script to manipulate the dhcp/conf

Question

i want to write a script to manipulate the dhcp.conf file. which in the sense, it has to read the file and it should ping all the lease IP and should give another text file, in which it should give the list of IPs, which devices are now existing.

EDIT: thanks for your swift reply. dhcpd.conf file is like this

lease 172.31.0.10 {
  some text
  some text1
}

lease 172.31.0.12 {
  some text
  some text1
}

lease 172.31.0.100 {
  some text
  some text1
}

so first i need to extract the ip address first and one by one we have to ping

Answer 1

Znip! (Moving the clarifiction to the question.)

Answer 2

Assumptions

First, there are two things you need to be aware of:

• Not all devices respond to ping. Quite a few PC firewalls disable ping replies. If you're on Ethernet, arping can be used instead and will even detect firewalled PCs.
• dhcpd leaves leases in the file which it /knows/ are no longer valid.

So, here is an example:

lease 192.168.66.132 {
  starts 4 2009/01/08 23:58:41;
  ends 5 2009/01/09 00:00:41;
  binding state free;
  hardware ethernet 00:e0:81:28:2d:56;
}
lease 192.168.66.133 {
  starts 5 2009/01/09 03:17:17;
  ends 2 2038/01/19 03:14:06;
  binding state active;
  next binding state free;
  hardware ethernet 00:e0:81:28:2d:57;
}

You can see that 132 is not in use (binding state free) and 133 is (binding state active). Another possibility is binding state backup, but that only occurs in a failover config.

A lease can also be abandoned, which means that the DHCP server was going to assign that IP, but found it was already in use (via ping).

This is all documented in dhcpd.leases(5).

Why are you wanting this?

1. The DHCP server already re-uses expired leases. Is there a good reason that you need to check its work? If you're running out of leases, have you considered lowering the lease time?

2. Does nmap -sP <start_ip>-<end_ip> do what you need? That'll also detect machines with static IP addresses.

Re-writing the question

So, given the above, and assuming you still want this, I'm going to answer this question instead:

Please write a script to find all leases which are either active or abandoned and determine if there is currently a machine using that IP address.

And so:

#!/usr/bin/perl
use File::Slurp qw(slurp);
use Data::Dump qw(pp);
use strict;

1 == @ARGV
    or die "Usage: $0 dhpcd.leases\n";
my $leases = slurp($ARGV[0]);
$leases =~ s/^#.*\n//mg;
my @leases = split(/lease (\d.+\d.+\d.+\d+) {/, $leases);
shift @leases;
my %lease = @leases;

while (my ($ip, $rec) = each %lease) {
    print $ip;
    $rec =~ /^\s*abandoned;\s*$/m and print " abandoned";
    $rec =~ /^\s*binding state free;\s*$/m and print " free";
    $rec =~ /^\s*binding state active;\s*$/m and print " active";
    print "\n";
}

This relatively ugly perl script will give you output like:

192.168.66.132 free
192.168.66.133 active

Which should be pretty easy for you to feed to arping.

Answer 3

the problem is, binding state active; does not means, the device is active. To make sure the device is active, we need to ping the devices first. there is no other way we can do it