Firewall basics (ufw, iptables) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable the UFW firewall.

Linux CLI

sudo ufw [1]

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

Adisable

Benable

Cstatus

Dreset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'disable' instead of 'enable' will turn the firewall off.

Using 'status' only shows the current state, it does not enable the firewall.

Use ufw enable to turn on the firewall.

2fill in blank

medium

Complete the code to allow incoming SSH connections using UFW.

Linux CLI

sudo ufw [1] ssh

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

Areject

Bdeny

Callow

Dlimit

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'deny' or 'reject' blocks SSH connections.

Using 'limit' restricts connections but does not fully allow them.

Use ufw allow ssh to permit SSH traffic through the firewall.

3fill in blank

hard

Fix the error in the iptables command to list all rules with line numbers.

Linux CLI

sudo iptables -[1] --line-numbers

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

AL

Blist

CS

DN

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'l' causes an error.

Using 'list' as a full word is not valid in iptables options.

The correct option to list rules is -L. It shows all rules with optional line numbers.

4fill in blank

hard

Fill both blanks to add a rule that drops incoming TCP packets on port 80 using iptables.

Linux CLI

sudo iptables -A INPUT -p [1] --dport [2] -j DROP

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

Atcp

Budp

C80

D22

Attempts:

3 left

💡 Hint

Common Mistakes

Using UDP instead of TCP will not match HTTP traffic.

Using port 22 blocks SSH, not HTTP.

Use protocol tcp and port 80 to drop HTTP traffic.

5fill in blank

hard

Fill all three blanks to create a UFW rule that limits incoming connections on port 22.

Linux CLI

sudo ufw [1] [2]/[3]

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

Alimit

B22

Ctcp

Ddeny

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'deny' blocks all connections instead of limiting.

Omitting the protocol may cause the rule to be less specific.

The command ufw limit 22/tcp limits SSH connections to prevent brute force attacks.

Practice

1. What is the primary purpose of a firewall like ufw or iptables on a Linux system?

easy

A. To update the Linux kernel automatically

B. To speed up the internet connection

C. To manage user accounts and passwords

D. To control network access and protect the system from unauthorized connections

Solution

1. Step 1: Understand firewall function

   A firewall controls network traffic to protect the system from unwanted access.

2. Step 2: Identify the correct purpose

   Options B, C, and D describe unrelated system tasks, so only To control network access and protect the system from unauthorized connections fits.

3. Final Answer:

   To control network access and protect the system from unauthorized connections -> Option D

4. Quick Check:

   Firewall = network protection [OK]

Hint: Firewalls block or allow network traffic to protect your system [OK]

Common Mistakes:

• Confusing firewall with system update tools
• Thinking firewall manages user accounts
• Assuming firewall speeds up internet

2. Which of the following is the correct command to enable the ufw firewall?

easy

A. ufw enable

B. ufw start

C. ufw on

D. ufw activate

Solution

1. Step 1: Recall ufw command to enable firewall

   The correct command to enable ufw is ufw enable.

2. Step 2: Check other options

   Commands like ufw start, ufw on, and ufw activate are invalid.

3. Final Answer:

   ufw enable -> Option A

4. Quick Check:

   Enable ufw = ufw enable [OK]

Hint: Remember: 'enable' starts ufw, not 'start' or 'on' [OK]

Common Mistakes:

• Using 'ufw start' instead of 'ufw enable'
• Typing 'ufw on' which is not a valid command
• Confusing with systemctl commands

3. What will be the output of the command sudo ufw status after running sudo ufw allow 22 and sudo ufw enable on a fresh system?

medium

A. Status: active\nTo Action From\n-- ------ ----\n80 ALLOW Anywhere

B. Status: active\nTo Action From\n-- ------ ----\n22 ALLOW Anywhere

C. Status: active\nNo rules configured

D. Status: inactive\nNo rules configured

Solution

1. Step 1: Understand commands run

   sudo ufw allow 22 opens port 22 (SSH). sudo ufw enable activates the firewall.

2. Step 2: Predict ufw status output

   The status will be active and show port 22 allowed from anywhere.

3. Final Answer:

   Status: active\nTo Action From\n-- ------ ----\n22 ALLOW Anywhere -> Option B

4. Quick Check:

   Allow 22 + enable = active with port 22 allowed [OK]

Hint: Allow port then enable ufw to see active status with rules [OK]

Common Mistakes:

• Expecting status inactive after enabling
• Confusing allowed ports (80 vs 22)
• Missing the 'active' status line

4. You wrote the command sudo ufw allow sshd but it returns an error. What is the likely cause and how to fix it?

medium

A. The service name 'sshd' is incorrect; use 'ssh' instead

B. You need to run the command without sudo

C. The firewall is disabled; enable it first

D. The port number must be specified instead of service name

Solution

1. Step 1: Identify the error cause

   The service name for SSH in ufw is 'ssh', not 'sshd'. Using 'sshd' causes an error.

2. Step 2: Correct the command

   Use sudo ufw allow ssh to allow SSH traffic properly.

3. Final Answer:

   The service name 'sshd' is incorrect; use 'ssh' instead -> Option A

4. Quick Check:

   Use correct service names like 'ssh' [OK]

Hint: Use 'ssh' not 'sshd' as ufw service name [OK]

Common Mistakes:

• Trying without sudo and expecting success
• Assuming firewall must be enabled before allowing ports
• Using port numbers when service names are accepted

5. You want to block all incoming traffic except SSH (port 22) and HTTP (port 80) using iptables. Which sequence of commands achieves this correctly?

hard

A. sudo iptables -P OUTPUT DROP\nsudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT\nsudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT

B. sudo iptables -P INPUT ACCEPT\nsudo iptables -A INPUT -p tcp --dport 22 -j DROP\nsudo iptables -A INPUT -p tcp --dport 80 -j DROP

C. sudo iptables -P INPUT DROP\nsudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT\nsudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT\nsudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

D. sudo iptables -P INPUT DROP\nsudo iptables -A OUTPUT -p tcp --dport 22 -j ACCEPT\nsudo iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT

Solution

1. Step 1: Set default policy to DROP incoming traffic

   sudo iptables -P INPUT DROP blocks all incoming by default.

2. Step 2: Allow incoming SSH and HTTP ports

   Commands to accept TCP traffic on ports 22 and 80 allow SSH and HTTP.

3. Step 3: Allow established connections

   sudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT lets responses through.

4. Final Answer:

   sudo iptables -P INPUT DROP\nsudo iptables -A INPUT -p tcp --dport 22 -j ACCEPT\nsudo iptables -A INPUT -p tcp --dport 80 -j ACCEPT\nsudo iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -> Option C

5. Quick Check:

   Default DROP + allow ports 22,80 + established accept = correct [OK]

Hint: Set default DROP, then allow needed ports and established states [OK]

Common Mistakes:

• Setting OUTPUT policy instead of INPUT
• Dropping allowed ports instead of accepting
• Forgetting to allow established connections