH encrypts the whole session: credentials, commands, output.

What SSH actually gives you

Remote terminal access — a shell on the other machine, indistinguishable from local.
File transfer — SCP, SFTP, and rsync-over-SSH all ride the same encrypted channel.
Port forwarding — tunnel arbitrary TCP through SSH (Track 3).
Authentication — password (weak) or key-based (strong, the rest of this track).

OpenSSH — the implementation that ships with everything

The standard implementation is OpenSSH, born in OpenBSD and now bundled with macOS, every Linux distro, and modern Windows. The current series (OpenSSH 10.x) added post-quantum key exchange (sntrup761x25519 and mlkem768x25519) by default and dropped DSA support entirely. The defaults are good — you almost never need to override them.

SSH defaults to port 22. The server side is sshd (the daemon). The client is ssh.

Code

Sanity-check your SSH stack·bash

# Client version
ssh -V

# Is SSH server running on this Mac?
sudo systemsetup -getremotelogin

# Enable SSH server (macOS — also: System Settings > General > Sharing > Remote Login)
sudo systemsetup -setremotelogin on

# Linux equivalent
sudo systemctl status ssh
sudo systemctl enable --now ssh

Exercise

Run ssh -V on every machine you have access to and write down which OpenSSH version each runs. Anything older than 9.0 should be flagged for upgrade. Then enable Remote Login on this Mac (System Settings or the systemsetup command above) and SSH from terminal: ssh $(whoami)@localhost. The first connection prompts you for a fingerprint — say yes, you've now used SSH end-to-end on one machine.