Port scanning basics in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this simple port scan script?

Consider this bash script that scans ports 80 and 443 on localhost. What output will it produce?

Bash Scripting

for port in 80 443; do
  timeout 1 bash -c "</dev/tcp/127.0.0.1/$port" && echo "Port $port is open" || echo "Port $port is closed"
done

A Port 80 is closed\nPort 443 is open

B Port 80 is closed\nPort 443 is closed

C Port 80 is open\nPort 443 is closed

D Port 80 is open\nPort 443 is open

💡 Hint

Think about which common services usually listen on ports 80 and 443 on localhost.

Ports 80 (HTTP) and 443 (HTTPS) are commonly open on localhost if a web server is running. The script tries to open a TCP connection to each port and prints if it is open or closed.

📝 Syntax

intermediate

Identify the syntax error in this port scanning snippet

Which option contains the correct syntax to scan port 22 on 192.168.1.1 using bash's /dev/tcp?

Bash Scripting

timeout 1 bash -c "</dev/tcp/192.168.1.1/22" && echo "Open" || echo "Closed"

A timeout 1 bash -c "</dev/tcp/192.168.1.1/22" && echo "Open" || echo "Closed"

B timeout 1 bash -c "</dev/tcp/192.168.1.1:22" && echo "Open" || echo "Closed"

C timeout 1 bash -c "</dev/tcp/192.168.1.1" 22 && echo "Open" || echo "Closed"

D timeout 1 bash -c "</dev/tcp/192.168.1.1-22" && echo "Open" || echo "Closed"

💡 Hint

Check the correct format for /dev/tcp host and port separation.

The correct syntax for bash's /dev/tcp is /dev/tcp/host/port without any colon or dash. Options A, B, and C use invalid separators or misplaced arguments causing syntax errors.

🔧 Debug

advanced

Why does this port scan script always report ports as closed?

This script tries to scan ports 20 to 22 on 10.0.0.1 but always prints 'Port X is closed'. What is the likely cause?

Bash Scripting

for port in {20..22}; do
  if timeout 1 bash -c "</dev/tcp/10.0.0.1/$port"; then
    echo "Port $port is open"
  else
    echo "Port $port is closed"
  fi
done

A The target host 10.0.0.1 is unreachable or blocking connections

B The timeout command is not installed, so the script fails silently

C The bash shell does not support /dev/tcp on this system

D The for loop syntax {20..22} is invalid in bash

💡 Hint

Check network connectivity and firewall rules on the target host.

If the target host is unreachable or firewall blocks connections, all attempts fail, so ports appear closed. The other options would cause errors or different failures.

🚀 Application

advanced

How to scan multiple ports efficiently in bash?

You want to scan ports 22, 80, 443, and 8080 on host 192.168.0.10 using bash. Which script snippet scans all ports and prints only open ones?

A for p in 22 80 443 8080; do timeout 1 bash -c "</dev/tcp/192.168.0.10/$p" && echo "Port $p open"; done

B for p in 22 80 443 8080; do timeout 1 bash -c "</dev/tcp/192.168.0.10/$p" || echo "Port $p open"; done

C for p in 22 80 443 8080; do if timeout 1 bash -c "</dev/tcp/192.168.0.10/$p"; then echo "Port $p open"; fi; done

D for p in 22 80 443 8080; do if ! timeout 1 bash -c "</dev/tcp/192.168.0.10/$p"; then echo "Port $p open"; fi; done

💡 Hint

Only print when the connection succeeds, not when it fails.

Option C correctly tests if the connection succeeds and prints 'Port X open'. Option C prints on success but also continues silently on failure. Options C and D invert logic causing wrong output.

🧠 Conceptual

expert

What is a limitation of using bash /dev/tcp for port scanning?

Which of the following is a true limitation when using bash's /dev/tcp feature for port scanning?

A It provides detailed service banners for each open port

B It cannot scan UDP ports because /dev/tcp only supports TCP connections

C It automatically scans all ports from 1 to 65535 without specifying ports

D It requires root privileges to open /dev/tcp sockets

💡 Hint

Think about the protocol support of /dev/tcp.

/dev/tcp in bash supports only TCP connections, so it cannot scan UDP ports. It does not require root privileges, does not scan all ports automatically, and does not provide service banners.

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'