Day-7: Linux SSH Authentication | 100 Days of DevOps

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When we work with remote servers, the most common way to connect is through SSH (Secure Shell). If you’ve ever managed Linux servers or deployed applications, chances are you’ve used it.
In this article, we’ll walk through what SSH is, how to use it for login, and finally how to set up password-less login across multiple servers.

What is SSH?

SSH (Secure Shell) is a protocol that let us securely connect to another machine over an insecure network. It’s widely used by developers, system admins, and DevOps engineers to:

Log in to remote servers
Run commands
Transfer files
Manage systems securely

Unlike older methods like Telnet, SSH encrypts all communication. This means our credentials and data are safe from eavesdropping.

How to Log In Using SSH

The basic syntax of an SSH login is:

ssh username@hostname

username → the user account on the remote server
hostname → the server’s IP address or domain name

Example

If we have a server at 172.16.238.10 with a user tony:

ssh tony@172.16.238.10

You’ll be prompted to enter the password for that user. Once authenticated, you’ll get a shell on the remote machine.

The Problem with Passwords

Typing a password every single time quickly becomes annoying, especially when we need to log into multiple servers or run automation scripts. Passwords also pose security risks if they are weak or reused.

This is where SSH keys come in.

Password-less SSH Login

SSH keys allow us to authenticate without typing a password. The idea is simple:

Generate a pair of keys on our local machine (private + public).
Copy the public key to the remote server.
When we connect, the server verifies our private key against the stored public key.

If they match, we can login without any password.

Step 1: Generate SSH Keys

On the client machine (in our case, the jump host with user thor):

ssh-keygen -t rsa -b 4096

Press Enter to accept the defaults.
Leave the passphrase empty (important for automation).

This creates two files:

~/.ssh/id_rsa → your private key (keep it safe!)
~/.ssh/id_rsa.pub → your public key (share this with servers)

Step 2: Copy the Public Key to the Server

To enable password-less login, copy the public key to the server:

ssh-copy-id -i ~/.ssh/id_rsa.pub username@hostname

Example

For a server at 172.16.238.10 with user tony:

ssh-copy-id -i ~/.ssh/id_rsa.pub tony@172.16.238.10

You’ll be asked for the password one last time. After that, you’ll be able to log in without it.

Step 3: Log In Without a Password

Now test it:

ssh tony@172.16.238.10

This time we can connect directly, without any password prompt.

Real-World Example: Multiple App Servers

Here’s a real setup where we had to configure password-less login for a jump host user (thor) to multiple application servers.

Server Name	IP	Hostname	User	Password	Purpose
stapp01	172.16.238.10	stapp01.stratos.xfusioncorp.com	tony	Ir0nM@n	Nautilus App 1
stapp02	172.16.238.11	stapp02.stratos.xfusioncorp.com	steve	Am3ric@	Nautilus App 2
stapp03	172.16.238.12	stapp03.stratos.xfusioncorp.com	banner	BigGr33n	Nautilus App 3

We ran:

ssh-copy-id -i ~/.ssh/id_rsa.pub tony@172.16.238.10
ssh-copy-id -i ~/.ssh/id_rsa.pub steve@172.16.238.11
ssh-copy-id -i ~/.ssh/id_rsa.pub banner@172.16.238.12

Now, the thor user can log in to all app servers without typing passwords.