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Linux Lab 14: SSH and SCP - Secure Remote Access

This lab covers the essential skills for managing remote servers securely. You will learn about the SSH protocol, how to connect to servers, how to set up password-less authentication for automation and ease of use, and how to transfer files securely using SCP.

Table of Contents

Key Concepts

  • SSH (Secure Shell): A cryptographic network protocol for operating network services securely over an unsecured network. It is the standard for remote server administration.
  • Port 22: The standard TCP port assigned to the SSH protocol.
  • Key Pair: A pair of cryptographic keys used for authentication.
    • Private Key (id_rsa): Kept secret on your local machine. It proves your identity.
    • Public Key (id_rsa.pub): Shared with remote servers. It allows them to verify your identity.
  • ssh-copy-id: A utility that simplifies the process of copying your public key to a remote server's authorized_keys file.
  • SCP (Secure Copy Protocol): A means of securely transferring computer files between a local host and a remote host or between two remote hosts.

[Image of ssh key authentication process]

Step-by-Step Walkthrough

1. SSH Default Port

Question: Which port number does the SSH service use by default? Answer: 22

Explanation: SSH listens on port 22 by default. This is a well-known port reserved for this service. You can verify this by looking at /etc/ssh/sshd_config or /etc/services.

2. Identifying the SSH User

Question: If you run ssh devapp01, which user are you using to connect? Answer: bob

Explanation: If you do not specify a username (e.g., ssh user@host), SSH defaults to using the current username of the local session. Since you are logged into caleston-lp10 as bob, ssh devapp01 attempts to connect as bob@devapp01.

3. Password-less SSH Setup Overview

Goal: Connect to devapp01 without typing the password caleston123 every time. Method: Use RSA key-based authentication.

4. Generating SSH Keys (ssh-keygen)

Task: Generate an RSA key pair. Command: 

ssh-keygen -t RSA
* -t RSA: Specifies the type of key to create. * Prompts: You can press Enter through all prompts (file location and passphrase) to accept defaults for this lab. * Result: Creates a private key (~/.ssh/id_rsa) and a public key (~/.ssh/id_rsa.pub).

5. Locating the Public Key

Question: What is the path to the public key created? Answer: /home/bob/.ssh/id_rsa.pub

Explanation: The output of the ssh-keygen command explicitly states: Your public key has been saved in /home/bob/.ssh/id_rsa.pub.

6. Copying the Key to the Server (ssh-copy-id)

Task: Install the public key on devapp01. Command: 

ssh-copy-id bob@devapp01
Explanation: * This script logs into the remote server (you will need to enter the password caleston123 one last time). * It automatically appends your public key to the remote user's ~/.ssh/authorized_keys file. * It sets the correct permissions on the file and directory.

Verification: Run ssh bob@devapp01. You should log in immediately without a password prompt.

7. Understanding Authorized Keys

Question: Which file on the target server is the public key copied into? Answer: authorized_keys

Explanation: The SSH daemon (sshd) on the remote server looks at the ~/.ssh/authorized_keys file to find a list of public keys that are allowed to access that account. ssh-copy-id manages this file for you.

8. Transferring Files with SCP

Task: Copy /home/bob/caleston-code.tar.gz to devapp01 in Bob's home directory. Command: 

scp /home/bob/caleston-code.tar.gz devapp01:/home/bob

Explanation: * scp: The command (Secure Copy). * Source: /home/bob/caleston-code.tar.gz (Local file). * Destination: devapp01:/home/bob (Remote host : Remote path). * Since password-less SSH is set up, this transfer happens instantly without prompting for a password.

Command Reference

Command Purpose Example
ssh user@host Connect to a remote server ssh bob@devapp01
ssh-keygen -t RSA Generate a new SSH key pair ssh-keygen -t RSA
ssh-copy-id user@host Copy public key to remote server for password-less login ssh-copy-id bob@devapp01
scp source dest Securely copy files between hosts scp file.txt user@host:/tmp
ls -a ~/.ssh List SSH configuration files (hidden) ls -la ~/.ssh