Inside the TCP Handshake

Introduction to port scanning

The concept of port scanning itself is based on the operation of the Transmission Control Protocol (TCP) and its counterpart, the User Datagram Protocol (UDP). While an IP address uniquely identifies a computer, these protocols define ports that allow communication with specific applications on a computer.

Port scanning is designed to identify which ports are open on a system and the services that are listening to them. In many cases, this is accomplished based on an understanding of the common ports associated with various services and how the TCP works.

The TCP handshake

We’ll be building a SYN scanner using Scapy. This type of port scanner takes advantage of the structure of the TCP handshake to determine which ports are open and listening on a system.