description = [[ Checks if a DNS server allows queries for third-party names. It is expected that recursion will be enabled on your own internal nameservers. ]] --- -- @output -- PORT STATE SERVICE REASON -- 53/udp open domain udp-response -- |_dns-recursion: Recursion appears to be enabled author = "Felix Groebert" license = "Same as Nmap--See http://nmap.org/book/man-legal.html" categories = {"default", "safe"} 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 nmap.set_port_state(host, port, "open") -- 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