Add support for overriding the NAT-PMP gateway with a nat_pmp_gateway config option

include/libtorrent/natpmp.hpp

@@ -91,7 +91,7 @@ struct TORRENT_EXTRA_EXPORT natpmp final
 {
 	natpmp(io_context& ios, aux::portmap_callback& cb, aux::listen_socket_handle ls);
 
-	void start(ip_interface const& ip);
+	void start(ip_interface const& ip, boost::optional<address> const& gateway);
 
 	// maps the ports, if a port is set to 0
 	// it will not be mapped

include/libtorrent/settings_pack.hpp

@@ -391,6 +391,11 @@ namespace aux {
 			// ``router.bt.ouinet.work:6881``,
 			dht_bootstrap_nodes,
 
+			// Overrides the NAT-PMP service gateway. When set, libtorrent won't try
+			// to resolve the default gateway and instead will send the requests to
+			// the address specified.
+			nat_pmp_gateway,
+
 			max_string_setting_internal
 		};
 

src/natpmp.cpp

@@ -152,31 +152,35 @@ natpmp::natpmp(io_context& ios
 	m_mappings.reserve(10);
 }
 
-void natpmp::start(ip_interface const& ip)
+void natpmp::start(ip_interface const& ip, boost::optional<address> const& gateway)
 {
 	TORRENT_ASSERT(is_single_thread());
 
 	// assume servers support PCP and fall back to NAT-PMP
 	// if necessary
 	m_version = version_pcp;
-
+	error_code ec;
 	address const& local_address = ip.interface_address;
 
-	error_code ec;
-	auto const routes = enum_routes(m_ioc, ec);
-	if (ec)
+	boost::optional<address> route = gateway;
+	if (!route)
 	{
-#ifndef TORRENT_DISABLE_LOGGING
-		if (should_log())
+
+		auto const routes = enum_routes(m_ioc, ec);
+		if (ec)
 		{
-			log("failed to enumerate routes: %s"
-				, convert_from_native(ec.message()).c_str());
+	#ifndef TORRENT_DISABLE_LOGGING
+			if (should_log())
+			{
+				log("failed to enumerate routes: %s"
+					, convert_from_native(ec.message()).c_str());
+			}
+	#endif
+			disable(ec);
 		}
-#endif
-		disable(ec);
-	}
 
-	auto const route = get_gateway(ip, routes);
+		route = get_gateway(ip, routes);
+	}
 
 	if (!route)
 	{

src/session_impl.cpp

@@ -5676,7 +5676,11 @@ namespace {
 			ip.netmask = s->netmask;
 			std::strncpy(ip.name, s->device.c_str(), sizeof(ip.name) - 1);
 			ip.name[sizeof(ip.name) - 1] = '\0';
-			s->natpmp_mapper->start(ip);
+
+			error_code ignored;
+			std::string gateway = m_settings.get_str(settings_pack::nat_pmp_gateway);
+			boost::optional<address> gateway_addr = make_address(gateway, ignored);
+			s->natpmp_mapper->start(ip, gateway == "" ? boost::none : gateway_addr);
 		}
 	}
 

src/settings_pack.cpp

@@ -149,7 +149,8 @@ constexpr int DISK_WRITE_MODE = settings_pack::enable_os_cache;
 		SET(proxy_password, "", &session_impl::update_proxy),
 		SET(i2p_hostname, "", &session_impl::update_i2p_bridge),
 		SET(peer_fingerprint, "-LT20B0-", nullptr),
-		SET(dht_bootstrap_nodes, "dht.libtorrent.org:25401", &session_impl::update_dht_bootstrap_nodes)
+		SET(dht_bootstrap_nodes, "dht.libtorrent.org:25401", &session_impl::update_dht_bootstrap_nodes),
+		SET(nat_pmp_gateway, "", &session_impl::update_upnp)
 	}});
 
 	CONSTEXPR_SETTINGS