🐧 Day 6: Linux Security, SSH, and Automation Basics
Purpose
This document introduces essential Linux security concepts every DevOps engineer must understand. It focuses on secure access using SSH, basic firewall awareness, and the automation mindset required to manage systems efficiently at scale.
Why Security Matters in DevOps
In DevOps, systems are:
Exposed to the internet
Accessed by multiple users and services
Continuously deployed and updated
Without basic security practices, systems become easy targets for misconfiguration, unauthorized access, or outages caused by human error.
Secure Remote Access with SSH
What Is SSH?
SSH (Secure Shell) is a protocol used to securely connect to remote Linux systems.
It is commonly used for:
Server administration
Deployments
Troubleshooting production issues
SSH Connection
Admin@DESKTOP-PRFKT2P MINGW64 ~
$ ssh saad@192.168.0.118
The authenticity of host '192.168.0.118 (192.168.0.118)' can't be established.
ED25519 key fingerprint is SHA256:Pc8EC10Te/WaUkGfZ7Q1P3tBVqU7TETyiJhOE/Nob8Q.
This host key is known by the following other names/addresses:
~/.ssh/known_hosts:1: 192.168.0.119
~/.ssh/known_hosts:4: [127.0.0.1]:2222
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '192.168.0.118' (ED25519) to the list of known hosts.
saad@192.168.0.118's password:
Welcome to Ubuntu 24.04.3 LTS (GNU/Linux 6.14.0-37-generic x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/pro
Expanded Security Maintenance for Applications is not enabled.
89 updates can be applied immediately.
3 of these updates are standard security updates.
To see these additional updates run: apt list --upgradable
Enable ESM Apps to receive additional future security updates.
See https://ubuntu.com/esm or run: sudo pro status
Last login: Tue Dec 16 19:48:39 2025 from 10.0.3.2
saad@saad-VirtualBox:~$
This command establishes an encrypted connection to a remote server.
SSH Key-Based Authentication
Using passwords for SSH access is insecure and does not scale in DevOps environments. SSH key-based authentication solves these problems by using cryptographic keys instead of passwords.
Why SSH Keys Are Used
Passwords can be guessed, reused, or leaked
Automated systems cannot safely store passwords
Keys provide stronger, non-brute-force authentication
Access can be revoked easily by removing a key
How SSH Key Authentication Works (Short)
A key pair is generated: a public key and a private key
The public key is stored on the server
The private key remains on the client and is never shared
During login, the server verifies the client using cryptographic proof
Only if the private key matches the public key on the server is access granted.
Generate SSH Key
Admin@DESKTOP-PRFKT2P MINGW64 ~
$ ssh-keygen
Generating public/private ed25519 key pair.
Copy Key to Server
Admin@DESKTOP-PRFKT2P MINGW64 ~
$ ssh-copy-id saad@192.168.0.118
/usr/bin/ssh-copy-id: INFO: Source of key(s) to be installed: "/c/Users/Admin/.ssh/id_ed25519.pub"
/usr/bin/ssh-copy-id: INFO: attempting to log in with the new key(s), to filter out any that are already installed
/usr/bin/ssh-copy-id: INFO: 1 key(s) remain to be installed -- if you are prompted now it is to install the new keys
saad@192.168.0.118's password:
Number of key(s) added: 1
Now try logging into the machine, with: "ssh 'saad@192.168.0.118'"
and check to make sure that only the key(s) you wanted were added.
Admin@DESKTOP-PRFKT2P MINGW64 ~
$ ssh saad@192.168.0.118
Welcome to Ubuntu 24.04.3 LTS (GNU/Linux 6.14.0-37-generic x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/pro
Expanded Security Maintenance for Applications is not enabled.
89 updates can be applied immediately.
3 of these updates are standard security updates.
To see these additional updates run: apt list --upgradable
Enable ESM Apps to receive additional future security updates.
See https://ubuntu.com/esm or run: sudo pro status
Last login: Thu Dec 18 12:45:41 2025 from 192.168.0.106
Benefits of SSH keys:
Stronger security
No password sharing
Automation-friendly
Basic Firewall Awareness
A firewall controls incoming and outgoing network traffic based on defined rules. It acts as a security layer between a system and the network.
In Linux, common firewall tools include:
iptables
iptables is a low-level firewall tool that directly manages packet filtering rules in the Linux kernel.
Very powerful and flexible
Rule-based and chain-based (INPUT, OUTPUT, FORWARD)
Commonly found on older and enterprise Linux systems
Because of its complexity, it is usually managed indirectly through higher-level tools.
nftables
nftables is the modern replacement for iptables.
Simpler and more efficient rule management
Better performance and scalability
Unified framework for packet filtering
Most modern Linux distributions are moving toward nftables.
ufw
ufw (Uncomplicated Firewall) is a user-friendly firewall management tool.
Designed for simplicity
Commonly used on Ubuntu systems
Suitable for basic firewall configurations
Example:
ufw allow 22
At a minimum, DevOps engineers must understand that:
Services can be running but still inaccessible
Firewall rules may block required ports
Security always involves controlled accessible
Firewall rules may block required ports
Security always involves controlled access
Principle of Least Privilege
Systems should grant only the permissions required to perform a task.
Examples:
Avoid logging in as root
Use sudo only when necessary
Restrict SSH access to specific users
This principle reduces the impact of security breaches and mistakes.
Automation Mindset
Manual work does not scale.
In DevOps:
Repeated tasks should be automated
Configuration should be consistent
Human error should be minimized
Linux provides the foundation for automation through:
Shell scripts
Cron jobs
Configuration management tools
Simple Automation Example
saad@saad-VirtualBox:/$ sudo touch script.sh
saad@saad-VirtualBox:/$ sudo vi script.sh
saad@saad-VirtualBox:/$ sudo chown saad:saad script.sh
saad@saad-VirtualBox:/$ ls -ltr | grep script.sh
-rwxr-xr-x 1 saad saad 65 Dec 18 13:10 script.sh
saad@saad-VirtualBox:/$ ./script.sh
Server health check
13:11:28 up 29 min, 2 users, load average: 0.13, 0.07, 0.03
Filesystem Size Used Avail Use% Mounted on
tmpfs 246M 1.6M 245M 1% /run
/dev/sda2 15G 13G 1.9G 87% /
tmpfs 1.3G 12K 1.3G 1% /dev/shm
tmpfs 5.0M 8.0K 5.0M 1% /run/lock
tmpfs 246M 120K 246M 1% /run/user/1000
total used free shared buff/cache available
Mem: 2.4Gi 1.3Gi 244Mi 28Mi 1.0Gi 1.1Gi
Swap: 2.4Gi 0B 2.4Gi
saad@saad-VirtualBox:/$
Even simple scripts can save time and reduce errors.
Security in DevOps Context
Linux security knowledge is applied when:
Hardening servers
Securing CI/CD runners
Managing cloud instances
Preparing systems for containerization
Security is not a one-time task, but a continuous responsibility.
Summary
Day 6 introduced essential Linux security practices including secure remote access with SSH, firewall awareness, least privilege principles, and the automation mindset. This marks the completion of Linux fundamentals required for DevOps work.
