id = "Nameserver open recursive queries"

description = [[
Checks if a DNS server allows queries for third-party names.
\n\n
It is expected that recursion will be enabled on your own internal nameservers.
]]

author = "Felix Groebert <felix@groebert.org>"

license = "Same as Nmap--See http://nmap.org/book/man-legal.html"

categories = {"default", "intrusive"}

require "bit"
require "comm"
require "shortport"

portrule = shortport.portnumber(53, "udp")

action = function(host, port)

    -- generate dns query, Transaction-ID 0xdead, www.wikipedia.org (type A, class IN)
	local request = string.char(0xde, 0xad, 0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03) ..  "www" .. string.char(0x09) .. "wikipedia" .. string.char(0x03) ..  "org" .. string.char(0x00, 0x00, 0x01, 0x00, 0x01)

	local status, result = comm.exchange(host, port, request, {proto="udp"})

	if not status then
		return
	end

    -- parse response for dns flags
    if (bit.band(string.byte(result,3), 0x80) == 0x80
    and bit.band(string.byte(result,4), 0x85) == 0x80)
    then
		return "Recursion appears to be enabled"
    end

	return
end
