Feature: allow the use of STUN to connect client and server together

This method doesn't require port-forwarding to be used, and works for
most common NAT routers in home setups. But, for sure it doesn't work
for all setups, and not everyone will be able to use this.

1 files changed, 140 insertions, 0 deletions

diff --git a/src/network/network_stun.cpp b/src/network/network_stun.cpp
new file mode 100644
index 000000000..4af9b1e9d
--- /dev/null
+++ b/src/network/network_stun.cpp
@@ -0,0 +1,140 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/** @file network_stun.cpp STUN sending/receiving part of the network protocol. */
+
+#include "../stdafx.h"
+#include "../debug.h"
+#include "network.h"
+#include "network_coordinator.h"
+#include "network_stun.h"
+
+#include "../safeguards.h"
+
+/** Connect to the STUN server. */
+class NetworkStunConnecter : public TCPConnecter {
+private:
+	ClientNetworkStunSocketHandler *stun_handler;
+	std::string token;
+	uint8 family;
+
+public:
+	/**
+	 * Initiate the connecting.
+	 * @param stun_handler The handler for this request.
+	 * @param connection_string The address of the server.
+	 */
+	NetworkStunConnecter(ClientNetworkStunSocketHandler *stun_handler, const std::string &connection_string, const std::string &token, uint8 family) :
+		TCPConnecter(connection_string, NETWORK_STUN_SERVER_PORT, NetworkAddress(), family),
+		stun_handler(stun_handler),
+		token(token),
+		family(family)
+	{
+	}
+
+	void OnFailure() override
+	{
+		this->stun_handler->connecter = nullptr;
+
+		/* Connection to STUN server failed. For example, the client doesn't
+		 * support IPv6, which means it will fail that attempt. */
+
+		_network_coordinator_client.StunResult(this->token, this->family, false);
+	}
+
+	void OnConnect(SOCKET s) override
+	{
+		this->stun_handler->connecter = nullptr;
+
+		assert(this->stun_handler->sock == INVALID_SOCKET);
+		this->stun_handler->sock = s;
+
+		/* Store the local address; later connects will reuse it again.
+		 * This is what makes STUN work for most NATs. */
+		this->stun_handler->local_addr = NetworkAddress::GetSockAddress(s);
+
+		/* We leave the connection open till the real connection is setup later. */
+	}
+};
+
+/**
+ * Connect to the STUN server over either IPv4 or IPv6.
+ * @param token The token as received from the Game Coordinator.
+ * @param family What IP family to use.
+ */
+void ClientNetworkStunSocketHandler::Connect(const std::string &token, uint8 family)
+{
+	this->token = token;
+	this->family = family;
+
+	this->connecter = new NetworkStunConnecter(this, NetworkStunConnectionString(), token, family);
+}
+
+/**
+ * Send a STUN packet to the STUN server.
+ * @param token The token as received from the Game Coordinator.
+ * @param family What IP family this STUN request is for.
+ * @return The handler for this STUN request.
+ */
+std::unique_ptr<ClientNetworkStunSocketHandler> ClientNetworkStunSocketHandler::Stun(const std::string &token, uint8 family)
+{
+	auto stun_handler = std::make_unique<ClientNetworkStunSocketHandler>();
+
+	stun_handler->Connect(token, family);
+
+	Packet *p = new Packet(PACKET_STUN_SERCLI_STUN);
+	p->Send_uint8(NETWORK_COORDINATOR_VERSION);
+	p->Send_string(token);
+	p->Send_uint8(family);
+
+	stun_handler->SendPacket(p);
+
+	return stun_handler;
+}
+
+NetworkRecvStatus ClientNetworkStunSocketHandler::CloseConnection(bool error)
+{
+	NetworkStunSocketHandler::CloseConnection(error);
+
+	/* If our connecter is still pending, shut it down too. Otherwise the
+	 * callback of the connecter can call into us, and our object is most
+	 * likely about to be destroyed. */
+	if (this->connecter != nullptr) {
+		this->connecter->Kill();
+		this->connecter = nullptr;
+	}
+
+	return NETWORK_RECV_STATUS_OKAY;
+}
+
+/**
+ * Check whether we received/can send some data from/to the STUN server and
+ * when that's the case handle it appropriately.
+ */
+void ClientNetworkStunSocketHandler::SendReceive()
+{
+	if (this->sock == INVALID_SOCKET) return;
+
+	/* We never attempt to receive anything on a STUN socket. After
+	 * connecting a STUN connection, the local address will be reused to
+	 * to establish the connection with the real server. If we would be to
+	 * read this socket, some OSes get confused and deliver us packets meant
+	 * for the real connection. It appears most OSes play best when we simply
+	 * never attempt to read it to start with (and the packets will remain
+	 * available on the other socket).
+	 * Protocol-wise, the STUN server will never send any packet back anyway. */
+
+	this->CanSendReceive();
+	if (this->SendPackets() == SPS_ALL_SENT && !this->sent_result) {
+		/* We delay giving the GC the result this long, as to make sure we
+		 * have sent the STUN packet first. This means the GC is more likely
+		 * to have the result ready by the time our StunResult() packet
+		 * arrives. */
+		this->sent_result = true;
+		_network_coordinator_client.StunResult(this->token, this->family, true);
+	}
+}