SSH connection basics in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this SSH command?

You run the command ssh -v user@192.168.1.10 to connect to a server. Which of the following lines will you see in the verbose output indicating a successful key exchange?

Linux CLI

ssh -v user@192.168.1.10

A Permission denied (password).

B debug1: Authentication succeeded (publickey).

C ssh: Could not resolve hostname 192.168.1.10: Name or service not known

D Connection timed out during banner exchange

💡 Hint

Look for messages that confirm authentication success in verbose SSH output.

The line 'debug1: Authentication succeeded (publickey).' shows that the SSH client successfully authenticated using a public key. Other options indicate errors or failures.

🧠 Conceptual

intermediate

Which SSH option disables strict host key checking?

You want to connect to a new SSH server without being prompted to confirm the server's fingerprint. Which SSH option disables this prompt?

A -o PasswordAuthentication=no

B -o UserKnownHostsFile=/dev/null

C -o StrictHostKeyChecking=no

D -o ConnectTimeout=5

💡 Hint

This option controls whether SSH asks to confirm the server's fingerprint.

The option '-o StrictHostKeyChecking=no' disables the prompt asking to confirm the server's fingerprint, allowing automatic connection.

🔧 Debug

advanced

Why does this SSH command fail with 'Permission denied'?

You run ssh user@host but get Permission denied (publickey,password). error. Which of the following is the most likely cause?

A The SSH server does not allow password authentication and your public key is not set up.

B The SSH client is using the wrong port number.

C The SSH server is down and unreachable.

D The username is misspelled in the command.

💡 Hint

Consider what authentication methods the server accepts and what keys you have.

The error means the server rejected both public key and password authentication. Usually, this happens if the server requires a public key but the client has none configured.

🚀 Application

advanced

How to copy a file securely using SSH?

You want to copy a file named report.txt from your local machine to the remote server server.example.com under the directory /home/user/docs. Which command correctly does this?

A scp report.txt [e-mail]:/home/user/docs/

B ssh [e-mail] cp report.txt /home/user/docs/

C ssh-copy-id report.txt [e-mail]:/home/user/docs/

D rsync report.txt [e-mail]:/home/user/docs/

💡 Hint

Think about which command is designed to copy files over SSH.

The scp command securely copies files over SSH. Option A tries to run a copy command on the remote server but does not transfer the file. Option A copies SSH keys, not files. Option A can copy files but requires rsync installed and configured.

💻 Command Output

expert

What is the output of this SSH command with a custom port?

You run ssh -p 2222 user@host but get the error message. Which option shows the exact error you will see if port 2222 is closed on the server?

Linux CLI

ssh -p 2222 user@host

A Connection timed out during banner exchange

B ssh: Could not resolve hostname host: Name or service not known

C Permission denied (publickey).

D ssh: connect to host host port 2222: Connection refused

💡 Hint

Think about what happens when the port is closed but the host is reachable.

If the port is closed, SSH cannot connect and shows 'Connection refused'. Other errors indicate DNS issues, authentication failure, or timeout.

Practice

1. What does the command ssh user@hostname do?

easy

A. Lists files on the remote computer.

B. Copies files from the remote computer to local.

C. Starts a secure connection to the remote computer as 'user'.

D. Deletes the user account on the remote computer.

Solution

1. Step 1: Understand the ssh command purpose

   The ssh command is used to securely connect to another computer over a network.

2. Step 2: Analyze the syntax ssh user@hostname

   This syntax means connect as 'user' to the computer identified by 'hostname'.

3. Final Answer:

   Starts a secure connection to the remote computer as 'user'. -> Option C

4. Quick Check:

   ssh user@hostname = Secure connection [OK]

Hint: ssh user@host means connect securely as user [OK]

Common Mistakes:

• Thinking ssh copies files instead of connecting
• Confusing ssh with file listing commands
• Assuming ssh deletes accounts

2. Which of these is the correct syntax to connect to a remote server with ssh?

easy

A. ssh user@hostname

B. ssh -connect user hostname

C. ssh connect user@hostname

D. ssh user hostname

Solution

1. Step 1: Recall ssh command syntax

   The correct syntax is ssh user@hostname without extra words or flags for basic connection.

2. Step 2: Check each option

   Options A, B, and D have incorrect syntax or missing '@' sign.

3. Final Answer:

   ssh user@hostname -> Option A

4. Quick Check:

   Correct ssh syntax = ssh user@hostname [OK]

Hint: ssh always uses user@hostname format [OK]

Common Mistakes:

• Adding extra words like 'connect' in command
• Omitting '@' between user and hostname
• Using flags incorrectly for basic ssh

3. What will happen if you run ssh alice@192.168.1.10 and the password is correct?

medium

A. You will see a list of files on your local computer.

B. The remote computer will shut down.

C. Your local files will be deleted.

D. You will be logged into the remote computer as user alice.

Solution

1. Step 1: Understand ssh login process

   Running ssh with user and IP tries to connect and log in to that remote machine.

2. Step 2: Check what happens on correct password

   If password is correct, you get a command prompt on the remote machine as that user.

3. Final Answer:

   You will be logged into the remote computer as user alice. -> Option D

4. Quick Check:

   ssh + correct password = remote login [OK]

Hint: Correct password means remote login success [OK]

Common Mistakes:

• Thinking ssh shuts down remote machine
• Confusing ssh with file deletion
• Assuming ssh lists local files

4. You try to connect with ssh [e-mail] but get the error: Permission denied (publickey). What is the likely problem?

medium

A. You typed the wrong hostname.

B. Your SSH key is missing or not accepted by the server.

C. The server is turned off.

D. You forgot to install ssh on your computer.

Solution

1. Step 1: Understand the error message

   "Permission denied (publickey)" means the server expects an SSH key for authentication but did not accept yours.

2. Step 2: Identify cause of error

   This usually means your SSH key is missing, not loaded, or not authorized on the server.

3. Final Answer:

   Your SSH key is missing or not accepted by the server. -> Option B

4. Quick Check:

   Permission denied (publickey) = SSH key issue [OK]

Hint: Permission denied (publickey) means SSH key problem [OK]

Common Mistakes:

• Assuming hostname typo causes this error
• Thinking server power state causes this error
• Believing ssh must be installed locally to connect

5. You want to connect to a remote server at example.com as user admin but the server uses a non-standard port 2222. Which command will connect correctly?

hard

A. ssh -p 2222 [e-mail]

B. ssh [e-mail]:2222

C. ssh [e-mail] -port 2222

D. ssh [e-mail]

Solution

1. Step 1: Recall how to specify a port in ssh

   The correct flag to specify a port is -p followed by the port number.

2. Step 2: Analyze each option

   ssh -p 2222 [e-mail] uses -p 2222 correctly. Options A, C, and D use incorrect syntax or wrong placement.

3. Final Answer:

   ssh -p 2222 [e-mail] -> Option A

4. Quick Check:

   Use -p to specify port in ssh [OK]

Hint: Use ssh -p port user@host for custom ports [OK]

Common Mistakes:

• Putting port after hostname with colon
• Using -port instead of -p
• Changing hostname to include port number