Port scanning basics in Bash Scripting - Mini Project: Build & Apply

Port scanning basics

📖 Scenario: You are a network technician who needs to check which ports are open on a local server to ensure the services are running correctly.

🎯 Goal: Build a simple Bash script that scans a range of ports on a given IP address and reports which ports are open.

📋 What You'll Learn

Create a variable holding the target IP address

Create a variable holding the start and end port numbers

Use a loop to scan each port in the range

Check if the port is open using Bash built-in tools

Print the list of open ports

💡 Why This Matters

🌍 Real World

Network administrators often need to check which ports are open on servers to troubleshoot connectivity or security issues.

💼 Career

Basic port scanning skills help in network monitoring, security auditing, and system administration roles.

1

Set the target IP address

Create a variable called target_ip and set it to the string "127.0.0.1".

Bash Scripting

# Create a variable called target_ip and set it to "127.0.0.1"
# Your code here

Hint

Use variable_name="value" syntax to create a variable in Bash.

2

Set the port range to scan

Create two variables called start_port and end_port and set them to 75 and 85 respectively.

Bash Scripting

target_ip="127.0.0.1"
# Create variables start_port and end_port with values 75 and 85
# Your code here

Hint

Assign numbers to variables without quotes in Bash.

3

Scan ports in the range

Use a for loop with variable port to iterate from start_port to end_port. Inside the loop, use timeout 1 bash -c " to try connecting to $target_ip on $port with /dev/tcp. If the connection succeeds, add the port number to a variable called open_ports separated by spaces.

Bash Scripting

target_ip="127.0.0.1"
start_port=75
end_port=85
# Use a for loop from start_port to end_port
# Check if port is open using timeout and /dev/tcp
# Add open ports to open_ports variable
# Your code here

Hint

Use seq to generate numbers and timeout 1 bash -c "echo > /dev/tcp/$target_ip/$port" to test ports.

4

Print the open ports

Print the text "Open ports:" followed by the value of the variable open_ports.

Bash Scripting

target_ip="127.0.0.1"
start_port=75
end_port=85
open_ports=""
for port in $(seq $start_port $end_port); do
  if timeout 1 bash -c "echo > /dev/tcp/$target_ip/$port" 2>/dev/null; then
    open_ports+="$port "
  fi
done
# Print "Open ports:" and the open_ports variable
# Your code here

Hint

Use printf or echo to display the open ports.

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'