description = [[ Checks a DNS server for the predictable-TXID DNS recursion vulnerability. Predictable TXID values can make a DNS server vulnerable to cache poisoning attacks (see CVE-2008-1447). The script works by querying txidtest.dns-oarc.net (see https://www.dns-oarc.net/oarc/services/txidtest). Be aware that any targets against which this script is run will be sent to and potentially recorded by one or more DNS servers and the txidtest server. In addition your IP address will be sent along with the txidtest query to the DNS server running on the target. ]] license = "Same as Nmap--See http://nmap.org/book/man-legal.html" author = [[ Script: Brandon Enright \n txidtest.dns-oarc.net: Duane Wessels ]] --- -- @output -- PORT STATE SERVICE REASON -- 53/udp open domain udp-response -- |_dns-random-txid: X.X.X.X is GREAT: 27 queries in 61.5 seconds from 27 txids with std dev 20509 -- This script uses (with permission) Duane Wessels' txidtest.dns-oarc.net -- service. Duane/OARC believe the service is valuable to the community -- and have no plans to ever turn the service off. -- The likely long-term availability makes this script a good candidate -- for inclusion in Nmap proper. categories = {"external", "intrusive"} require "bit" require "comm" require "shortport" portrule = shortport.portnumber(53, "udp") action = function(host, port) -- TXID: 0xbabe -- Flags: 0x0100 -- Questions: 1 -- Answer RRs: 0 -- Authority RRs: 0 -- Additional RRs: 0 -- Query: -- Name: txidtest, dns-oarc, net -- Type: TXT (0x0010) -- Class: IN (0x0001) local query = string.char( 0xba, 0xbe, -- TXID 0x01, 0x00, -- Flags 0x00, 0x01, -- Questions 0x00, 0x00, -- Answer RRs 0x00, 0x00, -- Authority RRs 0x00, 0x00, -- Additional RRs 0x08) .. "txidtest" .. string.char( 0x08) .. "dns-oarc" .. string.char( 0x03) .. "net" .. string.char( 0x00, -- Name terminator 0x00, 0x10, -- Type (TXT) 0x00, 0x01) -- Class (IN) local status, result = comm.exchange(host, port, query, {proto="udp", timeout=20000}) -- Fail gracefully if not status then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: TIMEOUT" else return end end -- Update the port nmap.set_port_state(host, port, "open") -- Now we need to "parse" the results to check to see if they are good -- We need a minimum of 5 bytes... if (#result < 5) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Malformed response" else return end end -- Check TXID if (string.byte(result, 1) ~= 0xba or string.byte(result, 2) ~= 0xbe) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Invalid Transaction ID" else return end end -- Check response flag and recursion if not (bit.band(string.byte(result, 3), 0x80) == 0x80 and bit.band(string.byte(result, 4), 0x80) == 0x80) then if (nmap.verbosity() >= 1 or nmap.debugging() >= 1) then return "ERROR: Server refused recursion" else return end end -- Check error flag if (bit.band(string.byte(result, 4), 0x0F) ~= 0x00) then if (nmap.verbosity() >= 1 or nmap.debugging() >= 1) then return "ERROR: Server failure" else return end end -- Check for two Answer RRs and 1 Authority RR if (string.byte(result, 5) ~= 0x00 or string.byte(result, 6) ~= 0x01 or string.byte(result, 7) ~= 0x00 or string.byte(result, 8) ~= 0x02) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Response did not include expected answers" else return end end -- We need a minimum of 128 bytes... if (#result < 128) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Truncated response" else return end end -- Here is the really fragile part. If the DNS response changes -- in any way, this won't work and will fail. -- Jump to second answer and check to see that it is TXT, IN -- then grab the length and display that text... -- Check for TXT if (string.byte(result, 118) ~= 0x00 or string.byte(result, 119) ~= 0x10) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Answer record not of type TXT" else return end end -- Check for IN if (string.byte(result, 120) ~= 0x00 or string.byte(result, 121) ~= 0x01) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Answer record not of type IN" else return end end -- Get TXT length local txtlen = string.byte(result, 128) -- We now need a minimum of 128 + txtlen bytes + 1... if (#result < 128 + txtlen) then if (nmap.verbosity() >= 2 or nmap.debugging() >= 1) then return "ERROR: Truncated response" else return end end -- GET TXT record local txtrd = string.sub(result, 129, 128 + txtlen) return txtrd end