File

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

-- public domain 20080404 lua@ztact.com


-- 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 ztact = require "util.ztact";
local timer = require "util.timer";

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, unpack =
      ipairs, next, pairs, print, setmetatable, tostring, assert, error, unpack;

local get, set = ztact.get, ztact.set;

local default_timeout = 15;

-------------------------------------------------- module dns
module('dns')
local dns = _M;


-- 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)    -- - - - - - - - - - - - - - - - - - - -  augment
	local a = {};
	for i,s in pairs(t) do
		a[i] = s;
		a[s] = s;
		a[string.lower(s)] = s;
	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 = {
	'A', 'NS', 'MD', 'MF', 'CNAME', 'SOA', 'MB', 'MG', 'MR', 'NULL', 'WKS',
	'PTR', 'HINFO', 'MINFO', 'MX', 'TXT',
	[ 28] = 'AAAA', [ 29] = 'LOC',   [ 33] = 'SRV',
	[252] = 'AXFR', [253] = 'MAILB', [254] = 'MAILA', [255] = '*' };


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


dns.type      = augment (dns.types);
dns.class     = augment (dns.classes);
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 pairs(rrs) do
		if rr.tod then
			-- rr.tod = rr.tod - 50    -- accelerated decripitude
			rr.ttl = math.floor(rr.tod - time);
			if rr.ttl <= 0 then
				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[i] = nil;
		end
	end
end


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


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

resolver.timeout = default_timeout;

local SRV_tostring;


local rr_metatable = {};   -- - - - - - - - - - - - - - - - - - -  rr_metatable
function rr_metatable.__tostring(rr)
	local s0 = string.format('%2s %-5s %6i %-28s', rr.class, rr.type, rr.ttl, rr.name);
	local s1 = '';
	if rr.type == 'A' then
		s1 = ' '..rr.a;
	elseif rr.type == 'MX' then
		s1 = string.format(' %2i %s', rr.pref, rr.mx);
	elseif rr.type == 'CNAME' then
		s1 = ' '..rr.cname;
	elseif rr.type == 'LOC' then
		s1 = ' '..resolver.LOC_tostring(rr);
	elseif rr.type == 'NS' then
		s1 = ' '..rr.ns;
	elseif rr.type == 'SRV' then
		s1 = ' '..SRV_tostring(rr);
	elseif rr.type == 'TXT' then
		s1 = ' '..rr.txt;
	else
		s1 = ' <UNKNOWN RDATA TYPE>';
	end
	return s0..s1;
end


local rrs_metatable = {};    -- - - - - - - - - - - - - - - - - -  rrs_metatable
function rrs_metatable.__tostring(rrs)
	local t = {};
	for i,rr in pairs(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


function resolver:new()    -- - - - - - - - - - - - - - - - - - - - - resolver
	local r = { active = {}, cache = {}, unsorted = {} };
	setmetatable(r, resolver);
	setmetatable(r.cache, cache_metatable);
	setmetatable(r.unsorted, { __mode = 'kv' });
	return r;
end


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


function dns.random(...)    -- - - - - - - - - - - - - - - - - - -  dns.random
	math.randomseed(math.floor(10000*socket.gettime()));
	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 = {};
	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


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 SRV_tostring(rr)    -- - - - - - - - - - - - - - - - - - SRV_tostring
	local s = rr.srv;
	return string.format( '%5d %5d %5d %s', s.priority, s.weight, s.port, s.target );
end


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

	if rr.ttl <= 0 then
		rr.tod = self.time + 30;
	else
		rr.tod = self.time + rr.ttl;
	end

	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 i = 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 i = 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
			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 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("#.*$", "")
					:match('^%s*nameserver%s+(.*)%s*$');
				if line then
					line:gsub("%f[%d.](%d+%.%d+%.%d+%.%d+)%f[^%d.]", function (address)
						self:addnameserver(address)
					end);
				end
			end
		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

	sock = socket.udp();
	if self.socket_wrapper then sock = self.socket_wrapper(sock, self); end
	sock:settimeout(0);
	-- todo: attempt to use a random port, fallback to 0
	sock:setsockname('*', 0);
	sock:setpeername(self.server[servernum], 53);
	self.socket[servernum] = sock;
	self.socketset[sock] = servernum;
	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
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));
	append(rrs, rr);

	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)    -- - - - - - - - - - - -  peek
	qname, qtype, qclass = standardize(qname, qtype, qclass);
	local rrs = get(self.cache, qclass, qtype, qname);
	if not rrs then return nil; 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); 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 = {}; end
end


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

	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;

  -- remember which coroutine wants the answer
	local co = coroutine.running();
	if co then
		set(self.wanted, qclass, qtype, qname, co, true);
		--set(self.yielded, co, qclass, qtype, qname, true);
	end

	local conn = self:getsocket(o.server)
	conn:send (o.packet)
	
	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 = self:getsocket(o.server);
					conn:send(o.packet);
					return self.timeout;
				else
					-- Tried everything, failed
					resolver:cancel(qclass, qtype, qname, co, true);
				end
			end
		end)
	end
end

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

	local num = self.socketset[sock]

	-- Socket is dead now
	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
					local _a = self:getsocket(o.server);
					if _a then _a:send(o.packet); end
				end
			end
		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
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 i,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 j,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
							set(self.yielded, co, q.class, q.type, q.name, nil);
							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)
	--print('receive'); print(self.socket);
	self.time = socket.gettime();

	local 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 j,rr in pairs(response.answer) do
			self:remember(rr, response.question[1].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
					set(self.yielded, co, q.class, q.type, q.name, nil);
					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, co, call_handler)
	local cos = get(self.wanted, qclass, qtype, qname);
	if cos then
		if call_handler then
			coroutine.resume(co);
		end
		cos[co] = 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


--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,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,s in pairs({'answer', 'authority', 'additional'}) do
		for i,rr in pairs(response[s]) do
			for j,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
	-- this function seems to be redundant with resolver.new ()

	local r = { active = {}, cache = {}, unsorted = {}, wanted = {}, yielded = {}, 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.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.socket_wrapper_set(...)    -- - - - - - - - -  socket_wrapper_set
	return _resolver:socket_wrapper_set(...);
end

return dns;