-- Prosody IM
-- This file is included with Prosody IM. It has modifications,
-- which are hereby placed in the public domain.


-- todo: quick (default) header generation
-- todo: nxdomain, error handling
-- todo: cache results of encodeName


-- reference: http://tools.ietf.org/html/rfc1035
-- reference: http://tools.ietf.org/html/rfc1876 (LOC)


local socket = require "socket";
local timer = require "util.timer";
local new_ip = require "util.ip".new_ip;
local have_util_net, util_net = pcall(require, "util.net");

local _, windows = pcall(require, "util.windows");
local is_windows = (_ and windows) or os.getenv("WINDIR");

local coroutine, io, math, string, table =
      coroutine, io, math, string, table;

local ipairs, next, pairs, print, setmetatable, tostring, assert, error, select, type =
      ipairs, next, pairs, print, setmetatable, tostring, assert, error, select, type;

local ztact = { -- public domain 20080404 lua@ztact.com
	get = function(parent, ...)
		local len = select('#', ...);
		for i=1,len do
			parent = parent[select(i, ...)];
			if parent == nil then break; end
		end
		return parent;
	end;
	set = function(parent, ...)
		local len = select('#', ...);
		local key, value = select(len-1, ...);
		local cutpoint, cutkey;

		for i=1,len-2 do
			local key = select (i, ...)
			local child = parent[key]

			if value == nil then
				if child == nil then
					return;
				elseif next(child, next(child)) then
					cutpoint = nil; cutkey = nil;
				elseif cutpoint == nil then
					cutpoint = parent; cutkey = key;
				end
			elseif child == nil then
				child = {};
				parent[key] = child;
			end
			parent = child
		end

		if value == nil and cutpoint then
			cutpoint[cutkey] = nil;
		else
			parent[key] = value;
			return value;
		end
	end;
};
local get, set = ztact.get, ztact.set;

local default_timeout = 15;

-------------------------------------------------- module dns
local _ENV = nil;
-- luacheck: std none
local dns = {};


-- dns type & class codes ------------------------------ dns type & class codes


local append = table.insert


local function highbyte(i)    -- - - - - - - - - - - - - - - - - - -  highbyte
	return (i-(i%0x100))/0x100;
end


local function augment (t, prefix)  -- - - - - - - - - - - - - - - - -  augment
	local a = {};
	for i,s in pairs(t) do
		a[i] = s;
		a[s] = s;
		a[string.lower(s)] = s;
	end
	setmetatable(a, {
		__index = function (_, i)
			if type(i) == "number" then
				return ("%s%d"):format(prefix, i);
			elseif type(i) == "string" then
				return i:upper();
			end
		end;
	})
	return a;
end


local function encode (t)    -- - - - - - - - - - - - - - - - - - - - -  encode
	local code = {};
	for i,s in pairs(t) do
		local word = string.char(highbyte(i), i%0x100);
		code[i] = word;
		code[s] = word;
		code[string.lower(s)] = word;
	end
	return code;
end


dns.types = {
	[1] = "A", -- a host address,[RFC1035],,
	[2] = "NS", -- an authoritative name server,[RFC1035],,
	[3] = "MD", -- a mail destination (OBSOLETE - use MX),[RFC1035],,
	[4] = "MF", -- a mail forwarder (OBSOLETE - use MX),[RFC1035],,
	[5] = "CNAME", -- the canonical name for an alias,[RFC1035],,
	[6] = "SOA", -- marks the start of a zone of authority,[RFC1035],,
	[7] = "MB", -- a mailbox domain name (EXPERIMENTAL),[RFC1035],,
	[8] = "MG", -- a mail group member (EXPERIMENTAL),[RFC1035],,
	[9] = "MR", -- a mail rename domain name (EXPERIMENTAL),[RFC1035],,
	[10] = "NULL", -- a null RR (EXPERIMENTAL),[RFC1035],,
	[11] = "WKS", -- a well known service description,[RFC1035],,
	[12] = "PTR", -- a domain name pointer,[RFC1035],,
	[13] = "HINFO", -- host information,[RFC1035],,
	[14] = "MINFO", -- mailbox or mail list information,[RFC1035],,
	[15] = "MX", -- mail exchange,[RFC1035],,
	[16] = "TXT", -- text strings,[RFC1035],,
	[17] = "RP", -- for Responsible Person,[RFC1183],,
	[18] = "AFSDB", -- for AFS Data Base location,[RFC1183][RFC5864],,
	[19] = "X25", -- for X.25 PSDN address,[RFC1183],,
	[20] = "ISDN", -- for ISDN address,[RFC1183],,
	[21] = "RT", -- for Route Through,[RFC1183],,
	[22] = "NSAP", -- "for NSAP address, NSAP style A record",[RFC1706],,
	[23] = "NSAP-PTR", -- "for domain name pointer, NSAP style",[RFC1348][RFC1637][RFC1706],,
	[24] = "SIG", -- for security signature,[RFC4034][RFC3755][RFC2535][RFC2536][RFC2537][RFC2931][RFC3110][RFC3008],,
	[25] = "KEY", -- for security key,[RFC4034][RFC3755][RFC2535][RFC2536][RFC2537][RFC2539][RFC3008][RFC3110],,
	[26] = "PX", -- X.400 mail mapping information,[RFC2163],,
	[27] = "GPOS", -- Geographical Position,[RFC1712],,
	[28] = "AAAA", -- IP6 Address,[RFC3596],,
	[29] = "LOC", -- Location Information,[RFC1876],,
	[30] = "NXT", -- Next Domain (OBSOLETE),[RFC3755][RFC2535],,
	[31] = "EID", -- Endpoint Identifier,[Michael_Patton][http://ana-3.lcs.mit.edu/~jnc/nimrod/dns.txt],,1995-06
	[32] = "NIMLOC", -- Nimrod Locator,[1][Michael_Patton][http://ana-3.lcs.mit.edu/~jnc/nimrod/dns.txt],,1995-06
	[33] = "SRV", -- Server Selection,[1][RFC2782],,
	[34] = "ATMA", -- ATM Address,"[ ATM Forum Technical Committee, ""ATM Name System, V2.0"", Doc ID: AF-DANS-0152.000, July 2000. Available from and held in escrow by IANA.]",,
	[35] = "NAPTR", -- Naming Authority Pointer,[RFC2915][RFC2168][RFC3403],,
	[36] = "KX", -- Key Exchanger,[RFC2230],,
	[37] = "CERT", -- CERT,[RFC4398],,
	[38] = "A6", -- A6 (OBSOLETE - use AAAA),[RFC3226][RFC2874][RFC6563],,
	[39] = "DNAME", -- DNAME,[RFC6672],,
	[40] = "SINK", -- SINK,[Donald_E_Eastlake][http://tools.ietf.org/html/draft-eastlake-kitchen-sink],,1997-11
	[41] = "OPT", -- OPT,[RFC6891][RFC3225],,
	[42] = "APL", -- APL,[RFC3123],,
	[43] = "DS", -- Delegation Signer,[RFC4034][RFC3658],,
	[44] = "SSHFP", -- SSH Key Fingerprint,[RFC4255],,
	[45] = "IPSECKEY", -- IPSECKEY,[RFC4025],,
	[46] = "RRSIG", -- RRSIG,[RFC4034][RFC3755],,
	[47] = "NSEC", -- NSEC,[RFC4034][RFC3755],,
	[48] = "DNSKEY", -- DNSKEY,[RFC4034][RFC3755],,
	[49] = "DHCID", -- DHCID,[RFC4701],,
	[50] = "NSEC3", -- NSEC3,[RFC5155],,
	[51] = "NSEC3PARAM", -- NSEC3PARAM,[RFC5155],,
	[52] = "TLSA", -- TLSA,[RFC6698],,
	[53] = "SMIMEA", -- S/MIME cert association,[RFC8162],SMIMEA/smimea-completed-template,2015-12-01
	-- [54] = "Unassigned", -- ,,,
	[55] = "HIP", -- Host Identity Protocol,[RFC8005],,
	[56] = "NINFO", -- NINFO,[Jim_Reid],NINFO/ninfo-completed-template,2008-01-21
	[57] = "RKEY", -- RKEY,[Jim_Reid],RKEY/rkey-completed-template,2008-01-21
	[58] = "TALINK", -- Trust Anchor LINK,[Wouter_Wijngaards],TALINK/talink-completed-template,2010-02-17
	[59] = "CDS", -- Child DS,[RFC7344],CDS/cds-completed-template,2011-06-06
	[60] = "CDNSKEY", -- DNSKEY(s) the Child wants reflected in DS,[RFC7344],,2014-06-16
	[61] = "OPENPGPKEY", -- OpenPGP Key,[RFC7929],OPENPGPKEY/openpgpkey-completed-template,2014-08-12
	[62] = "CSYNC", -- Child-To-Parent Synchronization,[RFC7477],,2015-01-27
	-- [63 .. 98] = "Unassigned", -- ,,,
	[99] = "SPF", -- ,[RFC7208],,
	[100] = "UINFO", -- ,[IANA-Reserved],,
	[101] = "UID", -- ,[IANA-Reserved],,
	[102] = "GID", -- ,[IANA-Reserved],,
	[103] = "UNSPEC", -- ,[IANA-Reserved],,
	[104] = "NID", -- ,[RFC6742],ILNP/nid-completed-template,
	[105] = "L32", -- ,[RFC6742],ILNP/l32-completed-template,
	[106] = "L64", -- ,[RFC6742],ILNP/l64-completed-template,
	[107] = "LP", -- ,[RFC6742],ILNP/lp-completed-template,
	[108] = "EUI48", -- an EUI-48 address,[RFC7043],EUI48/eui48-completed-template,2013-03-27
	[109] = "EUI64", -- an EUI-64 address,[RFC7043],EUI64/eui64-completed-template,2013-03-27
	-- [110 .. 248] = "Unassigned", -- ,,,
	[249] = "TKEY", -- Transaction Key,[RFC2930],,
	[250] = "TSIG", -- Transaction Signature,[RFC2845],,
	[251] = "IXFR", -- incremental transfer,[RFC1995],,
	[252] = "AXFR", -- transfer of an entire zone,[RFC1035][RFC5936],,
	[253] = "MAILB", -- "mailbox-related RRs (MB, MG or MR)",[RFC1035],,
	[254] = "MAILA", -- mail agent RRs (OBSOLETE - see MX),[RFC1035],,
	[255] = "*", -- A request for all records the server/cache has available,[RFC1035][RFC6895],,
	[256] = "URI", -- URI,[RFC7553],URI/uri-completed-template,2011-02-22
	[257] = "CAA", -- Certification Authority Restriction,[RFC6844],CAA/caa-completed-template,2011-04-07
	[258] = "AVC", -- Application Visibility and Control,[Wolfgang_Riedel],AVC/avc-completed-template,2016-02-26
	[259] = "DOA", -- Digital Object Architecture,[draft-durand-doa-over-dns],DOA/doa-completed-template,2017-08-30
	-- [260 .. 32767] = "Unassigned", -- ,,,
	[32768] = "TA", -- DNSSEC Trust Authorities,"[Sam_Weiler][http://cameo.library.cmu.edu/][ Deploying DNSSEC Without a Signed Root.  Technical Report 1999-19, Information Networking Institute, Carnegie Mellon University, April 2004.]",,2005-12-13
	[32769] = "DLV", -- DNSSEC Lookaside Validation,[RFC4431],,
	-- [32770 .. 65279] = "Unassigned", -- ,,,
	-- [65280 .. 65534] = "Private use", -- ,,,
	-- [65535] = "Reserved", -- ,,,
}

dns.classes = { 'IN', 'CS', 'CH', 'HS', [255] = '*' };


dns.type      = augment (dns.types, "TYPE");
dns.class     = augment (dns.classes, "CLASS");
dns.typecode  = encode  (dns.types);
dns.classcode = encode  (dns.classes);



local function standardize(qname, qtype, qclass)    -- - - - - - - standardize
	if string.byte(qname, -1) ~= 0x2E then qname = qname..'.';  end
	qname = string.lower(qname);
	return qname, dns.type[qtype or 'A'], dns.class[qclass or 'IN'];
end


local function prune(rrs, time, soft)    -- - - - - - - - - - - - - - -  prune
	time = time or socket.gettime();
	for i,rr in ipairs(rrs) do
		if rr.tod then
			if rr.tod < time then
				rrs[rr[rr.type:lower()]] = nil;
				table.remove(rrs, i);
				return prune(rrs, time, soft); -- Re-iterate
			end
		elseif soft == 'soft' then    -- What is this?  I forget!
			assert(rr.ttl == 0);
			rrs[rr[rr.type:lower()]] = nil;
			table.remove(rrs, i);
		end
	end
end


-- metatables & co. ------------------------------------------ metatables & co.


local resolver = {};
resolver.__index = resolver;

resolver.timeout = default_timeout;

local function default_rr_tostring(rr)
	local rr_val = rr.type and rr[rr.type:lower()];
	if type(rr_val) ~= "string" then
		return "<UNKNOWN RDATA TYPE>";
	end
	return rr_val;
end

local special_tostrings = {
	LOC = resolver.LOC_tostring;
	MX  = function (rr)
		return string.format('%2i %s', rr.pref, rr.mx);
	end;
	SRV = function (rr)
		local s = rr.srv;
		return string.format('%5d %5d %5d %s', s.priority, s.weight, s.port, s.target);
	end;
};

local rr_metatable = {};   -- - - - - - - - - - - - - - - - - - -  rr_metatable
function rr_metatable.__tostring(rr)
	local rr_string = (special_tostrings[rr.type] or default_rr_tostring)(rr);
	return string.format('%2s %-5s %6i %-28s %s', rr.class, rr.type, rr.ttl, rr.name, rr_string);
end


local rrs_metatable = {};    -- - - - - - - - - - - - - - - - - -  rrs_metatable
function rrs_metatable.__tostring(rrs)
	local t = {};
	for _, rr in ipairs(rrs) do
		append(t, tostring(rr)..'\n');
	end
	return table.concat(t);
end


local cache_metatable = {};    -- - - - - - - - - - - - - - - -  cache_metatable
function cache_metatable.__tostring(cache)
	local time = socket.gettime();
	local t = {};
	for class,types in pairs(cache) do
		for type,names in pairs(types) do
			for name,rrs in pairs(names) do
				prune(rrs, time);
				append(t, tostring(rrs));
			end
		end
	end
	return table.concat(t);
end


-- packet layer -------------------------------------------------- packet layer


function dns.random(...)    -- - - - - - - - - - - - - - - - - - -  dns.random
	math.randomseed(math.floor(10000*socket.gettime()) % 0x80000000);
	dns.random = math.random;
	return dns.random(...);
end


local function encodeHeader(o)    -- - - - - - - - - - - - - - -  encodeHeader
	o = o or {};
	o.id = o.id or dns.random(0, 0xffff); -- 16b	(random) id

	o.rd = o.rd or 1;		--  1b  1 recursion desired
	o.tc = o.tc or 0;		--  1b	1 truncated response
	o.aa = o.aa or 0;		--  1b	1 authoritative response
	o.opcode = o.opcode or 0;	--  4b	0 query
				--  1 inverse query
				--	2 server status request
				--	3-15 reserved
	o.qr = o.qr or 0;		--  1b	0 query, 1 response

	o.rcode = o.rcode or 0;	--  4b  0 no error
				--	1 format error
				--	2 server failure
				--	3 name error
				--	4 not implemented
				--	5 refused
				--	6-15 reserved
	o.z = o.z  or 0;		--  3b  0 resvered
	o.ra = o.ra or 0;		--  1b  1 recursion available

	o.qdcount = o.qdcount or 1;	-- 16b	number of question RRs
	o.ancount = o.ancount or 0;	-- 16b	number of answers RRs
	o.nscount = o.nscount or 0;	-- 16b	number of nameservers RRs
	o.arcount = o.arcount or 0;	-- 16b  number of additional RRs

	-- string.char() rounds, so prevent roundup with -0.4999
	local header = string.char(
		highbyte(o.id), o.id %0x100,
		o.rd + 2*o.tc + 4*o.aa + 8*o.opcode + 128*o.qr,
		o.rcode + 16*o.z + 128*o.ra,
		highbyte(o.qdcount),  o.qdcount %0x100,
		highbyte(o.ancount),  o.ancount %0x100,
		highbyte(o.nscount),  o.nscount %0x100,
		highbyte(o.arcount),  o.arcount %0x100
	);

	return header, o.id;
end


local function encodeName(name)    -- - - - - - - - - - - - - - - - encodeName
	local t = {};
	for part in string.gmatch(name, '[^.]+') do
		append(t, string.char(string.len(part)));
		append(t, part);
	end
	append(t, string.char(0));
	return table.concat(t);
end


local function encodeQuestion(qname, qtype, qclass)    -- - - - encodeQuestion
	qname  = encodeName(qname);
	qtype  = dns.typecode[qtype or 'a'];
	qclass = dns.classcode[qclass or 'in'];
	return qname..qtype..qclass;
end


function resolver:byte(len)    -- - - - - - - - - - - - - - - - - - - - - byte
	len = len or 1;
	local offset = self.offset;
	local last = offset + len - 1;
	if last > #self.packet then
		error(string.format('out of bounds: %i>%i', last, #self.packet));
	end
	self.offset = offset + len;
	return string.byte(self.packet, offset, last);
end


function resolver:word()    -- - - - - - - - - - - - - - - - - - - - - -  word
	local b1, b2 = self:byte(2);
	return 0x100*b1 + b2;
end


function resolver:dword ()    -- - - - - - - - - - - - - - - - - - - - -  dword
	local b1, b2, b3, b4 = self:byte(4);
	--print('dword', b1, b2, b3, b4);
	return 0x1000000*b1 + 0x10000*b2 + 0x100*b3 + b4;
end


function resolver:sub(len)    -- - - - - - - - - - - - - - - - - - - - - - sub
	len = len or 1;
	local s = string.sub(self.packet, self.offset, self.offset + len - 1);
	self.offset = self.offset + len;
	return s;
end


function resolver:header(force)    -- - - - - - - - - - - - - - - - - - header
	local id = self:word();
	--print(string.format(':header  id  %x', id));
	if not self.active[id] and not force then return nil; end

	local h = { id = id };

	local b1, b2 = self:byte(2);

	h.rd      = b1 %2;
	h.tc      = b1 /2%2;
	h.aa      = b1 /4%2;
	h.opcode  = b1 /8%16;
	h.qr      = b1 /128;

	h.rcode   = b2 %16;
	h.z       = b2 /16%8;
	h.ra      = b2 /128;

	h.qdcount = self:word();
	h.ancount = self:word();
	h.nscount = self:word();
	h.arcount = self:word();

	for k,v in pairs(h) do h[k] = v-v%1; end

	return h;
end


function resolver:name()    -- - - - - - - - - - - - - - - - - - - - - -  name
	local remember, pointers = nil, 0;
	local len = self:byte();
	local n = {};
	if len == 0 then return "." end -- Root label
	while len > 0 do
		if len >= 0xc0 then    -- name is "compressed"
			pointers = pointers + 1;
			if pointers >= 20 then error('dns error: 20 pointers'); end;
			local offset = ((len-0xc0)*0x100) + self:byte();
			remember = remember or self.offset;
			self.offset = offset + 1;    -- +1 for lua
		else    -- name is not compressed
			append(n, self:sub(len)..'.');
		end
		len = self:byte();
	end
	self.offset = remember or self.offset;
	return table.concat(n);
end


function resolver:question()    -- - - - - - - - - - - - - - - - - -  question
	local q = {};
	q.name  = self:name();
	q.type  = dns.type[self:word()];
	q.class = dns.class[self:word()];
	return q;
end


function resolver:A(rr)    -- - - - - - - - - - - - - - - - - - - - - - - -  A
	local b1, b2, b3, b4 = self:byte(4);
	rr.a = string.format('%i.%i.%i.%i', b1, b2, b3, b4);
end

if have_util_net and util_net.ntop then
	function resolver:A(rr)
		rr.a = util_net.ntop(self:sub(4));
	end
end

function resolver:AAAA(rr)
	local addr = {};
	for _ = 1, rr.rdlength, 2 do
		local b1, b2 = self:byte(2);
		table.insert(addr, ("%02x%02x"):format(b1, b2));
	end
	addr = table.concat(addr, ":"):gsub("%f[%x]0+(%x)","%1");
	local zeros = {};
	for item in addr:gmatch(":[0:]+:[0:]+:") do
		table.insert(zeros, item)
	end
	if #zeros == 0 then
		rr.aaaa = addr;
		return
	elseif #zeros > 1 then
		table.sort(zeros, function(a, b) return #a > #b end);
	end
	rr.aaaa = addr:gsub(zeros[1], "::", 1):gsub("^0::", "::"):gsub("::0$", "::");
end

if have_util_net and util_net.ntop then
	function resolver:AAAA(rr)
		rr.aaaa = util_net.ntop(self:sub(16));
	end
end

function resolver:CNAME(rr)    -- - - - - - - - - - - - - - - - - - - -  CNAME
	rr.cname = self:name();
end


function resolver:MX(rr)    -- - - - - - - - - - - - - - - - - - - - - - -  MX
	rr.pref = self:word();
	rr.mx   = self:name();
end


function resolver:LOC_nibble_power()    -- - - - - - - - - -  LOC_nibble_power
	local b = self:byte();
	--print('nibbles', ((b-(b%0x10))/0x10), (b%0x10));
	return ((b-(b%0x10))/0x10) * (10^(b%0x10));
end


function resolver:LOC(rr)    -- - - - - - - - - - - - - - - - - - - - - -  LOC
	rr.version = self:byte();
	if rr.version == 0 then
		rr.loc           = rr.loc or {};
		rr.loc.size      = self:LOC_nibble_power();
		rr.loc.horiz_pre = self:LOC_nibble_power();
		rr.loc.vert_pre  = self:LOC_nibble_power();
		rr.loc.latitude  = self:dword();
		rr.loc.longitude = self:dword();
		rr.loc.altitude  = self:dword();
	end
end


local function LOC_tostring_degrees(f, pos, neg)    -- - - - - - - - - - - - -
	f = f - 0x80000000;
	if f < 0 then pos = neg; f = -f; end
	local deg, min, msec;
	msec = f%60000;
	f    = (f-msec)/60000;
	min  = f%60;
	deg = (f-min)/60;
	return string.format('%3d %2d %2.3f %s', deg, min, msec/1000, pos);
end


function resolver.LOC_tostring(rr)    -- - - - - - - - - - - - -  LOC_tostring
	local t = {};

	--[[
	for k,name in pairs { 'size', 'horiz_pre', 'vert_pre', 'latitude', 'longitude', 'altitude' } do
		append(t, string.format('%4s%-10s: %12.0f\n', '', name, rr.loc[name]));
	end
	--]]

	append(t, string.format(
		'%s    %s    %.2fm %.2fm %.2fm %.2fm',
		LOC_tostring_degrees (rr.loc.latitude, 'N', 'S'),
		LOC_tostring_degrees (rr.loc.longitude, 'E', 'W'),
		(rr.loc.altitude - 10000000) / 100,
		rr.loc.size / 100,
		rr.loc.horiz_pre / 100,
		rr.loc.vert_pre / 100
	));

	return table.concat(t);
end


function resolver:NS(rr)    -- - - - - - - - - - - - - - - - - - - - - - -  NS
	rr.ns = self:name();
end


function resolver:SOA(rr)    -- - - - - - - - - - - - - - - - - - - - - -  SOA
end


function resolver:SRV(rr)    -- - - - - - - - - - - - - - - - - - - - - -  SRV
	  rr.srv = {};
	  rr.srv.priority = self:word();
	  rr.srv.weight   = self:word();
	  rr.srv.port     = self:word();
	  rr.srv.target   = self:name();
end

function resolver:PTR(rr)
	rr.ptr = self:name();
end

function resolver:TXT(rr)    -- - - - - - - - - - - - - - - - - - - - - -  TXT
	rr.txt = self:sub (self:byte());
end


function resolver:rr()    -- - - - - - - - - - - - - - - - - - - - - - - -  rr
	local rr = {};
	setmetatable(rr, rr_metatable);
	rr.name     = self:name(self);
	rr.type     = dns.type[self:word()] or rr.type;
	rr.class    = dns.class[self:word()] or rr.class;
	rr.ttl      = 0x10000*self:word() + self:word();
	rr.rdlength = self:word();

	rr.tod = self.time + math.max(rr.ttl, 1);

	local remember = self.offset;
	local rr_parser = self[dns.type[rr.type]];
	if rr_parser then rr_parser(self, rr); end
	self.offset = remember;
	rr.rdata = self:sub(rr.rdlength);
	return rr;
end


function resolver:rrs (count)    -- - - - - - - - - - - - - - - - - - - - - rrs
	local rrs = {};
	for _ = 1, count do append(rrs, self:rr()); end
	return rrs;
end


function resolver:decode(packet, force)    -- - - - - - - - - - - - - - decode
	self.packet, self.offset = packet, 1;
	local header = self:header(force);
	if not header then return nil; end
	local response = { header = header };

	response.question = {};
	local offset = self.offset;
	for _ = 1, response.header.qdcount do
		append(response.question, self:question());
	end
	response.question.raw = string.sub(self.packet, offset, self.offset - 1);

	if not force then
		if not self.active[response.header.id] or not self.active[response.header.id][response.question.raw] then
			self.active[response.header.id] = nil;
			return nil;
		end
	end

	response.answer     = self:rrs(response.header.ancount);
	response.authority  = self:rrs(response.header.nscount);
	response.additional = self:rrs(response.header.arcount);

	return response;
end


-- socket layer -------------------------------------------------- socket layer


resolver.delays = { 1, 3 };


function resolver:addnameserver(address)    -- - - - - - - - - - addnameserver
	self.server = self.server or {};
	append(self.server, address);
end


function resolver:setnameserver(address)    -- - - - - - - - - - setnameserver
	self.server = {};
	self:addnameserver(address);
end


function resolver:adddefaultnameservers()    -- - - - -  adddefaultnameservers
	if is_windows then
		if windows and windows.get_nameservers then
			for _, server in ipairs(windows.get_nameservers()) do
				self:addnameserver(server);
			end
		end
		if not self.server or #self.server == 0 then
			-- TODO log warning about no nameservers, adding opendns servers as fallback
			self:addnameserver("208.67.222.222");
			self:addnameserver("208.67.220.220");
		end
	else -- posix
		local resolv_conf = io.open("/etc/resolv.conf");
		if resolv_conf then
			for line in resolv_conf:lines() do
				line = line:gsub("#.*$", "")
					:match('^%s*nameserver%s+([%x:%.]*%%?%S*)%s*$');
				if line then
					local ip = new_ip(line);
					if ip then
						self:addnameserver(ip.addr);
					end
				end
			end
			resolv_conf:close();
		end
		if not self.server or #self.server == 0 then
			-- TODO log warning about no nameservers, adding localhost as the default nameserver
			self:addnameserver("127.0.0.1");
		end
	end
end


function resolver:getsocket(servernum)    -- - - - - - - - - - - - - getsocket
	self.socket = self.socket or {};
	self.socketset = self.socketset or {};

	local sock = self.socket[servernum];
	if sock then return sock; end

	local ok, err;
	local peer = self.server[servernum];
	if peer:find(":") then
		sock, err = socket.udp6();
	else
		sock, err = (socket.udp4 or socket.udp)();
	end
	if sock and self.socket_wrapper then sock, err = self.socket_wrapper(sock, self); end
	if not sock then
		return nil, err;
	end
	sock:settimeout(0);
	-- todo: attempt to use a random port, fallback to 0
	self.socket[servernum] = sock;
	self.socketset[sock] = servernum;
	-- set{sock,peer}name can fail, eg because of local routing table
	-- if so, try the next server
	ok, err = sock:setsockname('*', 0);
	if not ok then return self:servfail(sock, err); end
	ok, err = sock:setpeername(peer, 53);
	if not ok then return self:servfail(sock, err); end
	return sock;
end

function resolver:voidsocket(sock)
	if self.socket[sock] then
		self.socketset[self.socket[sock]] = nil;
		self.socket[sock] = nil;
	elseif self.socketset[sock] then
		self.socket[self.socketset[sock]] = nil;
		self.socketset[sock] = nil;
	end
	sock:close();
end

function resolver:socket_wrapper_set(func)  -- - - - - - - socket_wrapper_set
	self.socket_wrapper = func;
end


function resolver:closeall ()    -- - - - - - - - - - - - - - - - - -  closeall
	for i,sock in ipairs(self.socket) do
		self.socket[i] = nil;
		self.socketset[sock] = nil;
		sock:close();
	end
end


function resolver:remember(rr, type)    -- - - - - - - - - - - - - -  remember
	--print ('remember', type, rr.class, rr.type, rr.name)
	local qname, qtype, qclass = standardize(rr.name, rr.type, rr.class);

	if type ~= '*' then
		type = qtype;
		local all = get(self.cache, qclass, '*', qname);
		--print('remember all', all);
		if all then append(all, rr); end
	end

	self.cache = self.cache or setmetatable({}, cache_metatable);
	local rrs = get(self.cache, qclass, type, qname) or
		set(self.cache, qclass, type, qname, setmetatable({}, rrs_metatable));
	if rr[qtype:lower()] and not rrs[rr[qtype:lower()]] then
		rrs[rr[qtype:lower()]] = true;
		append(rrs, rr);
	end

	if type == 'MX' then self.unsorted[rrs] = true; end
end


local function comp_mx(a, b)    -- - - - - - - - - - - - - - - - - - - comp_mx
	return (a.pref == b.pref) and (a.mx < b.mx) or (a.pref < b.pref);
end


function resolver:peek (qname, qtype, qclass, n)    -- - - - - - - - - - - -  peek
	qname, qtype, qclass = standardize(qname, qtype, qclass);
	local rrs = get(self.cache, qclass, qtype, qname);
	if not rrs then
		if n then if n <= 0 then return end else n = 3 end
		rrs = get(self.cache, qclass, "CNAME", qname);
		if not (rrs and rrs[1]) then return end
		return self:peek(rrs[1].cname, qtype, qclass, n - 1);
	end
	if prune(rrs, socket.gettime()) and qtype == '*' or not next(rrs) then
		set(self.cache, qclass, qtype, qname, nil);
		return nil;
	end
	if self.unsorted[rrs] then table.sort (rrs, comp_mx); self.unsorted[rrs] = nil; end
	return rrs;
end


function resolver:purge(soft)    -- - - - - - - - - - - - - - - - - - -  purge
	if soft == 'soft' then
		self.time = socket.gettime();
		for class,types in pairs(self.cache or {}) do
			for type,names in pairs(types) do
				for name,rrs in pairs(names) do
					prune(rrs, self.time, 'soft')
				end
			end
		end
	else self.cache = setmetatable({}, cache_metatable); end
end


function resolver:query(qname, qtype, qclass)    -- - - - - - - - - - -- query
	qname, qtype, qclass = standardize(qname, qtype, qclass)

	local co = coroutine.running();
	local q = get(self.wanted, qclass, qtype, qname);
	if co and q then
		-- We are already waiting for a reply to an identical query.
		set(self.wanted, qclass, qtype, qname, co, true);
		return true;
	end

	if not self.server then self:adddefaultnameservers(); end

	local question = encodeQuestion(qname, qtype, qclass);
	local peek = self:peek (qname, qtype, qclass);
	if peek then return peek; end

	local header, id = encodeHeader();
	--print ('query  id', id, qclass, qtype, qname)
	local o = {
		packet = header..question,
		server = self.best_server,
		delay  = 1,
		retry  = socket.gettime() + self.delays[1]
	};

	-- remember the query
	self.active[id] = self.active[id] or {};
	self.active[id][question] = o;

	local conn, err = self:getsocket(o.server)
	if not conn then
		return nil, err;
	end
	conn:send (o.packet)

	-- remember which coroutine wants the answer
	if co then
		set(self.wanted, qclass, qtype, qname, co, true);
	end
	
	if timer and self.timeout then
		local num_servers = #self.server;
		local i = 1;
		timer.add_task(self.timeout, function ()
			if get(self.wanted, qclass, qtype, qname, co) then
				if i < num_servers then
					i = i + 1;
					self:servfail(conn);
					o.server = self.best_server;
					conn, err = self:getsocket(o.server);
					if conn then
						conn:send(o.packet);
						return self.timeout;
					end
				end
				-- Tried everything, failed
				self:cancel(qclass, qtype, qname);
			end
		end)
	end
	return true;
end

function resolver:servfail(sock, err)
	-- Resend all queries for this server

	local num = self.socketset[sock]

	-- Socket is dead now
	sock = self:voidsocket(sock);

	-- Find all requests to the down server, and retry on the next server
	self.time = socket.gettime();
	for id,queries in pairs(self.active) do
		for question,o in pairs(queries) do
			if o.server == num then -- This request was to the broken server
				o.server = o.server + 1 -- Use next server
				if o.server > #self.server then
					o.server = 1;
				end

				o.retries = (o.retries or 0) + 1;
				if o.retries >= #self.server then
					--print('timeout');
					queries[question] = nil;
				else
					sock, err = self:getsocket(o.server);
					if sock then sock:send(o.packet); end
				end
			end
		end
		if next(queries) == nil then
			self.active[id] = nil;
		end
	end

	if num == self.best_server then
		self.best_server = self.best_server + 1;
		if self.best_server > #self.server then
			-- Exhausted all servers, try first again
			self.best_server = 1;
		end
	end
	return sock, err;
end

function resolver:settimeout(seconds)
	self.timeout = seconds;
end

function resolver:receive(rset)    -- - - - - - - - - - - - - - - - -  receive
	--print('receive');  print(self.socket);
	self.time = socket.gettime();
	rset = rset or self.socket;

	local response;
	for _, sock in pairs(rset) do

		if self.socketset[sock] then
			local packet = sock:receive();
			if packet then
				response = self:decode(packet);
				if response and self.active[response.header.id]
					and self.active[response.header.id][response.question.raw] then
					--print('received response');
					--self.print(response);

					for _, rr in pairs(response.answer) do
						if rr.name:sub(-#response.question[1].name, -1) == response.question[1].name then
							self:remember(rr, response.question[1].type)
						end
					end

					-- retire the query
					local queries = self.active[response.header.id];
					queries[response.question.raw] = nil;
					
					if not next(queries) then self.active[response.header.id] = nil; end
					if not next(self.active) then self:closeall(); end

					-- was the query on the wanted list?
					local q = response.question[1];
					local cos = get(self.wanted, q.class, q.type, q.name);
					if cos then
						for co in pairs(cos) do
							if coroutine.status(co) == "suspended" then coroutine.resume(co); end
						end
						set(self.wanted, q.class, q.type, q.name, nil);
					end
				end
				
			end
		end
	end

	return response;
end


function resolver:feed(sock, packet, force)
	--print('receive'); print(self.socket);
	self.time = socket.gettime();

	local response = self:decode(packet, force);
	if response and self.active[response.header.id]
		and self.active[response.header.id][response.question.raw] then
		--print('received response');
		--self.print(response);

		for _, rr in pairs(response.answer) do
			self:remember(rr, rr.type);
		end

		for _, rr in pairs(response.additional) do
			self:remember(rr, rr.type);
		end

		-- retire the query
		local queries = self.active[response.header.id];
		queries[response.question.raw] = nil;
		if not next(queries) then self.active[response.header.id] = nil; end
		if not next(self.active) then self:closeall(); end

		-- was the query on the wanted list?
		local q = response.question[1];
		if q then
			local cos = get(self.wanted, q.class, q.type, q.name);
			if cos then
				for co in pairs(cos) do
					if coroutine.status(co) == "suspended" then coroutine.resume(co); end
				end
				set(self.wanted, q.class, q.type, q.name, nil);
			end
		end
	end

	return response;
end

function resolver:cancel(qclass, qtype, qname)
	local cos = get(self.wanted, qclass, qtype, qname);
	if cos then
		for co in pairs(cos) do
			if coroutine.status(co) == "suspended" then coroutine.resume(co); end
		end
		set(self.wanted, qclass, qtype, qname, nil);
	end
end

function resolver:pulse()    -- - - - - - - - - - - - - - - - - - - - -  pulse
	--print(':pulse');
	while self:receive() do end
	if not next(self.active) then return nil; end

	self.time = socket.gettime();
	for id,queries in pairs(self.active) do
		for question,o in pairs(queries) do
			if self.time >= o.retry then

				o.server = o.server + 1;
				if o.server > #self.server then
					o.server = 1;
					o.delay = o.delay + 1;
				end

				if o.delay > #self.delays then
					--print('timeout');
					queries[question] = nil;
					if not next(queries) then self.active[id] = nil; end
					if not next(self.active) then return nil; end
				else
					--print('retry', o.server, o.delay);
					local _a = self.socket[o.server];
					if _a then _a:send(o.packet); end
					o.retry = self.time + self.delays[o.delay];
				end
			end
		end
	end

	if next(self.active) then return true; end
	return nil;
end


function resolver:lookup(qname, qtype, qclass)    -- - - - - - - - - -  lookup
	self:query (qname, qtype, qclass)
	while self:pulse() do
		local recvt = {}
		for i, s in ipairs(self.socket) do
			recvt[i] = s
		end
		socket.select(recvt, nil, 4)
	end
	--print(self.cache);
	return self:peek(qname, qtype, qclass);
end

function resolver:lookupex(handler, qname, qtype, qclass)    -- - - - - - - - - -  lookup
	return self:peek(qname, qtype, qclass) or self:query(qname, qtype, qclass);
end

function resolver:tohostname(ip)
	return dns.lookup(ip:gsub("(%d+)%.(%d+)%.(%d+)%.(%d+)", "%4.%3.%2.%1.in-addr.arpa."), "PTR");
end

--print ---------------------------------------------------------------- print


local hints = {    -- - - - - - - - - - - - - - - - - - - - - - - - - - - hints
	qr = { [0]='query', 'response' },
	opcode = { [0]='query', 'inverse query', 'server status request' },
	aa = { [0]='non-authoritative', 'authoritative' },
	tc = { [0]='complete', 'truncated' },
	rd = { [0]='recursion not desired', 'recursion desired' },
	ra = { [0]='recursion not available', 'recursion available' },
	z  = { [0]='(reserved)' },
	rcode = { [0]='no error', 'format error', 'server failure', 'name error', 'not implemented' },

	type = dns.type,
	class = dns.class
};


local function hint(p, s)    -- - - - - - - - - - - - - - - - - - - - - - hint
	return (hints[s] and hints[s][p[s]]) or '';
end


function resolver.print(response)    -- - - - - - - - - - - - - resolver.print
	for _, s in pairs { 'id', 'qr', 'opcode', 'aa', 'tc', 'rd', 'ra', 'z',
						'rcode', 'qdcount', 'ancount', 'nscount', 'arcount' } do
		print( string.format('%-30s', 'header.'..s), response.header[s], hint(response.header, s) );
	end

	for i,question in ipairs(response.question) do
		print(string.format ('question[%i].name         ', i), question.name);
		print(string.format ('question[%i].type         ', i), question.type);
		print(string.format ('question[%i].class        ', i), question.class);
	end

	local common = { name=1, type=1, class=1, ttl=1, rdlength=1, rdata=1 };
	local tmp;
	for _, s in pairs({'answer', 'authority', 'additional'}) do
		for i,rr in pairs(response[s]) do
			for _, t in pairs({ 'name', 'type', 'class', 'ttl', 'rdlength' }) do
				tmp = string.format('%s[%i].%s', s, i, t);
				print(string.format('%-30s', tmp), rr[t], hint(rr, t));
			end
			for j,t in pairs(rr) do
				if not common[j] then
					tmp = string.format('%s[%i].%s', s, i, j);
					print(string.format('%-30s  %s', tostring(tmp), tostring(t)));
				end
			end
		end
	end
end


-- module api ------------------------------------------------------ module api


function dns.resolver ()    -- - - - - - - - - - - - - - - - - - - - - resolver
	local r = { active = {}, cache = {}, unsorted = {}, wanted = {}, best_server = 1 };
	setmetatable (r, resolver);
	setmetatable (r.cache, cache_metatable);
	setmetatable (r.unsorted, { __mode = 'kv' });
	return r;
end

local _resolver = dns.resolver();
dns._resolver = _resolver;

function dns.lookup(...)    -- - - - - - - - - - - - - - - - - - - - -  lookup
	return _resolver:lookup(...);
end

function dns.tohostname(...)
	return _resolver:tohostname(...);
end

function dns.purge(...)    -- - - - - - - - - - - - - - - - - - - - - -  purge
	return _resolver:purge(...);
end

function dns.peek(...)    -- - - - - - - - - - - - - - - - - - - - - - -  peek
	return _resolver:peek(...);
end

function dns.query(...)    -- - - - - - - - - - - - - - - - - - - - - -  query
	return _resolver:query(...);
end

function dns.feed(...)    -- - - - - - - - - - - - - - - - - - - - - - -  feed
	return _resolver:feed(...);
end

function dns.cancel(...)  -- - - - - - - - - - - - - - - - - - - - - -  cancel
	return _resolver:cancel(...);
end

function dns.settimeout(...)
	return _resolver:settimeout(...);
end

function dns.cache()
	return _resolver.cache;
end

function dns.socket_wrapper_set(...)    -- - - - - - - - -  socket_wrapper_set
	return _resolver:socket_wrapper_set(...);
end

return dns;
