Connecting with SSH

SSH (Secure SHell) lets you type commands on a computer that's somewhere else: in the next room, in a school server closet, or in a data center across the world. It's how almost every Linux server gets managed.

The big idea

Think of SSH as a secure phone call between two computers:

• Your computer is the client. It makes the call using the ssh command.
• The remote computer is the server. It answers the call with a program called sshd (the “SSH daemon”) that listens on port 22.
• Everything you send is encrypted. Someone snooping on the Wi-Fi only sees scrambled nonsense, not your password.

Step 1: make sure the server is listening

Most servers have SSH turned on already. If you're setting up your own machine (for example, a spare laptop running Linux), this is where the families differ:

sudo dnf install openssh-server
sudo systemctl enable --now sshd
sudo firewall-cmd --list-services

sudo apt update
sudo apt install openssh-server
systemctl status ssh

Then find the server's IP address, which is its number on the network:

hostname -I

(Linux Sysadmin, lesson 1 explains IP addresses, ip a and the old ifconfig properly.)

Step 2: connect from your computer

This part is the same everywhere: Windows PowerShell, Mac Terminal or Linux. (Prefer an app like PuTTY or iTerm2? That's lesson 4.)

The very first time, you'll see this question:

The authenticity of host '192.168.1.50 (192.168.1.50)' can't be established.
ED25519 key fingerprint is SHA256:Zk3dQ1v8mX4pR2yLw9cT0aB7nE5fH6jK1uS3gV8qW2o.
Are you sure you want to continue connecting (yes/no/[fingerprint])?

That's SSH saying “I've never talked to this server before. Is this really the one you meant?” It's like caller ID for computers. On your own network, type the whole word yes. SSH remembers the server from then on. If that fingerprint ever changes, SSH shows a big scary warning, because someone could be pretending to be your server. When that happens, stop and ask whoever runs the server.

Next, type your password. Nothing appears while you type, not even dots. That's on purpose, so nobody can count the letters. Type it and press Enter anyway.

When you're done, type exit to hang up and return to your own computer.

Try it

This terminal starts on your laptop (a pretend one). Somewhere on the network there's a server at 192.168.1.50. Username: student. Password: linux.

Level up: SSH keys

Passwords can be guessed. SSH keys are much safer, and they save you typing. A key comes as a pair:

• The public key (id_ed25519.pub) is like a padlock. You can hand out copies to any server you want to log in to.
• The private key (id_ed25519) is the only key that opens those padlocks. It stays on your computer. Never share it, email it, or paste it anywhere, including into an AI chat.

# 1. make a key pair (press Enter to accept the defaults)
ssh-keygen -t ed25519
# 2. put your padlock on the server (asks for your password one last time)
ssh-copy-id student@192.168.1.50
# 3. log in, no password needed
ssh student@192.168.1.50

When it doesn't work

Quick check