Socket Programming in PERL

In this article, Rahul shows us how to create a client-server socket program in Perl and then demonstrates it by pinging the server.

What is a socket? Just another bit of computer jargon? Devling a little into networking history, it is a Berkeley UNIX mechanism of creating a virtual duplex connection between processes. This was later ported on to every known OS enabling communication between systems across geographical location running on different OS software. If not for the socket, most of the network communication between systems would never ever have happened.

Taking a closer look; a typical computer system on a network receives and sends information as desired by the various applications running on it. This information is routed to the system, since a unique IP address is designated to it. On the system, this information is given to the relevant applications which listen on different ports. For example a net browser listens on port 80 for information. Also we can write applications which listen and send information on a specific port number.

For now, let’s sum up that a socket is an IP address and a port, enabling connection.{mospagebreak title=Types of Sockets&toc=1} There are just two types of sockets: connection oriented and connection-less. There are other types but this classification is fair enough to get started, a socket has a domain (UNIX or internet), a connection type (connection oriented or connection less) and a protocol (TCP or UDP).

A connection oriented or a stream socket is a reliable two way communication. If you send three packets, say 1, 2 and 3, they are received in the same order they were sent. They achieve this high level of transmission quality by using TCP for error free reliable communication. The ubiquitous telnet application uses stream sockets.

The connection-less sockets or stream-less sockets use IP for routing but use the UDP. They are connectionless, since the connection need not be open as in stream sockets, the packet formed is given a destination IP address and than transmitted. This method is mostly used for packet-to-packet transfer as in ftp applications.

How a Packet is Formed

We will use an example of UDP packet for some light stuff on networking theory. It’s Data Encapsulation. A packet is formed by encapsulating the data in a header at each level as it passes through the layers (protocol stack). At the receiving end the headers are stripped off as the packet travels up the layers to get the data.

Basically, at each layer, the protocol adds a header to the payload to perform the required functionality.

Client Server Architecture

It’s a client-server world, today. Just about everything on the network deals with client processes talking to server processes and vice versa. Take the ubiquitous telnet, for instance. When you connect to a remote host on port 23 with telnet (the client), a program on that host (called telnetd, the server) springs to life. It handles the incoming telnet connection, sets up a login prompt, etc. Note that the client-server pair can speak in streaming or stream-less, or anything else (as long as they are speaking the same thing).

Some good examples of client-server pairs are telnet/telnetd, ftp/ftpd.

{mospagebreak title=Client–Server Script in PERL&toc=1} Comfortable so far? Let’s dive head-first in to coding a simple client server interaction in PERL. Client/server network programming requires a server running on one machine to serve one or more clients running on either the same machine or different machines. These different machines can be located anywhere on the network.

To create a server, simply perform the following steps using the built-in Perl function indicated:
  1. Create a socket with socket.
  2. Bind the socket to a port address with bind.
  3. Listen to the socket at the port address with listen.
  4. Accept client connections with accept.
Establishing a client is even easier:
  1. Create a socket with socket.
  2. Connect (the socket) to the remote machine with connect.
A Simple Server

1. #! /usr/bin/perl -w
2. #
3. #——————–

4. use strict;
5. use Socket;

6. # use port 7890 as default
7. my $port = shift || 7890;
8. my $proto = getprotobyname(‘tcp’);

9. # create a socket, make it reusable
10. socket(SERVER, PF_INET, SOCK_STREAM, $proto) or die “socket: $!”;
11. setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) or die “setsock: $!”;

12. # grab a port on this machine
13. my $paddr = sockaddr_in($port, INADDR_ANY);

14. # bind to a port, then listen
15. bind(SERVER, $paddr) or die “bind: $!”;
16. listen(SERVER, SOMAXCONN) or die “listen: $!”;
17. print “SERVER started on port $port “;

18. # accepting a connection
19. my $client_addr;
20. while ($client_addr = accept(CLIENT, SERVER))
21. {
22. # find out who connected
23. my ($client_port, $client_ip) = sockaddr_in($client_addr);
24. my $client_ipnum = inet_ntoa($client_ip);
25. my $client_host = gethostbyaddr($client_ip, AF_INET);
26. # print who has connected
27. print “got a connection from: $client_host”,”[$client_ipnum] “;
28. # send them a message, close connection
29. print CLIENT “Smile from the server”;
30. close CLIENT;
31. }


This simple server can run just on one machine that can service only one client program at a time connecting from the same or a different machine. Recall that the steps for creating a server were to create a socket, bind it to a port, listen at the port and accept client connections.

Line 1 and 4
It is generally a good idea to compile a Perl script using strict. This requires all variables to be declared with the “my” function before they are used. Using “my” may be inconvenient, but it can catch many common syntactically correct yet logically incorrect programming bugs.

Line 7
The variable $port is assigned the first command-line argument or port 7890 as the default. When choosing a port for your server, pick one that is unused on your machine.

Line 10 and 11
The socket is created using the socket function. A socket is like a file handle-it can be read from, written to or both. The function setsockopt is called to ensure that the port will be immediately reusable.

Line 13
The sockaddr_in function obtains a port on the server. The argument INADDR_ANY chooses one of the server’s virtual IP addresses. You could instead decide to bind only one of the virtual IP addresses by replacing INADDR_ANY with inet_aton(“”) or gethostbyname (‘localhost’)

Line 15
The bind function binds the socket to the port, i.e., plugs the socket into that port.

Line 16
The listen function causes the server to begin listening at the port. The second argument to the listen function is the maximum queue length or the maximum number of pending client connections. The value SOMAXCONN is the maximum queue length for the machine being used.

Line 20
Once the server begins listening at the port, it can accept client connections using the accept function. When the client is accepted, a new socket is created named CLIENT which can be used like a file handle. Reading from the socket reads the client’s output and printing to the socket sends data to the client. The return value of the accept function is the Internet address of the client in a packed format.

Line 24 and 25
The function sockaddr_in takes the packed format and returns the client’s port number and the client’s numeric Internet address in a packed format. The packed numeric Internet address can be converted to a text string representing the numeric IP using inet_ntoa (numeric to ASCII). To convert the packed numeric address to a host name, the function gethostbyaddr is used.

Start the script on a localhost. I ran these scripts on a Windows 2000 machine with Active PERL ( binary build 631 PERL v5.6.1). The output looks something like this.

SERVER started on port 7890

The server is now listening at port 7890 on the local host, waiting for clients to connect.

A Simple Client

1. #! /usr/bin/perl -w
2. # – a simple client
3. #—————-

4. use strict;
5. use Socket;

6. # initialize host and port
7. my $host = shift || ‘localhost’;
8. my $port = shift || 7890;

9. my $proto = getprotobyname(‘tcp’);

10. # get the port address
11. my $iaddr = inet_aton($host);
12. my $paddr = sockaddr_in($port, $iaddr);
13. # create the socket, connect to the port
14. socket(SOCKET, PF_INET, SOCK_STREAM, $proto)
a. or die “socket: $!”;
15. connect(SOCKET, $paddr) or die “connect: $!”;

16. my $line;
17. while ($line = )
18. {
19. print $line;
20. }
21. close SOCKET or die “close: $!”;


Line 7 and 8
Takes the command-line arguments of host name and port number or if no arguments are passed initializes variables with the default values.

Line 11 and 12
The host name and the port number are used to generate the port address using inet_aton (ASCII to numeric) and sockaddr_in.

Line 14 and 15
A socket is created using socket and the client connects the socket to the port address using connect.

Line 17 and 21
The while loop then reads the data the server sends to the client until the end-of-file is reached, printing this input to STDOUT. Then the socket is closed.

Output on the server:

SERVER started on port 7890
got a connection from: SHILPA[]

Output on the client:

C:> rahul
Smile from the server
{mospagebreak title=PERL Makes Life Easy&toc=1} Those scripts were a lot of PERL code just to send a few bytes of information across. It is possible to write some cute and short socket programs by using OO Concepts and the IO::Socket module. We shall see now, how it is possible.

Sample Script Server Using IO

1. #!/usr/bin/perl -w
2. #
3. # server using IO::Socket
4. #———————
5. use strict;
6. use IO::Socket;

7. my $sock = new IO::Socket::INET(
LocalHost => ‘localhost’,
LocalPort => 7890,
Proto => ‘tcp’,
Listen => SOMAXCONN,
Reuse => 1);

8. $sock or die “no socket :$!”;
9. my($new_sock, $c_addr, $buf);

10. while (($new_sock, $c_addr) = $sock->accept())
11. {
my ($client_port, $c_ip) =sockaddr_in($c_addr);
my $client_ipnum = inet_ntoa($c_ip);
my $client_host =gethostbyaddr($c_ip, AF_INET);
print “got a connection from: $client_host”,” [$client_ipnum] “;

while (defined ($buf = <$new_sock>))
print $buf;
12. }


Line 7
A new IO::Socket::INET object is created using the new method The new method returns a socket that is assigned to $sock

Line 10
A client connection is accepted using the accept method. The accept method returns the client socket when evaluated in scalar context and the client’s socket and IP address when evaluated in list context.

Sample Client Script Using IO

1. #!/usr/bin/perl -w
2. #
3. # a simple client using IO:Socket
4. #—————-

5. use strict;
6. use IO::Socket;

7. my $host = shift || ‘localhost’;
8. my $port = shift || 7890;
9. my $sock = new IO::Socket::INET( PeerAddr => $host, PeerPort => $port, Proto => ‘tcp’);
10. $sock or die “no socket :$!”;
11. foreach my $i (1..10)
12. {
13. print $sock “$i”,scalar(localtime),” “;
14. sleep(1);
15. }
16. close $sock;


A new object is created which connects to the server and sends 10 timestamp strings with 1 second delay in between.

Output of the new cute scripts:

D:GalantPerl>start perl

On the server window:

got a connection from: RAHUL []
1Thu Feb 6 12:52:57 2003
2Thu Feb 6 12:52:58 2003
3Thu Feb 6 12:52:59 2003
4Thu Feb 6 12:53:00 2003
5Thu Feb 6 12:53:01 2003
6Thu Feb 6 12:53:02 2003
7Thu Feb 6 12:53:03 2003
8Thu Feb 6 12:53:04 2003
9Thu Feb 6 12:53:05 2003
10Thu Feb 6 12:53:06 2003

Pinging in PERL

Perl makes life easy, with it various modules available. Let’s demonstrate this by writing a short and quick ping program.

Sample Ping Program

1. #!/usr/bin/perl
2. #
3. # a simple ping program
4. #—————

5. use Net::Ping;

6. $pinghost = shift||”gsmgprs”;
7. print “Pinging $pinghost “;

8. $p = Net::Ping->new(“icmp”, 2);

9. for (;;) # infinite loop
10. {
11. unless ($p->ping($pinghost))
12. {
13. print “Fail: “, scalar(localtime), ” “;
14. } else
15. {
16. print “Success: “, scalar(localtime), ” “;
17. }
18. sleep 10;
19. }


The Net::Ping makes pinging remote hosts easy and quick. An object of the class created. Here we use the icmp protocol (you can also choose tcp or udp).

A sample output:

Pinging gsmgprs
Success: Thu Feb 6 14:22:01 2003
Success: Thu Feb 6 14:22:11 2003
Success: Thu Feb 6 14:22:21 2003
Success: Thu Feb 6 14:22:31 2003
Success: Thu Feb 6 14:22:41 2003
Success: Thu Feb 6 14:22:51 2003
Success: Thu Feb 6 14:23:01 2003
Success: Thu Feb 6 14:23:11 2003
Success: Thu Feb 6 14:23:21 2003
Success: Thu Feb 6 14:23:31 2003
{mospagebreak title=Conclusion&toc=1} Perl is a powerful, easy to learn, widely accepted and supported language. The freely available modules on CPAN (including the modules for network programming) cater to all development needs.

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