robertdavidgraham/masscan

Masscan is an extremely fast TCP port scanner, a tool that discovers which network ports are open on one or more IP addresses. A port is like a numbered door on a networked computer, different services (web servers, databases, email servers) listen on specific port numbers. Port scanners are used by network administrators and security professionals to inventory what services are exposed on their infrastructure, and by security researchers to study the open internet. What sets Masscan apart is its speed: it can scan the entire public internet in under five minutes by sending 10 million network packets per second from a single machine. It achieves this using asynchronous (non-waiting) packet transmission, meaning it fires off probes without waiting for replies before sending the next one. Its command-line interface is deliberately similar to nmap, the most widely known port scanner, so users familiar with nmap can switch with minimal relearning. Masscan implements its own internal TCP/IP stack rather than using the operating system's, which gives it independence and speed but requires some extra configuration to avoid conflicts with the host system's networking. Beyond just detecting open ports, it can also grab "banner" information, connecting to services like HTTP, SSH, FTP, and SSL to read their initial response. It is written in C, builds with just a C compiler, and runs on Linux, macOS, Windows, and BSD systems.