Linux · SSH: Keys, Config & Hardening

Which PAM line enables Google Authenticator 2FA for SSH, and what does the nullok option change about who it applies to?

Answer locked. Get the free KnowCard app to reveal it — plus spaced-repetition review so it actually sticks.

This is one card from the KnowCard library — thousands more across SAP, Linux, Python and more. In the app you get the answer, AI explanations, and cards that come back right before you would forget them. Free to start on iOS, Android or the web.

More in SSH: Keys, Config & Hardening

Why are telnet and rlogin considered obsolete, and what does ssh do differently to replace them?

The very first time you ssh to a new host, it prints a fingerprint and asks you to confirm. What is that step actually protecting you against, and what does answering yes do?

You reinstalled a server but kept its old hostname; now ssh screams REMOTE HOST IDENTIFICATION HAS CHANGED and refuses to connect. What is it warning about and how do you clear it once you know the change is legitimate?

How do you run a single command on a remote host without opening an interactive shell, versus feeding a whole batch of commands to it non-interactively?

For automating ssh across many fresh hosts, the first-connect trust prompt gets in the way. Which option skips it, and which config file do such client options belong in?

Which ssh option lets a graphical program running on the remote host display on your local screen, and does it still work under Wayland?

Start learning today

Free to start — download the app or use it in your browser.