How do you identify open ports using Masscan

Question

Masscan is known for its speed in scanning large address ranges. How does it work to detect open ports, and what makes it different from tools like Nmap?

Answer 1

Masscan is a high-performance, open-source port scanner designed to rapidly detect open ports across large network ranges. Its capability to scan the entire IPv4 address space in minutes sets it apart from other tools.

How Masscan Detects Open Ports:

Masscan employs an asynchronous scanning technique, allowing it to send multiple packets without waiting for individual responses. Specifically, it transmits TCP SYN packets to target ports:​

1. SYN Packet Transmission: Masscan sends a SYN packet to the specified port of the target IP address.​

2. Response Handling:

   • SYN-ACK Response: Indicates the port is open. Masscan records this as an open port and sends a RST packet to terminate the connection before completion.​

   • RST Response or No Response: Suggests the port is closed or filtered.​

This method allows Masscan to achieve exceptionally high scanning speeds, transmitting millions of packets per second from a single machine.

Differences Between Masscan and Nmap:

While both Masscan and Nmap are utilized for port scanning, they differ in several key aspects:​

1. Scanning Speed:

   • Masscan: Optimized for speed, capable of scanning the entire internet rapidly.​

   • Nmap: Offers comprehensive scanning features but operates at a slower pace due to its synchronous approach.​

2. Scanning Techniques:

   • Masscan: Utilizes asynchronous scanning, sending packets without awaiting responses, enabling high-speed operations.​

   • Nmap: Employs synchronous scanning, waiting for each response before proceeding, which enhances accuracy but reduces speed.​

3. Feature Set:

   • Masscan: Primarily focuses on identifying open ports quickly.​

   • Nmap: Provides extensive features, including service and OS detection, version detection, and scriptable interactions with the target.​

4. Host Discovery:

   • Masscan: Does not perform host discovery; assumes all specified hosts are online and attempts to scan them.​

   • Nmap: Performs host discovery to determine active hosts before initiating scans.​

Practical Usage:

To perform a basic scan with Masscan, identifying open ports on a target IP, the following command can be used:​

sudo masscan -p80,443 192.168.1.1 --rate=1000

In this command:​

• -p80,443: Specifies the ports to scan (HTTP and HTTPS).​

• 192.168.1.1: Target IP address.​

• --rate=1000: Sets the rate of packets per second.​

Considerations:

• Accuracy vs. Speed: Masscan's emphasis on speed may lead to missed packets or false negatives, especially on networks with packet loss or rate limiting.​

• Legal and Ethical Use: Ensure you have authorization before scanning networks or hosts, as unauthorized scanning can be illegal and unethical.​

• Firewall and IDS Evasion: Due to its high-speed nature, Masscan may trigger intrusion detection systems (IDS) or be blocked by firewalls.​

In summary, Masscan is a powerful tool for rapid port scanning across extensive network ranges, offering unparalleled speed compared to traditional scanners like Nmap. However, its use requires careful consideration of accuracy, network conditions, and legal implications.