Port scanning basics in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to scan port 80 on a target IP using netcat.

Bash Scripting

nc -zv [1] 80

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

A192.168.1.1

B255.255.255.0

Cgoogle.com

Dlocalhost

Attempts:

3 left

💡 Hint

Common Mistakes

Using a network mask like 255.255.255.0 instead of an IP address.

Using a domain name that might not respond on port 80.

The nc -zv command scans the specified port on the given host. Here, localhost refers to your own machine.

2fill in blank

medium

Complete the code to scan ports 20 to 25 on a target IP using a for loop.

Bash Scripting

for port in [1]; do nc -zv 127.0.0.1 $port; done

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

A{20..25}

B20-25

C20 21 22 23 24 25

D20,21,22,23,24,25

Attempts:

3 left

💡 Hint

Common Mistakes

Using a dash like '20-25' which is not valid for bash loops.

Using commas which are not recognized as separators in bash loops.

The brace expansion {20..25} generates the sequence of numbers from 20 to 25, which the loop iterates over.

3fill in blank

hard

Fix the error in the code to scan port 443 on a target IP stored in variable $target.

Bash Scripting

nc -zv [1] 443

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

Atarget

B$target

C${target}

Dtarget$

Attempts:

3 left

💡 Hint

Common Mistakes

Using the variable name without the dollar sign.

Placing the dollar sign after the variable name.

Using ${target} ensures the variable is expanded correctly in bash.

4fill in blank

hard

Fill both blanks to create a loop that scans ports 80 to 82 on IP 10.0.0.1.

Bash Scripting

for port in [1]; do nc -zv [2] $port; done

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

A{80..82}

B10.0.0.2

C10.0.0.1

D80 81 82

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong IP address.

Using a space-separated list instead of brace expansion for ports.

The brace expansion {80..82} generates ports 80, 81, and 82. The IP 10.0.0.1 is the target to scan.

5fill in blank

hard

Fill all three blanks to create a dictionary-like output of ports and their status using a loop and conditional.

Bash Scripting

for port in [1]; do
  nc -z [2] [3] $port && echo "$port: open" || echo "$port: closed"
done

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

A{22..24}

B127.0.0.1

C-v

D192.168.0.1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong IP address.

Omitting the verbose flag which helps see connection status.

The loop scans ports 22 to 24 on localhost (127.0.0.1) with verbose flag -v. The conditional prints if the port is open or closed.

Practice

1. What is the main purpose of port scanning in network security?

easy

A. To install software remotely

B. To delete files on a remote server

C. To speed up internet connection

D. To find open ports on a computer

Solution

1. Step 1: Understand port scanning basics

   Port scanning is used to check which network ports on a computer are open and accepting connections.

2. Step 2: Identify the correct purpose

   Among the options, only finding open ports matches the purpose of port scanning.

3. Final Answer:

   To find open ports on a computer -> Option D

4. Quick Check:

   Port scanning = find open ports [OK]

Hint: Port scanning checks open ports, not files or speed [OK]

Common Mistakes:

• Confusing port scanning with file operations
• Thinking it improves internet speed
• Assuming it installs software

2. Which of the following bash commands correctly scans if port 80 is open on host 192.168.1.1?

easy

A. nc -zv 192.168.1.1 80

B. nc -xv 192.168.1.1 80

C. nc -z 192.168.1.1 -p 80

D. nc -v 192.168.1.1

Solution

1. Step 1: Recall correct netcat syntax for port scanning

   The option '-z' tells nc to scan without sending data, and '-v' enables verbose output. The host and port follow these flags.

2. Step 2: Check each option

   Only nc -zv 192.168.1.1 80 uses the correct flags '-z' for scanning and '-v' for verbose, followed by host and port. Other options either lack necessary flags, use incorrect flags like -xv or misuse -p for source port, or omit the port specification.

3. Final Answer:

   nc -zv 192.168.1.1 80 -> Option A

4. Quick Check:

   Correct nc flags = -zv [OK]

Hint: Use nc -zv host port to scan a port [OK]

Common Mistakes:

• Using wrong flags like -xv
• Misplacing port with -p flag
• Omitting port number

3. What will be the output of this bash snippet scanning ports 22 and 80 on localhost?

for port in 22 80; do
  nc -zv localhost $port
 done

medium

A. It will produce a syntax error

B. It will show if ports 22 and 80 are open or closed on localhost

C. It will scan ports 22 and 80 on a remote host

D. It will scan all ports from 22 to 80

Solution

1. Step 1: Analyze the for loop and command

   The loop iterates over ports 22 and 80, running 'nc -zv localhost $port' for each port.

2. Step 2: Understand the command effect

   Netcat checks if each specified port on localhost is open, printing the result.

3. Final Answer:

   It will show if ports 22 and 80 are open or closed on localhost -> Option B

4. Quick Check:

   Loop scans specified ports on localhost [OK]

Hint: Loop scans listed ports on localhost, not a range [OK]

Common Mistakes:

• Thinking it scans a range of ports
• Assuming remote host scanning
• Expecting syntax errors

4. Identify the error in this bash script intended to scan ports 20 to 25 on 127.0.0.1:

for port in {20..25} do
  nc -zv 127.0.0.1 $port
 done

medium

A. The loop syntax is incorrect; it should use parentheses

B. The loop variable $port is not referenced correctly

C. The script is missing a semicolon after the for statement

D. The loop block is missing a 'do' keyword

Solution

1. Step 1: Check for correct bash for-loop syntax

   Bash requires a semicolon or newline after the for statement before 'do'. Here, 'for port in {20..25}' is not followed by a semicolon or newline before 'do'.

2. Step 2: Verify other syntax elements

   The loop variable and block are correct, and 'do' is present. The missing semicolon causes a syntax error.

3. Final Answer:

   The script is missing a semicolon after the for statement -> Option C

4. Quick Check:

   Missing semicolon after for statement causes error [OK]

Hint: Add semicolon or newline after for statement before do [OK]

Common Mistakes:

• Omitting semicolon after for line
• Misplacing 'do' keyword
• Incorrect variable usage

5. You want to scan all ports from 80 to 85 on host 10.0.0.5 and save only open ports to a file named open_ports.txt. Which bash script snippet achieves this?

hard

A. for port in {80..85}; do nc -zv 10.0.0.5 $port 2>&1 | grep 'succeeded' >> open_ports.txt; done

B. for port in 80 81 82 83 84 85; do nc -zv 10.0.0.5 $port > open_ports.txt; done

C. nc -zv 10.0.0.5 80-85 > open_ports.txt

D. for port in {80..85}; do nc -zv 10.0.0.5 $port | grep 'failed' >> open_ports.txt; done

Solution

1. Step 1: Understand the scanning and filtering requirements

   The script must scan ports 80 to 85, detect open ports, and save only those results to a file.

2. Step 2: Analyze each option

   for port in {80..85}; do nc -zv 10.0.0.5 $port 2>&1 | grep 'succeeded' >> open_ports.txt; done loops over ports 80 to 85, runs nc with verbose and zero-I/O flags, redirects stderr to stdout, filters lines containing 'succeeded' (indicating open ports), and appends to open_ports.txt. for port in 80 81 82 83 84 85; do nc -zv 10.0.0.5 $port > open_ports.txt; done overwrites the file each time and does not filter open ports. nc -zv 10.0.0.5 80-85 > open_ports.txt uses invalid port range syntax. for port in {80..85}; do nc -zv 10.0.0.5 $port | grep 'failed' >> open_ports.txt; done filters 'failed' ports, opposite of requirement.

3. Final Answer:

   for port in {80..85}; do nc -zv 10.0.0.5 $port 2>&1 | grep 'succeeded' >> open_ports.txt; done -> Option A

4. Quick Check:

   Loop + grep 'succeeded' + append to file = correct [OK]

Hint: Filter 'succeeded' lines and append output to file [OK]

Common Mistakes:

• Overwriting output file inside loop
• Using invalid port range syntax
• Filtering 'failed' instead of 'succeeded'