SSH

PAM: login checks, password policy and lockout

The module stack in /etc/pam.d decides whether to let a user in, which password to accept and when to lock them out.

ssh, scp — connecting and copying over SSH

A client for logging in and copying files over SSH; host aliases in ~/.ssh/config remove repeated connection parameters.

sshd — the SSH server

The OpenSSH server; edit the config with a second session open — a mistake in PermitRootLogin locks you out immediately.

SSH keys and ssh-agent

Key pairs for passwordless login; sshd rejects ~/.ssh if it is writable by the group or by others.

ssh-keyscan — collecting host keys

Pre-populate known_hosts with server keys — skip the interactive "yes" on first ssh.

Ansible: inventory, modules and one-off commands

Configures servers over SSH: from an inventory and playbooks it brings machines to a described state, no agent needed.

ansible-playbook: running a playbook

Runs an Ansible playbook on the inventory machines and reports ok, changed, skipping or failed for each task.