mod_http: Make RFC 7239 Forwarded opt-in for now to be safe

Supporting both methods at the same time may open to spoofing attacks,
whereby a client sends a Forwarded header that is not stripped by a
reverse proxy, leading Prosody to use that instead of the X-Forwarded-*
headers actually sent by the proxy.

By only supporting one at a time, it can be configured to match what the
proxy uses.

Disabled by default since implementations are sparse and X-Forwarded-*
are everywhere.

CHANGES

@@ -42,7 +42,7 @@ TRUNK
 - mod_blocklist: New option 'migrate_legacy_blocking' to disable migration from mod_privacy
 - Ability to use SQLite3 storage using LuaSQLite3 instead of LuaDBI
 - Moved all modules into the Lua namespace `prosody.`
-- Forwarded header from RFC 7239 supported
+- Forwarded header from RFC 7239 supported, disabled by default
 
 ## Removed
 

plugins/mod_http.lua

@@ -333,11 +333,17 @@ local function get_forwarded_connection_info(request) --> ip:string, secure:bool
 				break
 			end
 		end
-
-		-- Ignore legacy X-Forwarded-For and X-Forwarded-Proto, handling both seems unfeasible.
-		return ip, secure;
 	end
 
+	return ip, secure;
+end
+
+-- TODO switch to RFC 7239 by default once support is more common
+if module:get_option_boolean("http_legacy_x_forwarded", true) then
+function get_forwarded_connection_info(request) --> ip:string, secure:boolean
+	local ip = request.ip;
+	local secure = request.secure; -- set by net.http.server
+
 	local forwarded_for = request.headers.x_forwarded_for;
 	if forwarded_for then
 		-- luacheck: ignore 631
@@ -360,6 +366,7 @@ local function get_forwarded_connection_info(request) --> ip:string, secure:bool
 
 	return ip, secure;
 end
+end
 
 module:wrap_object_event(server._events, false, function (handlers, event_name, event_data)
 	local request = event_data.request;