How to Secure a port opened in the host? - perl

Question

Our goal is to monitor the server up/down from a remote(central) server. For this we are opening a port in the target host using the below (Client_port.pl) script. On the server we have other script to monitor this port, to make sure the server is up and port is up. Both the server wouldn't be on same network, so we need to open the client port to the world. How would we secure the client port from security hole and other possible harmful issues?
Client_port.pl
use IO::Socket::INET;
$server = IO::Socket::INET->new(LocalPort => $server_port,
Type => SOCK_STREAM,
Reuse => 1,
Listen => 10 ) # or SOMAXCONN
or die "Couldn't be a tcp server on port $server_port: $!\n";
while ($client = $server->accept()) {
# $client is the new connection
}
close($server);
Server_port_listener.pl
use strict;
use Socket;
# set time until connection attempt times out
my $timeout = 3;
if ($#ARGV != 1) {
print "usage: is_tcp_port_listening hostname portnumber\n";
exit 2;
}
my $hostname = $ARGV[0];
my $portnumber = $ARGV[1];
my $host = shift || $hostname;
my $port = shift || $portnumber;
my $proto = getprotobyname('tcp');
my $iaddr = inet_aton($host);
my $paddr = sockaddr_in($port, $iaddr);
socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
eval {
local $SIG{ALRM} = sub { die "timeout" };
alarm($timeout);
connect(SOCKET, $paddr) || error();
alarm(0);
};
if ($#) {
close SOCKET || die "close: $!";
print "$hostname is NOT listening on tcp port $portnumber.\n";
exit 1;
}
else {
close SOCKET || die "close: $!";
print "$hostname is listening on tcp port $portnumber.\n";
exit 0;
}