there're 2 fixs needed when using this script:
- in check_mem(): calculate swapused must check for totalswap > 0 because by default on VPS there is no swap so we get the "division by zero"
        my $swapused = 0;
        if ($mem->{swaptotal} > 0) {
		    $swapused = sprintf("%.2f", ($mem->{swapused}/$mem->{swaptotal})*100);
        }

- in check_net(): all the units is B (bytes) not KB (kilobytes), and the error number has no unit:
				$return_str .= $device.":".$ttbyt."B ";
                                $perfdata .= " ".$device."_txbyt=".$txbyt."B ".$device."_txerrs=".$txerrs." ".$device."_rxbyt=".$rxbyt."B ".$device."_rxerrs=".$rxerrs;