There are numerous, different ways that target discovery can be performed at layer 4. Scanning can be performed with either User Datagram Protocol (UDP) or Transmission Control Protocol (TCP). Scapy can be used to craft custom requests using both of these transport protocols and can be used in conjunction with Python scripting to develop useful discovery tools. This recipe will demonstrate how to use Scapy to perform layer 4 discovery with both TCP and UDP.
Using Scapy to perform layer 4 discovery does not require a lab environment, as many systems on the Internet will reply to both TCP and UDP traffic. However, it is highly recommended that you perform any type of network scanning exclusively in your own lab unless you are thoroughly familiar with the legal regulations imposed by any governing authorities to whom you are subject. If you wish to perform this technique within your lab, you will need to have at least one system that will...