--- Simple DNS library supporting packet creation, encoding, decoding,
-- and querying.
-- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html

module(... or "dns", package.seeall)

require("ipOps")
require("stdnse")

get_servers = nmap.get_dns_servers


types = {
   A = 1,
   AAAA = 28,
   NS = 2,
   SOA = 6,
   CNAME = 5,
   PTR = 12,
   HINFO = 13,
   MX = 15,
   TXT = 16,
   SRV = 33,
   SSHFP = 44,
   AXFR = 252,
   ANY = 255
}


---
-- Table of error codes.
-- @name err
-- @class table
err = {
   noSuchName = 3,
   noServers = 9
}


---
-- Repeatedly sends UDP packets to host, waiting for an answer.
-- @param data Data to be sent.
-- @param host Host to connect to.
-- @param port Port to connect to.
-- @param timeout Number of ms to wait for a response.
-- @param cnt Number of tries.
-- @return Status (true or false).
-- @return Response (if status is true).
local function sendPackets(data, host, port, timeout, cnt)
   local socket = nmap.new_socket()
   socket:set_timeout(timeout)
   socket:connect(host, port, "udp")

   for i = 1, cnt do 
      socket:send(data)
      local response
      local status, response = socket:receive_bytes(1)
      
      if (status) then
         socket:close()
         return true, response
      end
   end
   socket:close()
   return false
end


---
-- Checks if a DNS response packet contains a useful answer.
-- @param rPkt Decoded DNS response packet.
-- @return True if useful, false if not.
local function gotAnswer(rPkt)
   -- have we even got answers?
   if #rPkt.answers > 0 then
      
      -- are those answers not just cnames?
      if rPkt.questions[1].dtype == types.A then
         for _, v in ipairs(rPkt.answers) do
            -- if at least one answer is an A record, it's an answer
            if v.dtype == types.A then
               return true
            end
         end
         -- if none was an A record, it's not really an answer
         return false
      else -- there was no A request, CNAMEs are not of interest
         return true
      end
   -- no such name is the answer   
   elseif rPkt.flags.RC3 and rPkt.flags.RC4 then
      return true
   -- really no answer
   else
      return false
   end
end


---
-- Tries to find the next nameserver with authority to get a result for
-- query.
-- @param rPkt Decoded DNS response packet
-- @return String or table of next server(s) to query, or false.
local function getAuthDns(rPkt)
   if #rPkt.auth == 0 then 
      if #rPkt.answers == 0 then
         return false 
      else
         if #rPkt.answers[1].dtype == types.CNAME then
            return {cname = rPkt.answers[1].domain}
         end
      end
   end
   if rPkt.auth[1].dtype == types.NS then
      if #rPkt.add > 0 then
         local hosts = {}
         for _, v in ipairs(rPkt.add) do
            if v.dtype == types.A then
               table.insert(hosts, v.ip)
            end
         end
         if #hosts > 0 then return hosts end
      end
      local next = query(rPkt.auth[1].domain, {dtype = "A" })
      return next
   end
   return false
end

---
-- Query DNS servers for a DNS record.
-- @param dname Desired domain name entry.
-- @param options A table containing any of the following fields:
-- * <code>dtype</code>: Desired DNS record type (default: <code>"A"</code>).
-- * <code>host</code>: DNS server to be queried (default: DNS servers known to Nmap).
-- * <code>port</code>: Port of DNS server to connect to (default: <code>53</code>).
-- * <code>tries</code>: How often should <code>query</code> try to contact another server (for non-recursive queries).
-- * <code>retAll</code>: Return all answers, not just the first.
-- * <code>retPkt</code>: Return the packet instead of using the answer-fetching mechanism.
-- * <code>norecurse</code> If true, do not set the recursion (RD) flag.
-- @return Nice answer string by an answer fetcher on success or false on error.
-- @return An error code on error.
-- @see dns.err
function query(dname, options)
   if not options then options = {} end

   local dtype, host, port, tries = options.dtype, options.host, options.port, options.tries

   if not tries then tries = 10 end -- don't get into an infinite loop

   if not options.sendCount then options.sendCount = 2 end
   
   if type( options.timeout ) ~= "number" then options.timeout = get_default_timeout() end

   if type(dtype) == "string" then
      dtype = types[dtype]
   end
   if not dtype then dtype = types.A end

   local srv
   local srvI = 1
   if not port then port = 53 end
   if not host then 
      -- IF PATCH NOT APPLIED!
      if type(get_servers) == "function" then
         srv = get_servers()
      end
      -- !
      
      if srv and srv[1] then 
         host = srv[1]
      else
         return false, err.noServers
      end
   elseif type(host) == "table" then
      srv = host
      host = srv[1]
   end

   local pkt = newPacket()
   addQuestion(pkt, dname, dtype)
   if options.norecurse then pkt.flags.RD = false end

   local data = encode(pkt)

   local status, response = sendPackets(data, host, port, options.timeout, options.sendCount)


   -- if working with know nameservers, try the others
   while((not status) and srv and srvI < #srv) do
      srvI = srvI + 1
      host = srv[srvI]
      status, response = sendPackets(data, host, port, options.sendCount)
   end

   -- if we got any response:
   if status then
      local rPkt = decode(response)
      -- is it a real answer?
      if gotAnswer(rPkt) then
         if (options.retPkt) then 
            return rPkt 
         else
            return findNiceAnswer(dtype, rPkt, options.retAll)
         end
      else -- if not, ask the next server in authority

         local next_server = getAuthDns(rPkt)
         
         -- if we got a CNAME, ask for the CNAME
         if type(next_server) == 'table' and next_server.cname then
            options.tries = tries - 1
            return query(next_server.cname, options)
         end

         -- only ask next server in authority, if 
         -- we got an auth dns and
         -- it isn't the one we just asked
         if next_server and next_server ~= host and tries > 1 then 
            options.host = next_server
            options.tries = tries - 1
            return query(dname, options) 
         end
      end
      
      -- nothing worked, maybe user finds decoded packet useful
      return false, rPkt 
   else
      return false
   end
end


---
-- Formats an IP address for reverse lookup.
-- @param ip IP address string.
-- @return "Domain"-style representation of IP as subdomain of in-addr.arpa or
-- ip6.arpa.
function reverse(ip)
   ip = ipOps.expand_ip(ip)
   if type(ip) ~= "string" then return nil end
   local delim = "%."
   local arpa = ".in-addr.arpa"
   if ip:match(":") then
      delim = ":"
      arpa = ".ip6.arpa"
   end
   local ipParts = stdnse.strsplit(delim, ip)
   if #ipParts == 8 then
      -- padding
      local mask = "0000"
      for i, part in ipairs(ipParts) do
          ipParts[i] = mask:sub(1, string.len(mask) - string.len(part)) .. part
      end
      -- 32 parts from 8
      local temp = {}
      for i, hdt in ipairs(ipParts) do
         for part in hdt:gmatch("%x") do
          temp[#temp+1] = part
         end
      end
      ipParts = temp
   end
   local ipReverse = {}
   for i = #ipParts, 1, -1 do
      table.insert(ipReverse, ipParts[i])
   end
   return table.concat(ipReverse, ".") .. arpa
end

---
-- Table for answer fetching functions.
local answerFetcher = {}

---
-- Answer fetcher for TXT records.
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return First entry (or all of them), treated as TXT.
answerFetcher[types.TXT] = function(dec, retAll)
   if not retAll then
      return string.sub(dec.answers[1].data, 2)
   else
      local answers = {}
      for _, v in ipairs(dec.answers) do
         if v.data then table.insert(answers, string.sub(v.data, 2)) end
      end
      return answers
   end
end

---
-- Answer fetcher for A records
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return First IP (or all of them) of response packet.
answerFetcher[types.A] = function(dec, retAll)
   local answers = {}
   for _, ans in ipairs(dec.answers) do
      if ans.dtype == types.A then
         if not retAll then
            return ans.ip
         else
            table.insert(answers, ans.ip)
         end
      end
   end
   if retAll then return answers end
   return dec
end


---
-- Answer fetcher for CNAME records.
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return Domain entry of first answer RR (or all of them) in response packet.
answerFetcher[types.CNAME] = function(dec, retAll)
   if not retAll then
      return dec.answers[1].domain
   else
      local answers = {}
      for _, v in ipairs(dec.answers) do
         if v.domain then table.insert(answers, v.domain) end
      end
      return answers
   end
end

---
-- Answer fetcher for MX records.
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return Domain entry of first answer RR (or all of them) in response packet.
answerFetcher[types.MX] = function(dec, retAll)
   if not retAll then
      if dec.answers[1] then
         return dec.answers[1].MX.pref .. ":" .. dec.answers[1].MX.server .. ":" .. dec.add[1].ip
      else
         return dec
      end
   else
      local answers = {}
      for _, v in ipairs(dec.answers) do
         if v.MX then table.insert(answers, v.MX.pref .. ":" .. v.MX.server .. ":" .. v.MX.ip) end
      end
      return answers
   end
end


---
-- Answer fetcher for NS records.
-- @name answerFetcher[types.NS]
-- @class function
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return Domain entry of first answer RR (or all of them) in response packet.
answerFetcher[types.NS] = answerFetcher[types.CNAME]

---
-- Answer fetcher for PTR records.
-- @name answerFetcher[types.PTR]
-- @class function
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return Domain entry of first answer RR (or all of them) in response packet.
answerFetcher[types.PTR] = answerFetcher[types.CNAME]

---
-- Answer fetcher for AAAA records.
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return First IPv6 (or all of them) of response packet.
answerFetcher[types.AAAA] = function(dec, retAll)
   local answers = {}
   for _, ans in ipairs(dec.answers) do
      if ans.dtype == types.AAAA then
         if not retAll then
            return ans.ipv6
         else
            table.insert(answers, ans.ipv6)
         end
      end
   end
   if retAll then return answers end
   return dec
end


---
-- Calls the answer fetcher for <code>dtype</code> or returns an error code in
-- case of a "no such name" error.
-- @param dtype DNS resource record type.
-- @param dec Decoded DNS response.
-- @param retAll If true, return all entries, not just the first.
-- @return Answer according to the answer fetcher for <code>dtype</code>,
-- <code>dec</code> if no answer fetcher is known for <code>dtype</code>, or
-- false if flags indicate an error.
-- @return Error code on error.
function findNiceAnswer(dtype, dec, retAll) 
   if (#dec.answers > 0) then
      if answerFetcher[dtype] then 
         return answerFetcher[dtype](dec, retAll)
      else 
         return dec
      end
   else
      if (dec.flags.RC3 and dec.flags.RC4) then return false, err.noSuchName
      end
   end
end


---
-- Encodes the question part of a DNS request.
-- @param questions Table of questions.
-- @return Encoded question string.
local function encodeQuestions(questions)
   if type(questions) ~= "table" then return nil end
   local encQ = ""
   for _, v in ipairs(questions) do
      local parts = stdnse.strsplit("%.", v.dname)
      for _, part in ipairs(parts) do
         encQ = encQ .. bin.pack("p", part)
      end
      encQ = encQ .. string.char(0)
      encQ = encQ .. bin.pack(">SS", v.dtype, v.class)
   end
   return encQ
end

---
-- Encodes DNS flags to a binary digit string.
-- @param flags Flag table, each entry representing a flag (QR, OCx, AA, TC, RD,
-- RA, RCx).
-- @return Binary digit string representing flags.
local function encodeFlags(flags)
   if type(flags) == "string" then return flags end
   if type(flags) ~= "table" then return nil end
   local fb = ""
   if flags.QR then fb = fb .. "1" else fb = fb .. "0" end
   if flags.OC1 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.OC2 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.OC3 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.OC4 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.AA then fb = fb .. "1" else fb = fb .. "0" end
   if flags.TC then fb = fb .. "1" else fb = fb .. "0" end
   if flags.RD then fb = fb .. "1" else fb = fb .. "0" end
   if flags.RA then fb = fb .. "1" else fb = fb .. "0" end
   fb = fb .. "000"
   if flags.RC1 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.RC2 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.RC3 then fb = fb .. "1" else fb = fb .. "0" end
   if flags.RC4 then fb = fb .. "1" else fb = fb .. "0" end
   return fb
end

---
-- Encode a DNS packet.
--
-- Caution: doesn't encode answer, authority and additional part.
-- @param pkt Table representing DNS packet, initialized by
-- <code>newPacket</code>.
-- @return Encoded DNS packet.
function encode(pkt)
   if type(pkt) ~= "table" then return nil end
   local encFlags = encodeFlags(pkt.flags)
   local encQs = encodeQuestions(pkt.questions)
   local encStr = bin.pack(">SBS4", pkt.id, encFlags, #pkt.questions, #pkt.answers, #pkt.auth, #pkt.additional) .. encQs
   return encStr
end


---
-- Decodes a domain in a DNS packet. Handles "compressed" data too.
-- @param data Complete DNS packet.
-- @param pos Starting position in packet.
-- @return Position after decoding.
-- @return Decoded domain, or <code>nil</code> on error.
function decStr(data, pos)
   local function dec(data, pos, limit)
      local partlen
      local parts = {}
      local part

      -- Avoid infinite recursion on malformed compressed messages.
      limit = limit or 10
      if limit < 0 then
         return pos, nil
      end

      pos, partlen = bin.unpack(">C", data, pos)
      while (partlen ~= 0) do
         if (partlen < 64) then 
            pos, part = bin.unpack("A" .. partlen, data, pos)
            if part == nil then
               return pos
            end
            table.insert(parts, part)
            pos, partlen = bin.unpack(">C", data, pos)
         else
            pos, partlen = bin.unpack(">S", data, pos - 1)
            local _, part = dec(data, partlen - 0xC000 + 1, limit - 1)
            if part == nil then
               return pos
            end
            table.insert(parts, part)
            partlen = 0
         end
      end
      return pos, table.concat(parts, ".")
   end

   return dec(data, pos)
end


---
-- Decodes questions in a DNS packet.
-- @param data Complete DNS packet.
-- @param count Value of question counter in header.
-- @param pos Starting position in packet.
-- @return Position after decoding.
-- @return Table of decoded questions.
local function decodeQuestions(data, count, pos)
   local q = {}
   for i = 1, count do
      local currQ = {}
      pos, currQ.dname = decStr(data, pos)
      pos, currQ.dtype, currQ.class = bin.unpack(">SS", data, pos)
      table.insert(q, currQ)
   end
   return pos, q
end


---
-- Table of functions to decode resource records
local decoder = {}

---
-- Decodes IP of A record, puts it in <code>entry.ip</code>.
-- @param entry RR in packet.
decoder[types.A] = function(entry)
   local ip = {}
   local _
   _, ip[1], ip[2], ip[3], ip[4] = bin.unpack(">C4", entry.data)
   entry.ip = table.concat(ip, ".")
end

---
-- Decodes IP of AAAA record, puts it in <code>entry.ipv6</code>.
-- @param entry RR in packet.
decoder[types.AAAA] = function(entry)
   local ip = {}
   local pos = 1
   local num
   for i = 1, 8 do
      pos, num = bin.unpack(">S", entry.data, pos)
      table.insert(ip, string.format('%x', num))
   end
   entry.ipv6 = table.concat(ip, ":")
end

---
-- Decodes SSH fingerprint record, puts it in <code>entry.SSHFP</code> as
-- defined in RFC 4255.
--
-- <code>entry.SSHFP</code> has the fields <code>algorithm</code>,
-- <code>fptype</code>, and <code>fingerprint</code>.
-- @param entry RR in packet.
decoder[types.SSHFP] = function(entry)
   local _
   entry.SSHFP = {}
   _, entry.SSHFP.algorithm, 
   entry.SSHFP.fptype, entry.SSHFP.fingerprint = bin.unpack(">C2H" .. (#entry.data - 2), entry.data)
end


---
-- Decodes SOA record, puts it in <code>entry.SOA</code>.
--
-- <code>entry.SOA</code> has the fields <code>mname</code>, <code>rname</code>,
-- <code>serial</code>, <code>refresh</code>, <code>retry</code>,
-- <code>expire</code>, and <code>minimum</code>.
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.SOA] = function(entry, data, pos)

   local np = pos - #entry.data

   entry.SOA = {}

   np, entry.SOA.mname = decStr(data, np)
   np, entry.SOA.rname = decStr(data, np)
   np, entry.SOA.serial, 
     entry.SOA.refresh, 
     entry.SOA.retry, 
     entry.SOA.expire, 
     entry.SOA.minimum 
      = bin.unpack(">I5", data, np)
end

---
-- Decodes records that consist only of one domain, for example CNAME, NS, PTR.
-- Puts result in <code>entry.domain</code>.
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
local function decDomain(entry, data, pos)
      local np = pos - #entry.data
      local _
      _, entry.domain = decStr(data, np)
   end

---
-- Decodes CNAME records.
-- Puts result in <code>entry.domain</code>.
-- @name decoder[types.CNAME]
-- @class function
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.CNAME] = decDomain

---
-- Decodes NS records.
-- Puts result in <code>entry.domain</code>.
-- @name decoder[types.NS]
-- @class function
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.NS] = decDomain

---
-- Decodes PTR records.
-- Puts result in <code>entry.domain</code>.
-- @name decoder[types.PTR]
-- @class function
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.PTR] = decDomain

---
-- Decodes TXT records.
-- Puts result in <code>entry.domain</code>.
-- @name decoder[types.TXT]
-- @class function
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.TXT] = function () end

---
-- Decodes MX record, puts it in <code>entry.MX</code>.
--
-- <code>entry.MX</code> has the fields <code>pref</code> and
-- <code>server</code>.
-- @param entry RR in packet.
-- @param data Complete encoded DNS packet.
-- @param pos Position in packet after RR.
decoder[types.MX] = 
   function(entry, data, pos)
      local np = pos - #entry.data + 2
      local _
      entry.MX = {}
      _, entry.MX.pref = bin.unpack(">S", entry.data)
      _, entry.MX.server = decStr(data, np)
   end


---
-- Decodes returned resource records (answer, authority, or additional part).
-- @param data Complete encoded DNS packet.
-- @param count Value of according counter in header.
-- @param pos Starting position in packet.
-- @return Table of RRs.
local function decodeRR(data, count, pos)
   local ans = {}
   for i = 1, count do
      local currRR = {}
      pos, currRR.dname = decStr(data, pos)
      pos, currRR.dtype, currRR.class, currRR.ttl = bin.unpack(">SSI", data, pos)

      local reslen
      pos, reslen = bin.unpack(">S", data, pos)

      pos, currRR.data = bin.unpack("A" .. reslen, data, pos)

      -- try to be smart: decode per type
      decoder[currRR.dtype](currRR, data, pos)

      table.insert(ans, currRR)
   end
   return pos, ans
end

---
-- Splits a string up into a table of single characters.
-- @param str String to be split up.
-- @return Table of characters.
local function str2tbl(str)
   local tbl = {}
   for i = 1, #str do
      table.insert(tbl, string.sub(str, i, i))
   end
   return tbl
end

---
-- Decodes DNS flags.
-- @param flgStr Flags as a binary digit string.
-- @return Table representing flags.
local function decodeFlags(flgStr)
   flags = {}
   flgTbl = str2tbl(flgStr)
   if flgTbl[1] == '1' then flags.QR = true end
   if flgTbl[2] == '1' then flags.OC1 = true end   
   if flgTbl[3] == '1' then flags.OC2 = true end
   if flgTbl[4] == '1' then flags.OC3 = true end   
   if flgTbl[5] == '1' then flags.OC4 = true end   
   if flgTbl[6] == '1' then flags.AA = true end   
   if flgTbl[7] == '1' then flags.TC = true end   
   if flgTbl[8] == '1' then flags.RD = true end   
   if flgTbl[9] == '1' then flags.RA = true end   
   if flgTbl[13] == '1' then flags.RC1 = true end   
   if flgTbl[14] == '1' then flags.RC2 = true end   
   if flgTbl[15] == '1' then flags.RC3 = true end   
   if flgTbl[16] == '1' then flags.RC4 = true end   
   return flags
end

---
-- Decodes a DNS packet.
-- @param data Encoded DNS packet.
-- @return Table representing DNS packet.
function decode(data)
   local pos
   local pkt = {}
   local encFlags
   local cnt = {}
   pos, pkt.id, encFlags, cnt.q, cnt.a, cnt.auth, cnt.add = bin.unpack(">SB2S4", data)
   -- for now, don't decode the flags
   pkt.flags = decodeFlags(encFlags)

   pos, pkt.questions = decodeQuestions(data, cnt.q, pos)

   pos, pkt.answers = decodeRR(data, cnt.a, pos)

   pos, pkt.auth = decodeRR(data, cnt.auth, pos)

   pos, pkt.add = decodeRR(data, cnt.add, pos)

   return pkt
end


---
-- Creates a new table representing a DNS packet.
-- @return Table representing a DNS packet.
function newPacket()
   local pkt = {}
   pkt.id = 1
   pkt.flags = {}
   pkt.flags.RD = true
   pkt.questions = {}
   pkt.answers = {}
   pkt.auth = {}
   pkt.additional = {}
   return pkt
end


---
-- Adds a question to a DNS packet table.
-- @param pkt Table representing DNS packet.
-- @param dname Domain name to be asked.
-- @param dtype RR to be asked.
function addQuestion(pkt, dname, dtype)
   if type(pkt) ~= "table" then return nil end
   if type(pkt.questions) ~= "table" then return nil end
   local q = {}
   q.dname = dname
   q.dtype = dtype
   q.class = 1
   table.insert(pkt.questions, q)
   return pkt
end


get_default_timeout = function()
  local timeout = {[0] = 10000, 7000, 5000, 4000, 4000, 4000}
  return timeout[nmap.timing_level()] or 4000
end

