Network Programming with Perl
Listing 2, client1.pl, shows a simple client. The first command-line argument to this program is the host name to which it should connect, which defaults to server.onsight.com. The second command-line argument is the port number which defaults to 7890.
The host name and the port number are used to generate the port address using inet_aton (ASCII to numeric) and sockaddr_in. A socket is then created using socket and the client connects the socket to the port address using connect.
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.
Let's assume all of the clients are started on the the machine named client.avue.com, although they could be executed from any machine on the network. To execute the client, type:
[james@client networking]$ client1.pl server.onsight.com Hello from the server: Tue Oct 27 09:48:40 1998
The following is the standard output from the server:
got a connection from: client.avue.com [192.168.1.2]
Creating sockets using the functions described above is good when you want to control how the socket is created, the protocol to be used, etc. But using the functions above is too hard; I prefer the easy way—IO::Socket.
The module IO::Socket provides an easy way to create sockets which can then be used like file handles. If you don't have it installed on your machine, it can be found on CPAN. To see this module's POD, type:
Listing 3, serverIO.pl, is a simple server using IO::Socket. A new IO::Socket::INET object is created using the new method. Note that the arguments to the method include the host name, port number, protocol, queue length and an option indicating we want this port to be immediately reusable. The new method returns a socket that is assigned to $sock. This socket can be used like a file handle—we can either read the client output from it, or write to it by sending data to the client.
A client connection is accepted using the accept method. Note the accept method returns the client socket when evaluated in scalar context:
$new_sock = $sock->accept()
and returns the client's socket and the client's IP address when evaluated in list context:
($new_sock, $client_addr) = $sock->accept()The client address is computed and printed the same as in Listing 1, server1.pl. Then the socket associated with that client is read until end-of-file. The data read is printed to STDOUT. This example illustrates that the server can read from a client using < > around the socket variable.
Listing 4, clientIO.pl, is a simple client using IO::Socket. This time, a new object is created that connects to a host at a port using the TCP protocol. Ten strings are then printed to that server, then the socket is closed.
If the server in Listing 3, serverIO.pl, is executed and then the client Listing 4, clientIO.pl, connects, the output would be:
[james@server networking]$ serverIO.pl
got a connection from: client.avue.com [192.168.1.2] hello, world: 1 hello, world: 2 hello, world: 3 hello, world: 4 hello, world: 5 hello, world: 6 hello, world: 7 hello, world: 8 hello, world: 9 hello, world: 10
It is possible to create servers and clients that communicate with one another in both directions. For instance, the client may send information to the server, then the server may send information back to the client. Therefore, network programs can be written so that the server and client follow some predetermined protocol.
Listing 5, server2way.pl, shows how a simple server can be created to read a command from a client, then print out an appropriate response to the client. The module Sys::Hostname provides a function named hostname that returns the host name of the server. To insure output is seen as we print, IO buffering is turned off for the STDOUT file handle using the autoflush function. Then a while loop is executed that accepts connections. When a client connects, the server reads a line from the client, chopping off the newline character. Then a switch statement is executed. (The switch is cleverly disguised as a foreach loop, which happens to be one of my favorite ways of writing a switch.) Depending on the input entered by the client, the server outputs an appropriate response. All lines from the client are read until end-of-file.
Listing 6, client2way.pl, shows the companion client. A connection to the server is made, then the client prints a few commands to the server reads the response and prints the response to STDOUT.
The following is the output of the client code in Listing 6:
[james@client networking]$ client2way.pl server.onsight.com Hi server.onsight.com Tue Oct 27 15:36:19 1998 DEFAULT
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- Sharing Admin Privileges for Many Hosts Securely
- HPC Cluster Grant Accepting Applications!
- Internet of Things Blows Away CES, and it May Be Hunting for YOU Next
- Designing with Linux
- Wondershaper—QOS in a Pinch
- Ideal Backups with zbackup
- Red Hat Enterprise Linux 7.1 beta available on IBM Power Platform
- January 2015 Issue of Linux Journal: Security
- Slow System? iotop Is Your Friend
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