Dumping output of a program

In perl, if you want to do a dump of a program, say SSH. You do the following

system ("SSH 192.168.1.102 1> /dev/null 2> /dev/null");

Basically 1> /dev/null means dump all the output of the program to /dev/null
and 2> /dev/null means dump all the error output to /dev/null

Establish SSH Tunnel and run VNCViewer using Perl

Using perl, I was trying to run SSH Tunnel first and then run VNCViewer through this tunnel. Of course it was not so simple as to just doing the following in perl:

#!/usr/bin/perl
system("ssh -l -L 10000:localhost:5901 ");
system("vncviewer localhost:10000");

If you try to do the above, it won't work! This is because the process gets stuck on the first line while in SSH. Basically if the tunnel is active, it can't possibly run the next command which is to launch vncviewer. But if the tunnel is terminated, so that we can run vncviewer, vncviewer won't work because there is no more binding on port 10000.

We need to use perls' build in fork function. Similar to running programs using the trailing & command in C. My implementation (thanks to estabroo from LQ) is as follows:

$pid = fork();
if ($pid == 0) {
system("ssh -l -L 10000:localhost:5901 ");
exit(0);
} elsif ($pid > 0) {
sleep(5);
system("vncviewer localhost:10000");
} else {
print "fork failed\n";
}

Basically what this does is, fork the process, execute SSH, wait for login and exit, sleep for 5 seconds, run the child process vncviewer. Done :)

Running VNCViewer using SSH Port Fowarding

I would like to start an SSH tunnel and then run vncviewer to connect the port. But as we all know, vncviewer pretty much sends everything in clear over the network, we have to introduce some 'security' to it. One common way is using SSH. In linux, its as simple as just two lines:

ssh -l -L 10000:localhost:5901
vncviewer localhost:10000
*in order for this work, you need to have vncserver already setup and serving on port 5901.

That's it! What it is tryinng to do is. Listen locally on port 10000 and forward any incoming connection to port 5901 of the remotehost. Localhost here pretty much refers to the remote host. It does not make any difference if you were to run the command as, it should still work:

ssh -l -L 10000::5901

Done. I will be posting shortly on how I did this in perl.