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

Checking local connections

It is often useful to know what connections are being made on your local machine. The netstat command can help with that. Netstat is available on Linux, macOS, and Windows. Each version differs a little in the command-line options and output, but the general usage principles are the same.

I recommend running netstat with the -n flag. This flag prevents netstat from doing reverse-DNS lookups on each address and has the effect of speeding it up significantly.

On Linux, we can use the following command to show open TCP connections:

netstat -nt

The following screenshot shows the result of running this command on Linux:

In the preceding screenshot, you can see that netstat shows six columns. These columns display the protocol, the sending and receiving queue, the local address, the foreign address, and the connection state. In this example, we see that there are three connections to port 80. It is likely that this computer is loading up three web pages (as HTTP uses port...