Book Image

Hands-On Network Programming with C

By : Lewis Van Winkle
Book Image

Hands-On Network Programming with C

By: Lewis Van Winkle

Overview of this book

Network programming enables processes to communicate with each other over a computer network, but it is a complex task that requires programming with multiple libraries and protocols. With its support for third-party libraries and structured documentation, C is an ideal language to write network programs. Complete with step-by-step explanations of essential concepts and practical examples, this C network programming book begins with the fundamentals of Internet Protocol, TCP, and UDP. You’ll explore client-server and peer-to-peer models for information sharing and connectivity with remote computers. The book will also cover HTTP and HTTPS for communicating between your browser and website, and delve into hostname resolution with DNS, which is crucial to the functioning of the modern web. As you advance, you’ll gain insights into asynchronous socket programming and streams, and explore debugging and error handling. Finally, you’ll study network monitoring and implement security best practices. By the end of this book, you’ll have experience of working with client-server applications and be able to implement new network programs in C. The code in this book is compatible with the older C99 version as well as the latest C18 and C++17 standards. You’ll work with robust, reliable, and secure code that is portable across operating systems, including Winsock sockets for Windows and POSIX sockets for Linux and macOS.
Table of Contents (26 chapters)
Title Page
About Packt

A DNS query program

We will now implement a utility to send DNS queries to a DNS server and receive the DNS response.

This should not normally be needed in the field. It is, however, a good opportunity to better understand the DNS protocol and to get experience of sending binary UDP packets.

We begin with a function to print a name from a DNS message.

Printing a DNS message name

DNS encodes names in a particular way. Normally, each label is indicated by its length, followed by its text. A number of labels can be repeated, and then the name is terminated with a single 0 byte.

If a length has its two highest bits set (that is, 0xc0), then it and the next byte should be interpreted as a pointer instead.

We must also be aware at all times that the DNS response from the DNS server could be ill-formed or corrupted. We must try to write our program in such a way that it won't crash if it receives a bad message. This is easier said than done.

The declaration for our name-printing function looks like this...