Scan Millions of IPs Instantly with Masscan: Speed, Power, and Critical Precautions

Masscan: When Speed is the Top Priority

Do you need to inventory every device in a /16 range (over 65,000 IPs), but Nmap estimates it will take… 2 days? Don’t push Nmap too hard in this scenario. Masscan was born to solve large-scale problems with blazing speed.

Try installing and running the command below on Ubuntu to see the difference:

sudo apt update && sudo apt install masscan -y
# Scan port 80 on the local network at a rate of 1000 packets/second
sudo masscan 192.168.1.0/24 -p80 --rate 1000

In just a few seconds, a list of IPs with port 80 open will appear. Speed is this tool’s greatest weapon.

Why is Nmap Slower than Masscan?

Nmap operates on a Synchronous mechanism. It sends a packet, waits for a response, and then continues processing. While it can run multiple connections in parallel, even after optimizing the Linux TCP stack, when the scale reaches millions of IPs, the accumulated timeouts result in a massive amount of time.

Masscan is different. It uses an Asynchronous architecture and builds its own TCP/IP stack to bypass the operating system kernel. It fires SYN packets continuously without waiting for a response. A separate module stays on standby to record returning SYN-ACK packets.

With a standard 1Gbps network card, Masscan is capable of scanning the entire Internet in just 6 minutes. However, this power comes with significant risks.

I once saw an intern set --rate 1000000 (1 million packets/second) on an internal network. Within 30 seconds, the SIEM system was flooded with hundreds of thousands of false alerts. Worse, the Core Switch’s MAC table was overloaded, causing the entire office to lose connectivity. Always start with a modest number and monitor Linux networks for signs of congestion.

Installation for Maximum Performance

Although it is available in the repositories, compiling from source ensures you have the latest updates and the most stable performance:

sudo apt-get install git gcc make libpcap-dev
git clone https://github.com/robertdavidgraham/masscan
cd masscan
make -j

Once finished, you should copy the executable to /usr/local/bin for easier command access.

Essential Parameters You Must Master

• -p: Target ports (e.g., -p80,443 or -p1-1024).
• --range: The IP range to scan.
• --rate: Packets per second. This is your “throttle.”
• --exclude: Exclude sensitive IPs like Firewalls or Gateways to avoid crashing the network.
• --wait: The time to wait for responses after all packets have been sent (usually set to 5-10 seconds).

Real-World Scanning Techniques and Data Handling

Watching results fly by on a terminal screen makes reporting or CLI network analysis difficult. You need to output the data into a machine-readable format.

Exporting Results to JSON for Automation

If you want to push data into Python or an ELK Stack, use the -oJ option:

sudo masscan 10.0.0.0/8 -p80,443,8080 --rate 5000 -oJ scan_results.json

Focusing on Common Ports

Don’t scan all 65,535 ports unless absolutely necessary. This is both slow and easily blocked by Intrusion Detection Systems (IDS) or services configured to hide SSH ports. Focus on key service ports:

# Scan common Web, SSH, and Database ports
sudo masscan 172.16.0.0/12 -p22,80,443,3306,8080 --rate 2000

Hard-Learned Lessons: Don’t Let Speed Backfire

I once handled a packet loss issue at a branch office. After two days of searching, the cause turned out to be an automated inventory script using Masscan. This script was running with an excessively high --rate over a narrow bandwidth VPN link. It consumed the router’s entire queue, causing user data to be dropped.

Practical Advice:

1. Recon First: Always start with --rate 100. Observe CPU load and bandwidth before gradually increasing it.
2. Calculate Bandwidth: A --rate 100000 consumes about 50Mbps. Ensure your infrastructure can handle this load.
3. The Hybrid Strategy: Use Masscan for a “discovery scan” to find live IPs. Then, pipe that list into Nmap (using -iL) for detailed service version detection. This is the standard workflow for security professionals.

Quick Comparison: Masscan vs. Nmap

Conclusion

Masscan does not replace Nmap; it is a companion. Whether testing in a professional network lab or on a live subnet, Masscan clears the path by scanning broad areas, while Nmap dives deep into specific targets. This combination allows you to manage thousands of devices without wasting excessive time. However, always notify the network operations team before scanning to avoid security misunderstandings.