linuxexpert-practice-labs/labs/linux-terminal/3-1-ssh-keys/README.md

Using Linux Terminal 3.1: SSH keys

An Ed25519 key with a passphrase, installing it for another user by hand, and what sshd does when a home directory is too open.

Before you start

The lab creates user student (password Lab-Pass-1). Make the key as student (sudo -iu student); use sudo for the steps that need root. Give the key the passphrase Lab-Phrase-1: le-lab check needs it to test the login. The lab checks that user bob doesn't exist yet, and le-lab reset removes him and ~/.ssh/lab_key.

Tasks

1. Make an Ed25519 key pair with a passphrase and the comment lab-key, saved as ~/.ssh/lab_key.
2. Create a user bob with a password, then install your lab key for him so you can log in to bob@localhost with it. Ubuntu cloud images turn off password logins over SSH, so ssh-copy-id can't log in there; use the manual steps instead. After eval "$(ssh-agent)"; ssh-add ~/.ssh/lab_key, ssh -o BatchMode=yes -o StrictHostKeyChecking=accept-new -i ~/.ssh/lab_key bob@localhost whoami prints bob.
3. Run sudo chmod 777 /home/bob, try the login again, and find out why it fails. Then fix it.

Hints

• ssh-keygen -t ed25519 -C lab-key -f ~/.ssh/lab_key; ssh-keygen -l -f.
• useradd -m, passwd; then /home/bob/.ssh (mode 700) and authorized_keys (mode 600), both owned by bob, holding the .pub line. BatchMode doesn't ask about a new host key either: without accept-new, the first try fails with "Host key verification failed."
• journalctl -u ssh -n 5 or /var/log/auth.log: "bad ownership or modes"; chmod 750 /home/bob.