dns.conf.reverse.erb

# Reverse lookup
lala=<%= scope.function_foobar(['192.168.0.0/24']) %>

foobar.rb

module Puppet::Parser::Functions
  newfunction(:foobar, :type => :rvalue) do |args|
    require 'ipaddr_extensions'
    return IPAddr.new(args[0]).reverses[0]
  end
end

reverse.pp

define dnsmasq::dns::reverse (
  $upstream_server,
  $source_address = undef,
) {
  $netmask = $title

  package { 'ipaddr_extensions':
    ensure   => installed,
    provider => gem,
  }

  file {'/etc/dnsmasq.d/foobar.conf': 
    content => template('dnsmasq/dns.conf.reverse.erb'),
    require => Package['ipaddr_extensions'],
  }
}