simplify natpmp gateway and local address discovery

include/libtorrent/enum_net.hpp

@@ -41,6 +41,8 @@ POSSIBILITY OF SUCH DAMAGE.
 #include <sys/socket.h> // for SO_BINDTODEVICE
 #endif
 
+#include <boost/optional.hpp>
+
 #include "libtorrent/aux_/disable_warnings_pop.hpp"
 
 #include "libtorrent/io_service_fwd.hpp"
@@ -94,6 +96,9 @@ namespace libtorrent {
 	TORRENT_EXTRA_EXPORT bool in_local_network(std::vector<ip_interface> const& net
 		, address const& addr);
 
+	TORRENT_EXTRA_EXPORT boost::optional<ip_route> get_default_route(io_service& ios
+		, string_view device, bool v6, error_code& ec);
+
 	// returns the first default gateway found if device is empty
 	TORRENT_EXTRA_EXPORT address get_default_gateway(io_service& ios
 		, string_view device, bool v6, error_code& ec);

src/enum_net.cpp

@@ -45,6 +45,7 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "libtorrent/aux_/disable_warnings_push.hpp"
 
 #include <boost/asio/ip/host_name.hpp>
+#include <boost/optional.hpp>
 
 #if TORRENT_USE_IFCONF
 #include <sys/ioctl.h>
@@ -844,10 +845,10 @@ int _System __libsocket_sysctl(int* mib, u_int namelen, void *oldp, size_t *oldl
 		return ret;
 	}
 
-	address get_default_gateway(io_service& ios
-		, string_view device, bool v6, error_code& ec)
+	boost::optional<ip_route> get_default_route(io_service& ios
+		, string_view const device, bool const v6, error_code& ec)
 	{
-		std::vector<ip_route> ret = enum_routes(ios, ec);
+		std::vector<ip_route> const ret = enum_routes(ios, ec);
 		auto const i = std::find_if(ret.begin(), ret.end()
 			, [device,v6](ip_route const& r)
 		{
@@ -855,8 +856,15 @@ int _System __libsocket_sysctl(int* mib, u_int namelen, void *oldp, size_t *oldl
 				&& r.destination.is_v6() == v6
 				&& (device.empty() || r.name == device);
 		});
-		if (i == ret.end()) return address();
-		return i->gateway;
+		if (i == ret.end()) return boost::none;
+		return *i;
+	}
+
+	address get_default_gateway(io_service& ios
+		, string_view const device, bool const v6, error_code& ec)
+	{
+		auto const default_route = get_default_route(ios, device, v6, ec);
+		return default_route ? default_route->gateway : address();
 	}
 
 	std::vector<ip_route> enum_routes(io_service& ios, error_code& ec)

src/natpmp.cpp

@@ -156,24 +156,58 @@ void natpmp::start(address local_address, std::string device)
 	// if necessary
 	m_version = version_pcp;
 
+	// we really want a device name to get the right default gateway
+	// try to find one even if the listen socket isn't bound to a device
+	if (device.empty())
+	{
+		device = device_for_address(local_address, m_socket.get_io_service(), ec);
+		// if this fails fall back to using the first default gateway in the
+		// routing table
+		ec.clear();
+	}
+
+	auto const route = get_default_route(m_socket.get_io_service()
+		, device, local_address.is_v6(), ec);
+
+	if (!route)
+	{
+#ifndef TORRENT_DISABLE_LOGGING
+		if (should_log())
+		{
+			log("failed to find default route for \"%s\" %s: %s"
+				, device.c_str(), local_address.to_string().c_str()
+				, convert_from_native(ec.message()).c_str());
+		}
+#endif
+		disable(ec);
+		return;
+	}
+
+	if (device.empty()) device = route->name;
+
+	TORRENT_ASSERT(!device.empty());
+
 	// PCP requires reporting the source address at the application
 	// layer so the socket MUST be bound to a specific address
 	// if the caller didn't specify one then get the first suitable
 	// address from the OS
 	if (local_address.is_unspecified())
 	{
-		for (auto const& a : enum_net_interfaces(m_socket.get_io_service(), ec))
-		{
-			if (a.interface_address.is_loopback()) continue;
-			if (a.interface_address.is_v4() != local_address.is_v4()) continue;
-			if (a.interface_address.is_v6() && is_local(a.interface_address)) continue;
-			if (!device.empty() && a.name != device) continue;
-			local_address = a.interface_address;
-			device = a.name;
-			break;
-		}
+		std::vector<ip_interface> const net = enum_net_interfaces(
+			m_socket.get_io_service(), ec);
 
-		if (local_address.is_unspecified())
+		auto const it = std::find_if(net.begin(), net.end(), [&](ip_interface const& i)
+		{
+			return i.interface_address.is_v4() == local_address.is_v4()
+				&& (i.interface_address.is_v4() || !is_local(i.interface_address))
+				&& i.name == device;
+		});
+
+		if (it != net.end())
+		{
+			local_address = it->interface_address;
+		}
+		else
 		{
 			// if we can't get a specific address to bind to we'll have
 			// to fall back to NAT-PMP
@@ -198,34 +232,9 @@ void natpmp::start(address local_address, std::string device)
 		}
 	}
 
-	// we really want a device name to get the right default gateway
-	// try to find one even if the listen socket isn't bound to a device
-	if (device.empty())
-	{
-		device = device_for_address(local_address, m_socket.get_io_service(), ec);
-		// if this fails fall back to using the first default gateway in the
-		// routing table
-		ec.clear();
-	}
-
-	address const gateway = get_default_gateway(m_socket.get_io_service()
-		, device, local_address.is_v6(), ec);
-	if (ec || gateway.is_unspecified())
-	{
-#ifndef TORRENT_DISABLE_LOGGING
-		if (should_log())
-		{
-			log("failed to find default route for %s: %s"
-				, local_address.to_string().c_str(), convert_from_native(ec.message()).c_str());
-		}
-#endif
-		disable(ec);
-		return;
-	}
-
 	m_disabled = false;
 
-	udp::endpoint nat_endpoint(gateway, 5351);
+	udp::endpoint const nat_endpoint(route->gateway, 5351);
 	if (nat_endpoint == m_nat_endpoint) return;
 	m_nat_endpoint = nat_endpoint;
 

test/enum_if.cpp

@@ -84,7 +84,7 @@ int main()
 
 	for (auto const& i : net)
 	{
-		address iface_def_gw = get_default_gateway(ios, i.name, i.interface_address.is_v6(), ec);
+		address const iface_def_gw = get_default_gateway(ios, i.name, i.interface_address.is_v6(), ec);
 		std::printf("%-34s%-45s%-20s%s%s%-20s%-34s%s %s\n"
 			, i.interface_address.to_string(ec).c_str()
 			, i.netmask.to_string(ec).c_str()