mod_http: Use RFC 7239 Forwarded header to find original client IP

Prefer over X-Forwarded-* since it has an actual specification.

Main practical difference is that Forwarded may carry more properties
than only the IP address since it is a structured header.

Since we parse it into an array, it is easier to do the logical thing
and iterate backwards trough proxies until an untrusted one is
encountered. Compare the handling of X-Forwarded-For.

The 'secure' field now accounts for the full chain of proxies, which
must be secure all the way to be considered secure.

CHANGES

@@ -42,6 +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
 
 ## Removed
 

plugins/mod_http.lua

@@ -15,7 +15,8 @@ local portmanager = require "prosody.core.portmanager";
 local moduleapi = require "prosody.core.moduleapi";
 local url_parse = require "socket.url".parse;
 local url_build = require "socket.url".build;
-local normalize_path = require "prosody.util.http".normalize_path;
+local http_util = require "prosody.util.http";
+local normalize_path = http_util.normalize_path;
 local set = require "prosody.util.set";
 local array = require "util.array";
 
@@ -319,6 +320,24 @@ end
 local 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 = http_util.parse_forwarded(request.headers.forwarded);
+	if forwarded then
+		request.forwarded = forwarded;
+		for i = #forwarded, 1, -1 do
+			local proxy = forwarded[i]
+			if is_trusted_proxy(ip) then
+				ip = normal_ip(proxy["for"]);
+				secure = secure and proxy.proto == "https";
+			else
+				break
+			end
+		end
+
+		-- Ignore legacy X-Forwarded-For and X-Forwarded-Proto, handling both seems unfeasible.
+		return ip, secure;
+	end
+
 	local forwarded_for = request.headers.x_forwarded_for;
 	if forwarded_for then
 		-- luacheck: ignore 631