-- mod_s2s_auth_dane

-- Copyright (C) 2013-2014 Kim Alvefur

--

-- This file is MIT/X11 licensed.

--

-- In your DNS, put

-- _xmpp-server.example.com. IN TLSA 3 0 1 <sha256 hash of certificate>

--

-- Known issues:

-- Race condition

-- Could be done much cleaner if mod_s2s was using util.async

--

-- TODO Things to test/handle:

-- Negative or bogus answers

-- No encryption offered

-- Different hostname before and after STARTTLS - mod_s2s should complain

-- Interaction with Dialback

module:set_global();

local type = type;

local set = require"util.set";

local dns_lookup = require"net.adns".lookup;

local hashes = require"util.hashes";

local base64 = require"util.encodings".base64;

local idna_to_ascii = require "util.encodings".idna.to_ascii;

local s2sout = module:depends"s2s".route_to_new_session.s2sout;

local pat = "%-%-%-%-%-BEGIN ([A-Z ]+)%-%-%-%-%-\r?\n"..

"([0-9A-Za-z=+/\r\n]*)\r?\n%-%-%-%-%-END %1%-%-%-%-%-";

local function pem2der(pem)

	local typ, data = pem:match(pat);

	if typ and data then

		return base64.decode(data), typ;

	end

end

local use_map = { ["DANE-EE"] = 3; ["DANE-TA"] = 2; ["PKIX-EE"] = 1; ["PKIX-CA"] = 0 }

local implemented_uses = set.new { "DANE-EE", "PKIX-EE" };

local configured_uses = module:get_option_set("dane_uses", { "DANE-EE" });

local enabled_uses = set.intersection(implemented_uses, configured_uses) / function(use) return use_map[use] end;

local function dane_lookup(host_session, name, cb, a,b,c)

	if host_session.dane ~= nil then return false; end

	local ascii_host = name and idna_to_ascii(name);

	if not ascii_host then return false; end

	host_session.dane = dns_lookup(function(answer)

			host_session.dane = answer;

		else

			host_session.dane = false;

		end

		if cb then return cb(a,b,c); end

	end, ("_xmpp-server.%s."):format(ascii_host), "TLSA");

	host_session.connecting = true;

	return true;

end

local _attempt_connection = s2sout.attempt_connection;

function s2sout.attempt_connection(host_session, err)

	if not err and dane_lookup(host_session, host_session.to_host, _attempt_connection, host_session, err) then

		return true;

	end

	return _attempt_connection(host_session, err);

end

function module.add_host(module)

	module:hook("s2s-stream-features", function(event)

		local origin = event.origin;

		dane_lookup(origin, origin.from_host);

	end, 1);

	module:hook("s2s-authenticated", function(event)

		local session = event.session;

		if session.dane and not session.secure then

			-- TLSA record but no TLS, not ok.

			-- TODO Optional?

			-- Bogus replies should trigger this path

			-- How does this interact with Dialback?

			session:close({

				condition = "policy-violation",

				text = "Encrypted server-to-server communication is required but was not "

					..((session.direction == "outgoing" and "offered") or "used")

			});

			return false;

		end

	end);

end

module:hook("s2s-check-certificate", function(event)

	local session, cert = event.session, event.cert;

	local dane = session.dane;

	if type(dane) == "table" then

		local use, select, match, tlsa, certdata, match_found, supported_found;

		for i = 1, #dane do

			tlsa = dane[i].tlsa;

			module:log("debug", "TLSA %s %s %s %d bytes of data", tlsa:getUsage(), tlsa:getSelector(), tlsa:getMatchType(), #tlsa.data);

			use, select, match, certdata = tlsa.use, tlsa.select, tlsa.match;

			if enabled_uses:contains(use) then

				-- PKIX-EE or DANE-EE

				if use == 1 or use == 3 then

					supported_found = true

					if select == 0 then

						certdata = pem2der(cert:pem());

					elseif select == 1 and cert.pubkey then

						certdata = pem2der(cert:pubkey()); -- Not supported in stock LuaSec

					else

						module:log("warn", "DANE selector %s is unsupported", tlsa:getSelector() or select);

					end

					if match == 1 then

						certdata = hashes.sha256(certdata);

					elseif match == 2 then

						certdata = hashes.sha512(certdata);

					elseif match ~= 0 then

						module:log("warn", "DANE match rule %s is unsupported", tlsa:getMatchType() or match);

						certdata = nil;

					end

					-- Should we check if the cert subject matches?

					if certdata and certdata == tlsa.data then

						(session.log or module._log)("info", "DANE validation successful");

						session.cert_identity_status = "valid";

						if use == 3 then -- DANE-EE, chain status equals DNSSEC chain status

							session.cert_chain_status = "valid";

							-- for usage 1, PKIX-EE, the chain has to be valid already

						end

						match_found = true;

						break;

					end

				end

			end

		end

		if supported_found and not match_found or dane.bogus then

			-- No TLSA matched or response was bogus

			(session.log or module._log)("warn", "DANE validation failed");

			session.cert_identity_status = "invalid";

			session.cert_chain_status = "invalid";

		end

	end

end);

function module.unload()

	-- Restore the original attempt_connection function

	s2sout.attempt_connection = _attempt_connection;

end