in reply to Fetching ARP table with NetPacket

Hi,

But i really need to do it without using this method.
I was curious why you have this limitation..

I am not sure if you refer to Windows perl module Win32::NetPacket or the unix packet.h stuff.

Whatever platform you're on already has a quick method to access the arp table without resorting to an extra layer of activity - the info is already right there!!

Curious why you might want to get the arp data - perhaps knowing your ultimate goal might help come up with a solution... realizing that the arp table stuff, except for some very specific reasons, is generally not all that useful- what with aging, looking at the arp table is not a precise indicator of what IP addresses are "out there" at the moment. Additionally interfaces that are in fact "out there" may not, at any given moment, be in the arp table.

-hsinclai

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Re: Re: Fetching ARP table with NetPacket
by RnC (Sexton) on Jun 01, 2004 at 16:24 UTC
    In this case, GNU/Linux, I could use the arp command. Indeed I don't need this script to know "who's online", or anything like it, but I do need it because I wanted an example of how I could fetch the mac address of the network interfaces I have installed on my machine using code (it's a research work, by the way), and not internalizing another program such as system("arp -a"). I also ocurred me that it could be useful to fetch the entire arp table, so I asked the question as above :) I imagine I could fetch it from /proc. But I'm not sure how portable this solution is. I know it stands almost for the same as fetching from the "arp -a" results, but it's more tolerable from the point of view of this research (not my idea). Again, any help is appreciated.
[reply]

      I'm sure there are shorter, much more elegant ways of doing this (hire a monk:), plus in a multi-interface system, you'd want more info about which "device" from the arp table.. but, here's a basic approach.. 
      
#!/usr/bin/perl -w

# kernels 2.4.x /proc/net/arp

use strict;

 my $ipad;
my $macad;
  my @arp;

if ( -r "/proc/net/arp" ) {
   open(ARP,"</proc/net/arp");
   @arp = <ARP>;
   close(ARP);
} else {
     die("Cannot access arp in proc $!");
}

foreach my $linein ( @arp ) {
  chomp($linein);
  next if $linein =~ /IP/i;
  $ipad  = (split(/\s+/,$linein))[0];
  $macad = (split(/\s+/,$linein))[3];
  print "IP address $ipad has MAC Address $macad\n";
}

      [download]

      to start with.. definitely not portable to Windows or BSD, just linux

      You could also split up each record getting a value for each field with something like 
      ($ipad,$macad,$hwtype,$flags,$mask,$device) = split(/\s+/,$linein);

      [download]
      -hsinclai
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