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
Dedication
About Packt
Contributors
Preface
Index

Port numbers


An IP address alone isn't quite enough. We need port numbers. To return to the telephone analogy, if IP addresses are phone numbers, then port numbers are like phone extensions.

Generally, an IP address gets a packet routed to a specific system, but a port number is used to route the packet to a specific application on that system.

For example, on your system, you may be running multiple web browsers, an email client, and a video-conferencing client. When your computer receives a TCP segment or UDP datagram, your operating system looks at the destination port number in that packet. That port number is used to look up which application should handle it.

Port numbers are stored as unsigned 16-bit integers. This means that they are between 0 and 65,535 inclusive.

Some port numbers for common protocols are as follows:

Port Number

Protocol

20, 21

TCP

File Transfer Protocol (FTP)

22

TCP

Secure Shell (SSH)

Chapter 11Establishing SSH Connections with libssh

23

TCP

Telnet

25

TCP

Simple Mail Transfer Protocol (SMTP)

Chapter 8Making Your Program Send Email

53

UDP

Domain Name System (DNS)

Chapter 5, Hostname Resolution and DNS

80

TCP

Hypertext Transfer Protocol (HTTP)

Chapter 6Building a Simple Web Client

Chapter 7Building a Simple Web Server

110

TCP

Post Office Protocol, Version 3 (POP3)

143

TCP

Internet Message Access Protocol (IMAP)

194

TCP

Internet Relay Chat (IRC)

443

TCP

HTTP over TLS/SSL (HTTPS)

Chapter 9Loading Secure Web Pages with HTTPS and OpenSSLChapter 10Implementing a Secure Web Server

993

TCP

IMAP over TLS/SSL (IMAPS)

 

995

TCP

POP3 over TLS/SSL (POP3S)

Each of these listed port numbers is assigned by the Internet Assigned Numbers Authority (IANA). They are responsible for the official assignments of port numbers for specific protocols. Unofficial port usage is very common for applications implementing custom protocols. In this case, the application should try to choose a port number that is not in common use to avoid conflict.