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mod_tls: Enable Prosody's certificate checking for incoming s2s connections (fixes #1916) (thanks Damian, Zash) Various options in Prosody allow control over the behaviour of the certificate verification process For example, some deployments choose to allow falling back to traditional "dialback" authentication (XEP-0220), while others verify via DANE, hard-coded fingerprints, or other custom plugins. Implementing this flexibility requires us to override OpenSSL's default certificate verification, to allow Prosody to verify the certificate itself, apply custom policies and make decisions based on the outcome. To enable our custom logic, we have to suppress OpenSSL's default behaviour of aborting the connection with a TLS alert message. With LuaSec, this can be achieved by using the verifyext "lsec_continue" flag. We also need to use the lsec_ignore_purpose flag, because XMPP s2s uses server certificates as "client" certificates (for mutual TLS verification in outgoing s2s connections). Commit 99d2100d2918 moved these settings out of the defaults and into mod_s2s, because we only really need these changes for s2s, and they should be opt-in, rather than automatically applied to all TLS services we offer. That commit was incomplete, because it only added the flags for incoming direct TLS connections. StartTLS connections are handled by mod_tls, which was not applying the lsec_* flags. It previously worked because they were already in the defaults. This resulted in incoming s2s connections with "invalid" certificates being aborted early by OpenSSL, even if settings such as `s2s_secure_auth = false` or DANE were present in the config. Outgoing s2s connections inherit verify "none" from the defaults, which means OpenSSL will receive the cert but will not terminate the connection when it is deemed invalid. This means we don't need lsec_continue there, and we also don't need lsec_ignore_purpose (because the remote peer is a "server"). Wondering why we can't just use verify "none" for incoming s2s? It's because in that mode, OpenSSL won't request a certificate from the peer for incoming connections. Setting verify "peer" is how you ask OpenSSL to request a certificate from the client, but also what triggers its built-in verification.

-- Prosody IM
-- Copyright (C) 2008-2010 Matthew Wild
-- Copyright (C) 2008-2010 Waqas Hussain
--
-- This project is MIT/X11 licensed. Please see the
-- COPYING file in the source package for more information.
--
-- luacheck: ignore 212

local new_sasl = require "prosody.util.sasl".new;
local datamanager = require "prosody.util.datamanager";
local hosts = prosody.hosts;

local allow_storage = module:get_option_boolean("allow_anonymous_storage", false);

-- define auth provider
local provider = {};

function provider.test_password(username, password)
	return nil, "Password based auth not supported.";
end

function provider.get_password(username)
	return nil, "Password not available.";
end

function provider.set_password(username, password)
	return nil, "Password based auth not supported.";
end

function provider.user_exists(username)
	return nil, "Only anonymous users are supported."; -- FIXME check if anonymous user is connected?
end

function provider.create_user(username, password)
	return nil, "Account creation/modification not supported.";
end

function provider.get_sasl_handler()
	local anonymous_authentication_profile = {
		anonymous = function(sasl, username, realm)
			return true; -- for normal usage you should always return true here
		end
	};
	return new_sasl(module.host, anonymous_authentication_profile);
end

function provider.users()
	return next, hosts[module.host].sessions, nil;
end

-- datamanager callback to disable writes
local function dm_callback(username, host, datastore, data)
	if host == module.host then
		return false;
	end
	return username, host, datastore, data;
end

if not module:get_option_boolean("allow_anonymous_s2s", false) then
	module:hook("route/remote", function (event)
		return false; -- Block outgoing s2s from anonymous users
	end, 300);
end

function module.load()
	if not allow_storage then
		datamanager.add_callback(dm_callback);
	end
end
function module.unload()
	if not allow_storage then
		datamanager.remove_callback(dm_callback);
	end
end

module:provides("auth", provider);