OpenSSH Server

• OpenSSH server is the program that allows remote login using SSH.
• The server process is called sshd.
• It typically starts automatically at boot.
• Its main configuration file is: /etc/ssh/sshd_config

What is an SSH key?

• SSH supports an authentication mechanism based on cryptographic keys, called public key authentication.
• An SSH key pair consists of:
  • Public key
  • Private key

---

Authorized keys

• One or more public keys can be configured on the server as authorized keys.
• These are stored in: ~/.ssh/authorized_keys (inside the user’s home directory on the server)

How authentication works

• The private key stays on the client machine.
• The public key is copied to the server.
• During login:
  • The server checks whether the client can prove it owns the private key
  • If it matches a public key in authorized_keys, access is granted
• The private key is never sent over the network

SSH key workflow (client → server)

On client (Windows, Linux, macOS)

Generate a key pair:

ssh-keygen -t rsa

This creates two files in:

~/.ssh/

• id_rsa → private key
• id_rsa.pub → public key

Client Machine Windows on CMD

On server (Ubuntu)

1. Ensure OpenSSH server is installed:

sudo apt install openssh-server

2. Make sure SSH service is running:

sudo systemctl start ssh

3. Copy the public key contents into:

~/.ssh/authorized_keys

Server Machine Ubuntu

SSH Connection Established

Connection Flow

[ Client (Windows / Linux) ]
   └── private key
        ↓ SSH connection
[ Network ]
        ↓
[ Linux Server (sshd) ]
   └── authorized_keys (public key)

If keys match → login succeeds without password

Key Points – Exam & Interview

• sshd = SSH server daemon
• SSH keys = public + private
• Public key → server (authorized_keys)
• Private key → client only
• More secure than passwords
• Config file: /etc/ssh/sshd_config

Click for Ubuntu Index