ReverseEchoer.pl - A Simple Socket Server Program

Perl Tutorials - Herong's Tutorial Examples

∟ReverseEchoer.pl - A Simple Socket Server Program

This section provides a tutorial example on how to write a simple socket communication server program, ReverseEchoer.pl, which listens and accepts for any client requests.

The following program called ReverseEchoer.pl is a simple socket communication server application, which listens for a connection request. Once connected, it reads what lines of text from the client application, reverses the text lines, and echoes back to the remote application:

#- ReverseEchoer.pl
#- Copyright (c) HerongYang.com. All Rights Reserved.
#
   $domain = 2; # Internet domain
   $type = 1; # Sequenced, reliable, two-way connection, byte streams
   $proto = 6; # Transmission Control Protocol (TCP)
   socket(SOCK,$domain,$type,$proto);
   $host = pack('C4', 0,0,0,0); # Local wildcard host id: 0.0.0.0
   $port = 8888;
   $address = pack('S n a4 x8', $domain, $port, $host);
   bind(SOCK, $address);
   $queueSize = 5; # Queue up to 5 connections
   listen(SOCK, $queueSize);
   print STDOUT "Server host: ",join('.',unpack('C4', $host)),"\n";
   print STDOUT "Server port: $port\n";
   $cAddress = accept(NEWSOCK,SOCK);
   ($cDomain, $cPort, $cHost) = unpack('S n a4 x8', $cAddress);
   print STDOUT "Client host: ",join('.',unpack('C4', $cHost)),"\n";
   print STDOUT "Client port: $cPort\n";
   select(NEWSOCK); $| = 1; select(STDOUT);
   print NEWSOCK "Welcome to Reverse Echo Server.\r\n";
   while ($m=<NEWSOCK>) {
      $m =~ s/\n|\r//g;
      last if ($m eq ".");
      $m = reverse($m);
      print NEWSOCK "$m\r\n";
   }
   close(NEWSOCK);
   exit;

Note that:

The socket is bound to the local system with the wildcard host address 0.0.0.0, which represents any IP address on the local host system.
Setting $| to 1 is to turn on auto flush to the default file handle. In order to turn on auto flush to NEWSOCK, I have to use select() function to switch default file handles.
If a text line received only contains a single character '.', the communication link will be terminated by the close() call.

Run ReverseEchoer.pl, you will get the following output on the console window:

Server host: 0.0.0.0
Server port: 8888

This tells us that the program is ready to accept request at address 0.0.0.0 and port 8888.

To test ReverseEchoer.pl, we can use an existing client program called telnet to initiate the communication request and talk to ReverseEchoer.pl. Open another command window and type in the following command:

telnet localhost 8888

Immediately, you will see more output on the console window of ReverseEchoer:

Client host: 127.0.0.1
Client port: 1032

This tells us that the server socket received a connection request, and a communication link has be established with the client application, which is the telnet program running on the same machine. The client host, 127.0.0.1, is the standard IP address for "localhost". The client port, 1032, was picked up by the telnet program.

In the telnet window, type in the following text:

Fish, I love you and respect you very much.
But I will kill you dead before this day ends.
.

The text will be reversed and returned back from the ReverseEchoer.pl:

Welcome to Reverse Echo Server.
.hcum yrev uoy tcepser dna uoy evol I ,hsiF
.sdne yad siht erofeb daed uoy llik lliw I tuB
.

Note that the telnet program is not displaying text you typed in. It only displays the text received from ReverseEchoer.pl program.

Table of Contents

About This Book

Perl on Linux Systems

ActivePerl on Windows Systems

Data Types: Values and Variables

Expressions, Operations and Simple Statements

User Defined Subroutines

Perl Built-in Debugger

Name Spaces and Perl Module Files

Symbolic (or Soft) References

Hard References - Addresses of Memory Objects

Objects (or References) and Classes (or Packages)

Typeglob and Importing Identifiers from Other Packages

String Built-in Functions and Performance

File Handles and Data Input/Output

Open Files in Binary Mode

Open Directories and Read File Names

File System Functions and Operations

Image and Picture Processing

Using DBM Database Files

Using MySQL Database Server

►Socket Communication Over the Internet

What Is Socket Communication?

connect() - Establishing a Socket Communication

►ReverseEchoer.pl - A Simple Socket Server Program

SocketClient.pl - A Simple Socket Client Program

gethostbyaddr() - Network Utility Functions

Socket.pm - The Socket Module

XML::Simple Module - XML Parser and Generator

XML Communication Model

SOAP::Lite - SOAP Server-Client Communication Module

Perl Programs as IIS Server CGI Scripts

CGI (Common Gateway Interface)

XML-RPC - Remote Procedure Call with XML and HTTP

RPC::XML - Perl Implementation of XML-RPC

Integrating Perl with Apache Web Server

CGI.pm Module for Building Web Pages

LWP::UserAgent and Web Site Testing

Converting Perl Script to Executable Binary

Managing Perl Engine and Modules on macOS

Archived Tutorials

References

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