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In trying to solve a problem setting up iptables on my Linux gateway box, I wrote a Perl script to display netfilter activity in real-time. Every second, it grabs output from the iptables list utility, including counters (which are then zeroed). It displays this data with extraneous lines removed, highlighting the lines with non-zero packet counts in color. It's designed to run in an xterm window and uses ANSI escape sequences to control the text. To be effective, the xterm window needs to be at least as high as the output, else scrolling will ruin the visual effect. Also, the code may be iptables version-dependent, as it matches certain keywords for the formatting. Once started, it can be stopped with a ctl-C, which will restore some of the display settings. Herewith is the code:

#!/usr/bin/perl
use strict;
use warnings;
my @types = qw/nat mangle filter/;
$SIG{INT} = sub{print "\e[?25h\e[u"; exit}; 
print "\e[40;37m\e[2J\e[?25l";
while (1) {
  print "\e[0;0H";
  my %output = map {$_ => scalar `iptables -t $_ -L -v -Z`} @types;
  foreach my $type (@types) {
    print "\e[01;34m------", uc($type), '-' x (73 - length($type)), "\
+n";
    $output{$type} =~ s/ pkts[^\n]*\n(\n|Zeroing)/$1/gs;
    foreach my $line (split /\n/, $output{$type}) {
      next if $line =~ m/^Zeroing/ || $line eq '';
      print $line =~ m/^\s*(\d+)/ || $line =~ m/(\d+) packets/
        ? ($1 > 0 
          ? ($line =~ m/DROP|DENY|REJECT/
            ? "\e[01;40;31m" 
            : "\e[01;40;32m")
          : "\e[00;40;37m")
        : "\e[00;40;33m";
      print "\e[K$line\e[01;40;37m\n"
    }
  }
  print "\e[s";
  sleep 1
}
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In reply to Real-time Iptables Monitor by Dr. Mu

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