Key-based authentication in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to generate a new SSH key pair.

Linux CLI

ssh-keygen -t [1] -f ~/.ssh/id_rsa

Drag options to blanks, or click blank then click option'

Aed25519

Brsa

Cecdsa

Ddsa

Attempts:

3 left

💡 Hint

Common Mistakes

Using dsa which is outdated and less secure.

Using rsa without specifying key size.

The ed25519 algorithm is recommended for SSH keys because it is secure and fast.

2fill in blank

medium

Complete the command to copy your public key to the remote server for passwordless login.

Linux CLI

ssh-copy-id -i ~/.ssh/id_rsa.pub [1]@remote_host

Drag options to blanks, or click blank then click option'

Aroot

Bguest

Cadmin

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Using root without permission.

Using guest which may not have SSH access.

You should replace user with your actual username on the remote server.

3fill in blank

hard

Fix the error in the SSH config file to specify the private key file.

Linux CLI

Host remote
  HostName example.com
  User user
  IdentityFile [1]

Drag options to blanks, or click blank then click option'

A~/.ssh/id_rsa

B~/.ssh/config

C~/.ssh/authorized_keys

D~/.ssh/id_rsa.pub

Attempts:

3 left

💡 Hint

Common Mistakes

Using the public key file instead of the private key.

Using unrelated files like config.

The IdentityFile should point to your private key, not the public key or other files.

4fill in blank

hard

Fill both blanks to create a directory and set correct permissions for SSH keys.

Linux CLI

mkdir -p ~/.ssh && chmod [1] ~/.ssh && chmod [2] ~/.ssh/id_rsa

Drag options to blanks, or click blank then click option'

A700

B600

C755

D644

Attempts:

3 left

💡 Hint

Common Mistakes

Setting 755 on private keys, making them world-readable.

Setting 644 on the .ssh directory.

The .ssh directory needs 700 permissions to be accessible only by you. The private key file needs 600 permissions to be readable and writable only by you.

5fill in blank

hard

Fill all three blanks to create an SSH config entry for a server with a custom port and identity file.

Linux CLI

Host myserver
  HostName [1]
  Port [2]
  IdentityFile [3]

Drag options to blanks, or click blank then click option'

A192.168.1.100

B2222

C~/.ssh/my_custom_key

Dexample.com

Attempts:

3 left

💡 Hint

Common Mistakes

Using the public key file for IdentityFile.

Using default port 22 when a custom port is required.

Use the server IP or hostname for HostName, the custom SSH port number for Port, and the path to your private key for IdentityFile.

Practice

1. What is the main purpose of key-based authentication in SSH?

easy

A. To encrypt files on the local computer

B. To allow secure login without typing a password every time

C. To speed up the internet connection

D. To create a backup of the server data

Solution

1. Step 1: Understand key-based authentication

   Key-based authentication uses a pair of keys to allow login without passwords.

2. Step 2: Identify the main benefit

   This method lets users connect securely without typing passwords each time.

3. Final Answer:

   To allow secure login without typing a password every time -> Option B

4. Quick Check:

   Key-based authentication = passwordless secure login [OK]

Hint: Key-based auth means no password typing needed [OK]

Common Mistakes:

• Confusing key-based auth with file encryption
• Thinking it speeds up internet
• Believing it backs up data

2. Which command correctly generates a new SSH key pair?

easy

A. ssh-keygen

B. ssh-copy-id

C. ssh-add

D. ssh-connect

Solution

1. Step 1: Identify the key generation command

   The command ssh-keygen creates a new SSH key pair.

2. Step 2: Differentiate from other commands

   ssh-copy-id copies keys, ssh-add adds keys to agent, ssh-connect is invalid.

3. Final Answer:

   ssh-keygen -> Option A

4. Quick Check:

   Generate keys = ssh-keygen [OK]

Hint: ssh-keygen always creates keys [OK]

Common Mistakes:

• Using ssh-copy-id to generate keys
• Confusing ssh-add with key creation
• Typing a non-existent ssh-connect command

3. What will happen if you run ssh-copy-id user@server after generating your SSH keys?

medium

A. The SSH service on the server restarts

B. Your private key is copied to the server

C. The server's keys are copied to your local machine

D. Your public key is copied to the server for passwordless login

Solution

1. Step 1: Understand ssh-copy-id purpose

   This command copies your public key to the server's authorized keys file.

2. Step 2: Identify what is copied

   Only the public key is copied, enabling passwordless login; private keys stay local.

3. Final Answer:

   Your public key is copied to the server for passwordless login -> Option D

4. Quick Check:

   ssh-copy-id copies public key to server [OK]

Hint: ssh-copy-id copies public key, not private [OK]

Common Mistakes:

• Thinking private key is copied
• Believing server keys copy locally
• Assuming ssh-copy-id restarts SSH

4. You tried to connect with SSH key authentication but still get a password prompt. Which is the most likely cause?

medium

A. The public key was not copied to the server

B. The private key file is missing on the server

C. The SSH server is turned off

D. The username is incorrect

Solution

1. Step 1: Check key placement

   If the public key is not on the server, SSH cannot authenticate without a password.

2. Step 2: Understand private key location

   The private key stays on the client, so missing it on the server is irrelevant.

3. Final Answer:

   The public key was not copied to the server -> Option A

4. Quick Check:

   Missing public key on server = password prompt [OK]

Hint: Public key must be on server for key login [OK]

Common Mistakes:

• Looking for private key on server
• Assuming SSH server off causes password prompt
• Blaming wrong username without checking keys

5. You want to automate SSH login to multiple servers without passwords. Which sequence of commands is correct?

hard

A. ssh user@server -> ssh-copy-id user@server -> ssh-keygen

B. ssh-copy-id user@server -> ssh-keygen -> ssh user@server

C. ssh-keygen -> ssh-copy-id user@server -> ssh user@server

D. ssh user@server -> ssh-keygen -> ssh-copy-id user@server

Solution

1. Step 1: Generate SSH keys first

   Use ssh-keygen to create your key pair locally.

2. Step 2: Copy public key to server

   Use ssh-copy-id user@server to add your public key to the server.

3. Step 3: Connect without password

   Now ssh user@server logs in using the key without asking for a password.

4. Final Answer:

   ssh-keygen -> ssh-copy-id user@server -> ssh user@server -> Option C

5. Quick Check:

   Generate keys, copy key, then connect [OK]

Hint: Generate keys first, then copy, then connect [OK]

Common Mistakes:

• Copying keys before generating them
• Trying to connect before copying keys
• Mixing command order