Ethical Hacking and Penetration Testing Guide

Understanding Network Protocols
Having a solid introduction about network protocols is fundamental in the server exploitation 
phase; you just cannot attack a protocol without knowing how it works. I will not be explaining 
the ins and outs of every protocol because there are good resources available where you can learn 
about them, so I don’t need to reinvent the wheel. However, in this chapter, I will give a brief 
introduction to network protocols.
As a penetration tester, most of the times, you would come across only three protocols:
1. TCP (Transmission Control Protocol)
2. UDP (User Datagram Protocol)
3. ICMP (Internet Control Messaging Protocol)

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Ethical Hacking and Penetration Testing Guide
Transmission Control Protocol
Most of the Internet’s traffic is based upon TCP since it guarantees a reliable communication 
unlike UDP. Most of the protocols that we encounter in our daily lives are based upon TCP. 
Common examples are FTP, SMTP, Telnet, and HTTP.
TCP is used whenever we need to perform a reliable communication between a client and 
a server. TCP performs a reliable communication via the three-way handshake, which we have 
already discussed thoroughly in the “Network Sniffing” chapter (Chapter 6).
User Datagram Protocol
UDP is the exact opposite of TCP. It is used for faster communications. An example would be for 
video streaming, such as Skype (VOIP) communication. The advantage of this protocol over TCP 
is that it’s much faster and efficient. The disadvantage of UDP is that it does not guarantee that the 
packet will reach the destination, since it does not perform the three-way handshake, thus causing 
reliability issues. Some of the common UDP protocols that we will run into as a penetration tester 
are DNS and SQL Server.