Network Programming with Perl
Perl has been called the glue that holds the Internet together because it is an extremely powerful text processing and CGI programming language. Although Perl was designed in the beginning to be a text manipulation language, it has evolved into a potent multi-purpose programming language. One area in which Perl has shown its power is that of network programming.
Perl makes network programming easy by providing built-in functions that can be used to create low-level client/server programs from scratch. Also, many modules are freely available that make programming common networking tasks simple and quick. These tasks include pinging remote machines, TELNET and FTP sessions. This article presents examples of each of these types of network programs.
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:
Create a socket with socket.
Bind the socket to a port address with bind.
Listen to the socket at the port address with listen.
Accept client connections with accept.
Establishing a client is even easier:
Create a socket with socket.
Connect (the socket) to the remote machine with connect.
Several other required functions and variables are defined in the Socket.pm module. This module is probably already installed on your machine, but if not, it is available at the Comprehensive Perl Archive Network (CPAN), the official Perl source code repository (see Resources). To use this module in our programs, the following statement is required at the top of the program:
This statement will locate the file Socket.pm and import all of its exported functions and variables.
All examples in this article use modules that are available at no cost from CPAN.
Perl modules are usually self-documenting. If the author of the module follows the generally accepted rules of creating a Perl module, they will add Plain Old Documentation (POD) to the module's .pm file. One way to view the POD for the Socket module (assuming Perl and Socket.pm were installed correctly) is to execute the following at the shell:
This command displays Socket.pm's POD converted to a man page. The output is a relatively thorough discussion of the functions and variables defined in this module.
Another way to view the documentation is to convert the POD to text using:
pod2text \ /usr/lib/perl5/i686-linux/5.00404/Socket.pm | more
The program pod2text is included in the Perl distribution, as are the programs pod2html, pod2man, pod2usage and pod2latex.
Our first programming example is a simple server running 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.
Listing 1, server1.pl, is the source code for this simple server. First, it is generally a good idea to compile using Perl's strict rules:
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.
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. Note that the only way to ensure you select a port that does not have a predefined use is to look at the appropriate RFC (see Resources).
Next, 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.
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
gethostbyname("server.onsight.com")The bind function binds the socket to the port, i.e., plugs the socket into that port. Then, 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.
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.
To read from a file handle or socket in Perl, wrap it in angle brackets (<FH>). To write to it, use the print function:
The return value of the accept function is the Internet address of the client in a packed format. The function sockaddr_in takes that 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.
Let's assume all of the servers referred to in this article are started on the machine named server.onsight.com. To start the server on this machine, execute:
[james@server networking]$ server1.pl SERVER started on port 7890
The server is now listening at port 7890 on server.onsight.com, waiting for clients to connect.
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