5Security

• 5.1Performing Security Administration Tasks

  Learn routine host security checks: auditing SUID/SGID files (find -perm), password aging (chage, passwd, usermod), discovering open ports (ss, netstat, nmap, lsof, fuser), login visibility (who, w, last), resource limits (ulimit), sudo (/etc/sudoers, su), and auto-logout (TMOUT).

• 5.2Host Security Configuration

  Learn to shrink the host attack surface: shadow passwords and blocking logins with /etc/nologin, stopping unused network services (systemctl, /etc/init.d/, chkconfig, service, legacy inetd/xinetd), and firewall overview (iptables, firewalld).

• 5.3Protecting Data with Encryption

  Learn protecting data and communication with public-key technology: OpenSSH client key management (ssh-keygen, ssh-agent, ssh-add), the major algorithms (RSA, ECDSA, Ed25519), SSH port forwarding (local/remote), and file encryption/signing with GnuPG (gpg, gpg-agent, ~/.gnupg/).

• 5.4Cloud Security Basics

  Learn what changes for Linux servers on public clouds: the division of responsibilities with on-premises (the management console, cloud-side firewalls, routing), the meaning of region selection (domestic/foreign constraints), ephemeral storage, provider-driven maintenance and reboots, and console access paths and authentication (multi-factor authentication, one-time passwords).