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Calculate DHCP option 121 (Classless Static Route Option)

Posted: Thu Jan 13, 2011 10:19 pm
by THG
I have a perl script to calculate routes to hexadecimal, so it works with MikroTik DHCP Server. I'm now asking if anyone has anything similar, Linux binary or source code etc.?

The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4.
#!/usr/bin/perl -w

use strict;

    print "\033[2J";    #clear the screen
    print "\033[0;0H";  #jump to 0,0

sub option_121 {
    my $gw = shift;
    my $string = '';
    my ($subnet, $mask, $b0, $b1, $b2, $b3);
    foreach my $cidr (@_) {
        ($subnet,  $mask) = split('/', $cidr);
        ($b0, $b1, $b2, $b3) = split(/\./, $subnet);
        $string .= sprintf('%02x', $mask);
        $string .= sprintf('%02x', $b0) if($mask > 0);
        $string .= sprintf('%02x', $b1) if($mask > 8);
        $string .= sprintf('%02x', $b2) if($mask > 16);
        $string .= sprintf('%02x', $b3) if($mask > 24);
        $string .= sprintf('%02x%02x%02x%02x', split(/\./, $gw));
    return $string;

if(@ARGV < 2)
    print "\n";
    print "Usage: $0 [gateway] [host|network]/[bitmask]\n\n";
    print "Example: $0\n\n";
elsif($ARGV[0] =~ /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/)
    print "DHCP option 121 (249) hex string: ".option_121(@ARGV)."\n";
    print "Invalid gateway IP address: '$ARGV[0]'\n";

Re: Calculate DHCP option 121 (Classless Static Route Option

Posted: Sat Jan 15, 2011 5:06 am
by astounding
No Linux binaries nor C/C++ code here. How about a quick Ruby translation using the excellent 'ipaddress' gem?
(See: for 'ipaddress' gem info...)

I ripped this from some DHCP Ruby code I've been using (a Ruby library that parses and generates DHCP packets) and added the hexification like your script.

Oh, this script makes an assumption that the GATEWAY IP address will always be ON or PART of the network. Therefore each argument is a gateway IP/mask where the mask can either be a four-octet version (i.e. "") or a CIDR integer number of mask bits (0 through 32 inclusive).

So if the network is and the gateway is at .1, I would do:
and would see output
NET:	GATEWAY:	DHCP(Opt. 121): 18c0a864c0a86401
Here's the Ruby script:
#!/usr/bin/env ruby

require 'rubygems'
require 'ipaddress'

if ARGV.size < 1
  STDERR.print "\n" +
    "Usage: #{$0} gateway/mask [gateway/mask ...]\n" +
    "\n" +
    "Example: #{$0}\n" +
  exit -1

ARGV.each do |gnet|
  gnet =
  hex  = (
    gnet.prefix.to_i.chr +                    ## Mask bits (single byte)[0,(gnet.prefix+7)/8] +  ## Compressed Network IP                                 ## Gateway IP
  puts "NET: #{}\tGATEWAY: #{gnet.to_s}\tDHCP(Opt. 121): #{hex}"
Unlike the prior posted Perl script, this won't handle a gateway IP that doesn't fall within the network without some adaptation. (Does that ever happen? Would it ever work?)

Aaron out.

Re: Calculate DHCP option 121 (Classless Static Route Option

Posted: Sat Jan 15, 2011 10:47 pm
by THG
Thank you for the script, I'll take a look at it after ruby is installed on my linux computer.

I'll also try to create a web formular using javascript.

Re: Calculate DHCP option 121 (Classless Static Route Option)

Posted: Wed Mar 07, 2018 9:08 pm
by cieplik206
You can try this generator if you want to use it.