Why build networking functionality into your Perl scripts? You might want to access your email remotely, or write a simple script that updates files on an FTP site . You might want to check up on your employees with a program that searches for Usenet postings that came from your site. You might want to check a web site for any recent changes, or even write your own home-grown web server. The network is the computer these days, and Perl makes network applications easy.

Perl programmers have their choice of modules for doing common tasks with network protocols; Chapter 14 through Chapter 17 cover the modules for writing email, news, FTP, and web applications in Perl. If you can do what you want with the available modules, you’re encouraged to jump to those chapters and skip this one. However, there will be times when you’ll have to wrestle with sockets directly, and that’s when this chapter comes in.

Sockets are the underlying mechanism for networking on the Internet. With sockets, one application (a server) sits on a port waiting for connections. Another application (the client) connects to that port and says hello; then the client and server have a chat. Their actual conversation is done with whatever protocol they choose—for example, a web client and server would use HTTP, an email server would use POP3 and SMTP, etc. But at the most basic level, you might say that all network programming comes down to opening a socket, reading and writing data, and closing the socket.

You can work with sockets in Perl at various levels. At the lowest level, Perl’s built-in functions include socket routines similar to the system calls in C of the same name. To make these routines easier to use, the Socket module in the standard library imports common definitions and constants specific to your system’s networking capabilities. Finally, the IO::Socket module provides an object interface to the socket functions through a standard set of methods and options for constructing both client and server communications programs.

Sockets provide a connection between systems or applications. They can be set up to handle streaming data or discrete data packets. Streaming data continually comes and goes over a connection. A transport protocol such as TCP (Transmission Control Protocol) is used to process streaming data so that all of the data is properly received and ordered . Packet-oriented communication sends data across the network in discrete chunks. The message-oriented protocol UDP (User Datagram Protocol) works on this type of connection. Although streaming sockets using TCP are widely used for applications, UDP sockets also have their uses.

Sockets exist in one of two address domains: the Internet domain and the Unix domain. Sockets used for Internet connections require the careful binding and assignment of the proper type of address dictated by the Internet Protocol (IP). These sockets are referred to as Internet-domain sockets.

Sockets in the Unix domain create connections between applications either on the same machine or within a LAN. The addressing scheme is less complicated, often just providing the name of the target process.

In Perl, sockets are attached to a filehandle after they have been created. Communication over the connection is then handled by standard Perl I/O functions.