Feature: join servers based on their invite code

This removes the need to know a server IP to join it. Invite codes
are small (~7 characters) indentifiers for servers, which can be
exchanged with other players to join the servers.

15 files changed, 422 insertions, 29 deletions

diff --git a/src/console_cmds.cpp b/src/console_cmds.cpp
index 9d03f3deb..0fcac4488 100644
--- a/src/console_cmds.cpp
+++ b/src/console_cmds.cpp
@@ -702,6 +702,7 @@ DEF_CONSOLE_CMD(ConServerInfo)
 		return true;
 	}
 
+	IConsolePrint(CC_DEFAULT, "Invite code:                {}", _network_server_invite_code);
 	IConsolePrint(CC_DEFAULT, "Current/maximum clients:    {:3d}/{:3d}", _network_game_info.clients_on, _settings_client.network.max_clients);
 	IConsolePrint(CC_DEFAULT, "Current/maximum companies:  {:3d}/{:3d}", Company::GetNumItems(), _settings_client.network.max_companies);
 	IConsolePrint(CC_DEFAULT, "Current/maximum spectators: {:3d}/{:3d}", NetworkSpectatorCount(), _settings_client.network.max_spectators);
diff --git a/src/lang/english.txt b/src/lang/english.txt
index 2e84d4b3a..813705958 100644
--- a/src/lang/english.txt
+++ b/src/lang/english.txt
@@ -2045,12 +2045,12 @@ STR_NETWORK_SERVER_LIST_SEARCH_SERVER_INTERNET_TOOLTIP          :{BLACK}Search i
 STR_NETWORK_SERVER_LIST_SEARCH_SERVER_LAN                       :{BLACK}Search LAN
 STR_NETWORK_SERVER_LIST_SEARCH_SERVER_LAN_TOOLTIP               :{BLACK}Search local area network for servers
 STR_NETWORK_SERVER_LIST_ADD_SERVER                              :{BLACK}Add server
-STR_NETWORK_SERVER_LIST_ADD_SERVER_TOOLTIP                      :{BLACK}Adds a server to the list which will always be checked for running games
+STR_NETWORK_SERVER_LIST_ADD_SERVER_TOOLTIP                      :{BLACK}Adds a server to the list. This can either be a server address or an invite code
 STR_NETWORK_SERVER_LIST_START_SERVER                            :{BLACK}Start server
 STR_NETWORK_SERVER_LIST_START_SERVER_TOOLTIP                    :{BLACK}Start your own server
 
 STR_NETWORK_SERVER_LIST_PLAYER_NAME_OSKTITLE                    :{BLACK}Enter your name
-STR_NETWORK_SERVER_LIST_ENTER_IP                                :{BLACK}Enter the address of the host
+STR_NETWORK_SERVER_LIST_ENTER_SERVER_ADDRESS                    :{BLACK}Enter server address or invite code
 
 # Start new multiplayer server
 STR_NETWORK_START_SERVER_CAPTION                                :{WHITE}Start new multiplayer game
@@ -2136,6 +2136,8 @@ STR_NETWORK_CLIENT_LIST_SERVER_NAME_EDIT_TOOLTIP                :{BLACK}Edit the
 STR_NETWORK_CLIENT_LIST_SERVER_NAME_QUERY_CAPTION               :Name of the server
 STR_NETWORK_CLIENT_LIST_SERVER_VISIBILITY                       :{BLACK}Visibility
 STR_NETWORK_CLIENT_LIST_SERVER_VISIBILITY_TOOLTIP               :{BLACK}Whether other people can see your server in the public listing
+STR_NETWORK_CLIENT_LIST_SERVER_INVITE_CODE                      :{BLACK}Invite code
+STR_NETWORK_CLIENT_LIST_SERVER_INVITE_CODE_TOOLTIP              :{BLACK}Invite code other players can use to join this server
 STR_NETWORK_CLIENT_LIST_SERVER_CONNECTION_TYPE                  :{BLACK}Connection type
 STR_NETWORK_CLIENT_LIST_SERVER_CONNECTION_TYPE_TOOLTIP          :{BLACK}Whether and how your server can be reached by others
 STR_NETWORK_CLIENT_LIST_PLAYER                                  :{BLACK}Player
diff --git a/src/network/core/config.h b/src/network/core/config.h
index d37210e19..06df93140 100644
--- a/src/network/core/config.h
+++ b/src/network/core/config.h
@@ -47,7 +47,7 @@ static const uint16 COMPAT_MTU                      = 1460;           ///< Numbe
 static const byte NETWORK_GAME_ADMIN_VERSION        =    1;           ///< What version of the admin network do we use?
 static const byte NETWORK_GAME_INFO_VERSION         =    4;           ///< What version of game-info do we use?
 static const byte NETWORK_COMPANY_INFO_VERSION      =    6;           ///< What version of company info is this?
-static const byte NETWORK_COORDINATOR_VERSION       =    1;           ///< What version of game-coordinator-protocol do we use?
+static const byte NETWORK_COORDINATOR_VERSION       =    2;           ///< What version of game-coordinator-protocol do we use?
 
 static const uint NETWORK_NAME_LENGTH               =   80;           ///< The maximum length of the server name and map name, in bytes including '\0'
 static const uint NETWORK_COMPANY_NAME_LENGTH       =  128;           ///< The maximum length of the company name, in bytes including '\0'
@@ -67,7 +67,10 @@ static const uint NETWORK_CONTENT_VERSION_LENGTH    =   16;           ///< The m
 static const uint NETWORK_CONTENT_URL_LENGTH        =   96;           ///< The maximum length of a content's url, in bytes including '\0'.
 static const uint NETWORK_CONTENT_DESC_LENGTH       =  512;           ///< The maximum length of a content's description, in bytes including '\0'.
 static const uint NETWORK_CONTENT_TAG_LENGTH        =   32;           ///< The maximum length of a content's tag, in bytes including '\0'.
-static const uint NETWORK_ERROR_DETAIL_LENGTH        = 100;           ///< The maximum length of the error detail, in bytes including '\0'
+static const uint NETWORK_ERROR_DETAIL_LENGTH       =  100;           ///< The maximum length of the error detail, in bytes including '\0'.
+static const uint NETWORK_INVITE_CODE_LENGTH        =   64;           ///< The maximum length of the invite code, in bytes including '\0'.
+static const uint NETWORK_INVITE_CODE_SECRET_LENGTH =   80;           ///< The maximum length of the invite code secret, in bytes including '\0'.
+static const uint NETWORK_TOKEN_LENGTH              =   64;           ///< The maximum length of a token, in bytes including '\0'.
 
 static const uint NETWORK_GRF_NAME_LENGTH           =   80;           ///< Maximum length of the name of a GRF
 
diff --git a/src/network/core/game_info.cpp b/src/network/core/game_info.cpp
index b4c487fba..17f00c5f5 100644
--- a/src/network/core/game_info.cpp
+++ b/src/network/core/game_info.cpp
@@ -141,7 +141,11 @@ void FillStaticNetworkServerGameInfo()
  */
 const NetworkServerGameInfo *GetCurrentNetworkServerGameInfo()
 {
-	/* Client_on is used as global variable to keep track on the number of clients. */
+	/* These variables are updated inside _network_game_info as if they are global variables:
+	 *  - clients_on
+	 *  - invite_code
+	 * These don't need to be updated manually here.
+	 */
 	_network_game_info.companies_on  = (byte)Company::GetNumItems();
 	_network_game_info.spectators_on = NetworkSpectatorCount();
 	_network_game_info.game_date     = _date;
diff --git a/src/network/core/tcp_connect.cpp b/src/network/core/tcp_connect.cpp
index d9b6bb781..0e8e2e125 100644
--- a/src/network/core/tcp_connect.cpp
+++ b/src/network/core/tcp_connect.cpp
@@ -13,6 +13,7 @@
 #include "../../thread.h"
 
 #include "tcp.h"
+#include "../network_coordinator.h"
 #include "../network_internal.h"
 
 #include <deque>
@@ -48,8 +49,7 @@ TCPServerConnecter::TCPServerConnecter(const std::string &connection_string, uin
 
 		case SERVER_ADDRESS_INVITE_CODE:
 			this->status = Status::CONNECTING;
-
-			// TODO -- The next commit will add this functionality.
+			_network_coordinator_client.ConnectToServer(this->server_address.connection_string, this);
 			break;
 
 		default:
diff --git a/src/network/core/tcp_coordinator.cpp b/src/network/core/tcp_coordinator.cpp
index bbcb59b14..dfd73147e 100644
--- a/src/network/core/tcp_coordinator.cpp
+++ b/src/network/core/tcp_coordinator.cpp
@@ -27,12 +27,18 @@ bool NetworkCoordinatorSocketHandler::HandlePacket(Packet *p)
 	PacketCoordinatorType type = (PacketCoordinatorType)p->Recv_uint8();
 
 	switch (type) {
-		case PACKET_COORDINATOR_GC_ERROR:        return this->Receive_GC_ERROR(p);
-		case PACKET_COORDINATOR_SERVER_REGISTER: return this->Receive_SERVER_REGISTER(p);
-		case PACKET_COORDINATOR_GC_REGISTER_ACK: return this->Receive_GC_REGISTER_ACK(p);
-		case PACKET_COORDINATOR_SERVER_UPDATE:   return this->Receive_SERVER_UPDATE(p);
-		case PACKET_COORDINATOR_CLIENT_LISTING:  return this->Receive_CLIENT_LISTING(p);
-		case PACKET_COORDINATOR_GC_LISTING:      return this->Receive_GC_LISTING(p);
+		case PACKET_COORDINATOR_GC_ERROR:              return this->Receive_GC_ERROR(p);
+		case PACKET_COORDINATOR_SERVER_REGISTER:       return this->Receive_SERVER_REGISTER(p);
+		case PACKET_COORDINATOR_GC_REGISTER_ACK:       return this->Receive_GC_REGISTER_ACK(p);
+		case PACKET_COORDINATOR_SERVER_UPDATE:         return this->Receive_SERVER_UPDATE(p);
+		case PACKET_COORDINATOR_CLIENT_LISTING:        return this->Receive_CLIENT_LISTING(p);
+		case PACKET_COORDINATOR_GC_LISTING:            return this->Receive_GC_LISTING(p);
+		case PACKET_COORDINATOR_CLIENT_CONNECT:        return this->Receive_CLIENT_CONNECT(p);
+		case PACKET_COORDINATOR_GC_CONNECTING:         return this->Receive_GC_CONNECTING(p);
+		case PACKET_COORDINATOR_SERCLI_CONNECT_FAILED: return this->Receive_SERCLI_CONNECT_FAILED(p);
+		case PACKET_COORDINATOR_GC_CONNECT_FAILED:     return this->Receive_GC_CONNECT_FAILED(p);
+		case PACKET_COORDINATOR_CLIENT_CONNECTED:      return this->Receive_CLIENT_CONNECTED(p);
+		case PACKET_COORDINATOR_GC_DIRECT_CONNECT:     return this->Receive_GC_DIRECT_CONNECT(p);
 
 		default:
 			Debug(net, 0, "[tcp/coordinator] Received invalid packet type {}", type);
@@ -82,3 +88,9 @@ bool NetworkCoordinatorSocketHandler::Receive_GC_REGISTER_ACK(Packet *p) { retur
 bool NetworkCoordinatorSocketHandler::Receive_SERVER_UPDATE(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_SERVER_UPDATE); }
 bool NetworkCoordinatorSocketHandler::Receive_CLIENT_LISTING(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_CLIENT_LISTING); }
 bool NetworkCoordinatorSocketHandler::Receive_GC_LISTING(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_GC_LISTING); }
+bool NetworkCoordinatorSocketHandler::Receive_CLIENT_CONNECT(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_CLIENT_CONNECT); }
+bool NetworkCoordinatorSocketHandler::Receive_GC_CONNECTING(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_GC_CONNECTING); }
+bool NetworkCoordinatorSocketHandler::Receive_SERCLI_CONNECT_FAILED(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_SERCLI_CONNECT_FAILED); }
+bool NetworkCoordinatorSocketHandler::Receive_GC_CONNECT_FAILED(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_GC_CONNECT_FAILED); }
+bool NetworkCoordinatorSocketHandler::Receive_CLIENT_CONNECTED(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_CLIENT_CONNECTED); }
+bool NetworkCoordinatorSocketHandler::Receive_GC_DIRECT_CONNECT(Packet *p) { return this->ReceiveInvalidPacket(PACKET_COORDINATOR_GC_DIRECT_CONNECT); }
diff --git a/src/network/core/tcp_coordinator.h b/src/network/core/tcp_coordinator.h
index e95916816..2d793b1b6 100644
--- a/src/network/core/tcp_coordinator.h
+++ b/src/network/core/tcp_coordinator.h
@@ -23,15 +23,22 @@
  * GC     -> packets from Game Coordinator to either Client or Server.
  * SERVER -> packets from Server to Game Coordinator.
  * CLIENT -> packets from Client to Game Coordinator.
+ * SERCLI -> packets from either the Server or Client to Game Coordinator.
  **/
 enum PacketCoordinatorType {
-	PACKET_COORDINATOR_GC_ERROR,        ///< Game Coordinator indicates there was an error.
-	PACKET_COORDINATOR_SERVER_REGISTER, ///< Server registration.
-	PACKET_COORDINATOR_GC_REGISTER_ACK, ///< Game Coordinator accepts the registration.
-	PACKET_COORDINATOR_SERVER_UPDATE,   ///< Server sends an set intervals an update of the server.
-	PACKET_COORDINATOR_CLIENT_LISTING,  ///< Client is requesting a listing of all public servers.
-	PACKET_COORDINATOR_GC_LISTING,      ///< Game Coordinator returns a listing of all public servers.
-	PACKET_COORDINATOR_END,             ///< Must ALWAYS be on the end of this list!! (period).
+	PACKET_COORDINATOR_GC_ERROR,              ///< Game Coordinator indicates there was an error.
+	PACKET_COORDINATOR_SERVER_REGISTER,       ///< Server registration.
+	PACKET_COORDINATOR_GC_REGISTER_ACK,       ///< Game Coordinator accepts the registration.
+	PACKET_COORDINATOR_SERVER_UPDATE,         ///< Server sends an set intervals an update of the server.
+	PACKET_COORDINATOR_CLIENT_LISTING,        ///< Client is requesting a listing of all public servers.
+	PACKET_COORDINATOR_GC_LISTING,            ///< Game Coordinator returns a listing of all public servers.
+	PACKET_COORDINATOR_CLIENT_CONNECT,        ///< Client wants to connect to a server based on an invite code.
+	PACKET_COORDINATOR_GC_CONNECTING,         ///< Game Coordinator informs the client of the token assigned to the connection attempt.
+	PACKET_COORDINATOR_SERCLI_CONNECT_FAILED, ///< Client/server tells the Game Coordinator the current connection attempt failed.
+	PACKET_COORDINATOR_GC_CONNECT_FAILED,     ///< Game Coordinator informs client/server it has given up on the connection attempt.
+	PACKET_COORDINATOR_CLIENT_CONNECTED,      ///< Client informs the Game Coordinator the connection with the server is established.
+	PACKET_COORDINATOR_GC_DIRECT_CONNECT,     ///< Game Coordinator tells client to directly connect to the hostname:port of the server.
+	PACKET_COORDINATOR_END,                   ///< Must ALWAYS be on the end of this list!! (period)
 };
 
 /**
@@ -49,6 +56,7 @@ enum ConnectionType {
 enum NetworkCoordinatorErrorType {
 	NETWORK_COORDINATOR_ERROR_UNKNOWN,             ///< There was an unknown error.
 	NETWORK_COORDINATOR_ERROR_REGISTRATION_FAILED, ///< Your request for registration failed.
+	NETWORK_COORDINATOR_ERROR_INVALID_INVITE_CODE, ///< The invite code given is invalid.
 };
 
 /** Base socket handler for all Game Coordinator TCP sockets. */
@@ -76,6 +84,8 @@ protected:
 	 *  uint8   Game Coordinator protocol version.
 	 *  uint8   Type of game (see ServerGameType).
 	 *  uint16  Local port of the server.
+	 *  string  Invite code the server wants to use (can be empty; coordinator will assign a new invite code).
+	 *  string  Secret that belongs to the invite code (empty if invite code is empty).
 	 *
 	 * @param p The packet that was just received.
 	 * @return True upon success, otherwise false.
@@ -85,6 +95,8 @@ protected:
 	/**
 	 * Game Coordinator acknowledges the registration.
 	 *
+	 *  string  Invite code that can be used to join this server.
+	 *  string  Secret that belongs to the invite code (only needed if reusing the invite code on next SERVER_REGISTER).
 	 *  uint8   Type of connection was detected (see ConnectionType).
 	 *
 	 * @param p The packet that was just received.
@@ -130,6 +142,79 @@ protected:
 	 */
 	virtual bool Receive_GC_LISTING(Packet *p);
 
+	/**
+	 * Client wants to connect to a Server.
+	 *
+	 *  uint8   Game Coordinator protocol version.
+	 *  string  Invite code of the Server to join.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_CLIENT_CONNECT(Packet *p);
+
+	/**
+	 * Game Coordinator informs the Client under what token it will start the
+	 * attempt to connect the Server and Client together.
+	 *
+	 *  string  Token to track the current connect request.
+	 *  string  Invite code of the Server to join.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_GC_CONNECTING(Packet *p);
+
+	/**
+	 * Client or Server failed to connect to the remote side.
+	 *
+	 *  uint8   Game Coordinator protocol version.
+	 *  string  Token to track the current connect request.
+	 *  uint8   Tracking number to track current connect request.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_SERCLI_CONNECT_FAILED(Packet *p);
+
+	/**
+	 * Game Coordinator informs the Client that it failed to find a way to
+	 * connect the Client to the Server. Any open connections for this token
+	 * should be closed now.
+	 *
+	 *  string  Token to track the current connect request.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_GC_CONNECT_FAILED(Packet *p);
+
+	/**
+	 * Client informs the Game Coordinator the connection with the Server is
+	 * established. The Client will disconnect from the Game Coordinator next.
+	 *
+	 *  uint8   Game Coordinator protocol version.
+	 *  string  Token to track the current connect request.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_CLIENT_CONNECTED(Packet *p);
+
+	/**
+	 * Game Coordinator requests that the Client makes a direct connection to
+	 * the indicated peer, which is a Server.
+	 *
+	 *  string  Token to track the current connect request.
+	 *  uint8   Tracking number to track current connect request.
+	 *  string  Hostname of the peer.
+	 *  uint16  Port of the peer.
+	 *
+	 * @param p The packet that was just received.
+	 * @return True upon success, otherwise false.
+	 */
+	virtual bool Receive_GC_DIRECT_CONNECT(Packet *p);
+
 	bool HandlePacket(Packet *p);
 public:
 	/**
diff --git a/src/network/network.cpp b/src/network/network.cpp
index 3a33e5096..f8138bbbc 100644
--- a/src/network/network.cpp
+++ b/src/network/network.cpp
@@ -589,9 +589,13 @@ void NetworkClose(bool close_admins)
 		ServerNetworkAdminSocketHandler::CloseListeners();
 
 		_network_coordinator_client.CloseConnection();
-	} else if (MyClient::my_client != nullptr) {
-		MyClient::SendQuit();
-		MyClient::my_client->CloseConnection(NETWORK_RECV_STATUS_CLIENT_QUIT);
+	} else {
+		if (MyClient::my_client != nullptr) {
+			MyClient::SendQuit();
+			MyClient::my_client->CloseConnection(NETWORK_RECV_STATUS_CLIENT_QUIT);
+		}
+
+		_network_coordinator_client.CloseAllTokens();
 	}
 
 	TCPConnecter::KillAll();
diff --git a/src/network/network_coordinator.cpp b/src/network/network_coordinator.cpp
index b3d304952..8bd81b6f6 100644
--- a/src/network/network_coordinator.cpp
+++ b/src/network/network_coordinator.cpp
@@ -1,4 +1,3 @@
-
 /*
  * 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.
@@ -27,13 +26,41 @@
 static const auto NETWORK_COORDINATOR_DELAY_BETWEEN_UPDATES = std::chrono::seconds(30); ///< How many time between updates the server sends to the Game Coordinator.
 ClientNetworkCoordinatorSocketHandler _network_coordinator_client; ///< The connection to the Game Coordinator.
 ConnectionType _network_server_connection_type = CONNECTION_TYPE_UNKNOWN; ///< What type of connection the Game Coordinator detected we are on.
+std::string _network_server_invite_code = ""; ///< Our invite code as indicated by the Game Coordinator.
+
+/** Connect to a game server by IP:port. */
+class NetworkDirectConnecter : public TCPConnecter {
+private:
+	std::string token;     ///< Token of this connection.
+	uint8 tracking_number; ///< Tracking number of this connection.
+
+public:
+	/**
+	 * Try to establish a direct (hostname:port based) connection.
+	 * @param hostname The hostname of the server.
+	 * @param port The port of the server.
+	 * @param token The token as given by the Game Coordinator to track this connection attempt.
+	 * @param tracking_number The tracking number as given by the Game Coordinator to track this connection attempt.
+	 */
+	NetworkDirectConnecter(const std::string &hostname, uint16 port, const std::string &token, uint8 tracking_number) : TCPConnecter(hostname, port), token(token), tracking_number(tracking_number) {}
+
+	void OnFailure() override
+	{
+		_network_coordinator_client.ConnectFailure(this->token, this->tracking_number);
+	}
+
+	void OnConnect(SOCKET s) override
+	{
+		_network_coordinator_client.ConnectSuccess(this->token, s);
+	}
+};
 
 /** Connect to the Game Coordinator server. */
 class NetworkCoordinatorConnecter : TCPConnecter {
 public:
 	/**
 	 * Initiate the connecting.
-	 * @param address The address of the Game Coordinator server.
+	 * @param connection_string The address of the Game Coordinator server.
 	 */
 	NetworkCoordinatorConnecter(const std::string &connection_string) : TCPConnecter(connection_string, NETWORK_COORDINATOR_SERVER_PORT) {}
 
@@ -73,6 +100,20 @@ bool ClientNetworkCoordinatorSocketHandler::Receive_GC_ERROR(Packet *p)
 			this->CloseConnection();
 			return false;
 
+		case NETWORK_COORDINATOR_ERROR_INVALID_INVITE_CODE: {
+			/* Find the connecter based on the invite code. */
+			auto connecter_it = this->connecter_pre.find(detail);
+			if (connecter_it == this->connecter_pre.end()) return true;
+			this->connecter_pre.erase(connecter_it);
+
+			/* Mark the server as offline. */
+			NetworkGameList *item = NetworkGameListAddItem(detail);
+			item->online = false;
+
+			UpdateNetworkGameWindow();
+			return true;
+		}
+
 		default:
 			Debug(net, 0, "Invalid error type {} received from Game Coordinator", error);
 			this->CloseConnection();
@@ -85,12 +126,21 @@ bool ClientNetworkCoordinatorSocketHandler::Receive_GC_REGISTER_ACK(Packet *p)
 	/* Schedule sending an update. */
 	this->next_update = std::chrono::steady_clock::now();
 
+	_settings_client.network.server_invite_code = p->Recv_string(NETWORK_INVITE_CODE_LENGTH);
+	_settings_client.network.server_invite_code_secret = p->Recv_string(NETWORK_INVITE_CODE_SECRET_LENGTH);
 	_network_server_connection_type = (ConnectionType)p->Recv_uint8();
 
 	if (_network_server_connection_type == CONNECTION_TYPE_ISOLATED) {
 		ShowErrorMessage(STR_NETWORK_ERROR_COORDINATOR_ISOLATED, STR_NETWORK_ERROR_COORDINATOR_ISOLATED_DETAIL, WL_ERROR);
 	}
 
+	/* Users can change the invite code in the settings, but this has no effect
+	 * on the invite code as assigned by the server. So
+	 * _network_server_invite_code contains the current invite code,
+	 * and _settings_client.network.server_invite_code contains the one we will
+	 * attempt to re-use when registering again. */
+	_network_server_invite_code = _settings_client.network.server_invite_code;
+
 	SetWindowDirty(WC_CLIENT_LIST, 0);
 
 	if (_network_dedicated) {
@@ -107,7 +157,10 @@ bool ClientNetworkCoordinatorSocketHandler::Receive_GC_REGISTER_ACK(Packet *p)
 		Debug(net, 3, "Your server is now registered with the Game Coordinator:");
 		Debug(net, 3, "  Game type:       Public");
 		Debug(net, 3, "  Connection type: {}", connection_type);
+		Debug(net, 3, "  Invite code:     {}", _network_server_invite_code);
 		Debug(net, 3, "----------------------------------------");
+	} else {
+		Debug(net, 3, "Game Coordinator registered our server with invite code '{}'", _network_server_invite_code);
 	}
 
 	return true;
@@ -130,7 +183,7 @@ bool ClientNetworkCoordinatorSocketHandler::Receive_GC_LISTING(Packet *p)
 		NetworkGameInfo ngi = {};
 		DeserializeNetworkGameInfo(p, &ngi);
 
-		/* Now we know the join-key, we can add it to our list. */
+		/* Now we know the connection string, we can add it to our list. */
 		NetworkGameList *item = NetworkGameListAddItem(connection_string);
 
 		/* Clear any existing GRFConfig chain. */
@@ -149,6 +202,58 @@ bool ClientNetworkCoordinatorSocketHandler::Receive_GC_LISTING(Packet *p)
 	return true;
 }
 
+bool ClientNetworkCoordinatorSocketHandler::Receive_GC_CONNECTING(Packet *p)
+{
+	std::string token = p->Recv_string(NETWORK_TOKEN_LENGTH);
+	std::string invite_code = p->Recv_string(NETWORK_INVITE_CODE_LENGTH);
+
+	/* Find the connecter based on the invite code. */
+	auto connecter_it = this->connecter_pre.find(invite_code);
+	if (connecter_it == this->connecter_pre.end()) {
+		this->CloseConnection();
+		return false;
+	}
+
+	/* Now store it based on the token. */
+	this->connecter[token] = connecter_it->second;
+	this->connecter_pre.erase(connecter_it);
+
+	return true;
+}
+
+bool ClientNetworkCoordinatorSocketHandler::Receive_GC_CONNECT_FAILED(Packet *p)
+{
+	std::string token = p->Recv_string(NETWORK_TOKEN_LENGTH);
+
+	auto connecter_it = this->connecter.find(token);
+	if (connecter_it != this->connecter.end()) {
+		connecter_it->second->SetFailure();
+		this->connecter.erase(connecter_it);
+	}
+
+	/* Close all remaining connections. */
+	this->CloseToken(token);
+
+	return true;
+}
+
+bool ClientNetworkCoordinatorSocketHandler::Receive_GC_DIRECT_CONNECT(Packet *p)
+{
+	std::string token = p->Recv_string(NETWORK_TOKEN_LENGTH);
+	uint8 tracking_number = p->Recv_uint8();
+	std::string hostname = p->Recv_string(NETWORK_HOSTNAME_LENGTH);
+	uint16 port = p->Recv_uint16();
+
+	/* Ensure all other pending connection attempts are killed. */
+	if (this->game_connecter != nullptr) {
+		this->game_connecter->Kill();
+		this->game_connecter = nullptr;
+	}
+
+	this->game_connecter = new NetworkDirectConnecter(hostname, port, token, tracking_number);
+	return true;
+}
+
 void ClientNetworkCoordinatorSocketHandler::Connect()
 {
 	/* We are either already connected or are trying to connect. */
@@ -172,13 +277,15 @@ NetworkRecvStatus ClientNetworkCoordinatorSocketHandler::CloseConnection(bool er
 	_network_server_connection_type = CONNECTION_TYPE_UNKNOWN;
 	this->next_update = {};
 
+	this->CloseAllTokens();
+
 	SetWindowDirty(WC_CLIENT_LIST, 0);
 
 	return NETWORK_RECV_STATUS_OKAY;
 }
 
 /**
- * Register our server to receive our join-key.
+ * Register our server to receive our invite code.
  */
 void ClientNetworkCoordinatorSocketHandler::Register()
 {
@@ -193,6 +300,13 @@ void ClientNetworkCoordinatorSocketHandler::Register()
 	p->Send_uint8(NETWORK_COORDINATOR_VERSION);
 	p->Send_uint8(SERVER_GAME_TYPE_PUBLIC);
 	p->Send_uint16(_settings_client.network.server_port);
+	if (_settings_client.network.server_invite_code.empty() || _settings_client.network.server_invite_code_secret.empty()) {
+		p->Send_string("");
+		p->Send_string("");
+	} else {
+		p->Send_string(_settings_client.network.server_invite_code);
+		p->Send_string(_settings_client.network.server_invite_code_secret);
+	}
 
 	this->SendPacket(p);
 }
@@ -230,6 +344,123 @@ void ClientNetworkCoordinatorSocketHandler::GetListing()
 }
 
 /**
+ * Join a server based on an invite code.
+ * @param invite_code The invite code of the server to connect to.
+ * @param connecter The connecter of the request.
+ */
+void ClientNetworkCoordinatorSocketHandler::ConnectToServer(const std::string &invite_code, TCPServerConnecter *connecter)
+{
+	assert(StrStartsWith(invite_code, "+"));
+
+	if (this->connecter_pre.find(invite_code) != this->connecter_pre.end()) {
+		/* If someone is hammering the refresh key, one can sent out two
+		 * requests for the same invite code. There isn't really a great way
+		 * of handling this, so just ignore this request. */
+		connecter->SetFailure();
+		return;
+	}
+
+	/* Initially we store based on invite code; on first reply we know the
+	 * token, and will start using that key instead. */
+	this->connecter_pre[invite_code] = connecter;
+
+	this->Connect();
+
+	Packet *p = new Packet(PACKET_COORDINATOR_CLIENT_CONNECT);
+	p->Send_uint8(NETWORK_COORDINATOR_VERSION);
+	p->Send_string(invite_code);
+
+	this->SendPacket(p);
+}
+
+/**
+ * Callback from a Connecter to let the Game Coordinator know the connection failed.
+ * @param token Token of the connecter that failed.
+ * @param tracking_number Tracking number of the connecter that failed.
+ */
+void ClientNetworkCoordinatorSocketHandler::ConnectFailure(const std::string &token, uint8 tracking_number)
+{
+	/* Connecter will destroy itself. */
+	this->game_connecter = nullptr;
+
+	Packet *p = new Packet(PACKET_COORDINATOR_SERCLI_CONNECT_FAILED);
+	p->Send_uint8(NETWORK_COORDINATOR_VERSION);
+	p->Send_string(token);
+	p->Send_uint8(tracking_number);
+
+	this->SendPacket(p);
+
+	auto connecter_it = this->connecter.find(token);
+	assert(connecter_it != this->connecter.end());
+
+	connecter_it->second->SetFailure();
+	this->connecter.erase(connecter_it);
+}
+
+/**
+ * Callback from a Connecter to let the Game Coordinator know the connection
+ * to the game server is established.
+ * @param token Token of the connecter that succeeded.
+ * @param sock The socket that the connecter can now use.
+ */
+void ClientNetworkCoordinatorSocketHandler::ConnectSuccess(const std::string &token, SOCKET sock)
+{
+	/* Connecter will destroy itself. */
+	this->game_connecter = nullptr;
+
+	assert(!_network_server);
+
+	Packet *p = new Packet(PACKET_COORDINATOR_CLIENT_CONNECTED);
+	p->Send_uint8(NETWORK_COORDINATOR_VERSION);
+	p->Send_string(token);
+	this->SendPacket(p);
+
+	auto connecter_it = this->connecter.find(token);
+	assert(connecter_it != this->connecter.end());
+
+	connecter_it->second->SetConnected(sock);
+	this->connecter.erase(connecter_it);
+
+	/* Close all remaining connections. */
+	this->CloseToken(token);
+}
+
+/**
+ * Close everything related to this connection token.
+ * @param token The connection token to close.
+ */
+void ClientNetworkCoordinatorSocketHandler::CloseToken(const std::string &token)
+{
+	/* Ensure all other pending connection attempts are also killed. */
+	if (this->game_connecter != nullptr) {
+		this->game_connecter->Kill();
+		this->game_connecter = nullptr;
+	}
+}
+
+/**
+ * Close all pending connection tokens.
+ */
+void ClientNetworkCoordinatorSocketHandler::CloseAllTokens()
+{
+	/* Ensure all other pending connection attempts are also killed. */
+	if (this->game_connecter != nullptr) {
+		this->game_connecter->Kill();
+		this->game_connecter = nullptr;
+	}
+
+	/* Mark any pending connecters as failed. */
+	for (auto &[token, it] : this->connecter) {
+		it->SetFailure();
+	}
+	for (auto &[invite_code, it] : this->connecter_pre) {
+		it->SetFailure();
+	}
+	this->connecter.clear();
+	this->connecter_pre.clear();
+}
+
+/**
  * Check whether we received/can send some data from/to the Game Coordinator server and
  * when that's the case handle it appropriately.
  */
diff --git a/src/network/network_coordinator.h b/src/network/network_coordinator.h
index 7399ce1fc..f846958bf 100644
--- a/src/network/network_coordinator.h
+++ b/src/network/network_coordinator.h
@@ -1,4 +1,3 @@
-
 /*
  * 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.
@@ -12,6 +11,7 @@
 #define NETWORK_COORDINATOR_H
 
 #include "core/tcp_coordinator.h"
+#include <map>
 
 /**
  * Game Coordinator communication.
@@ -25,17 +25,32 @@
  * For clients (listing):
  *  - Client sends CLIENT_LISTING.
  *  - Game Coordinator returns the full list of public servers via GC_LISTING (multiple packets).
+ *
+ * For clients (connecting):
+ *  - Client sends CLIENT_CONNECT.
+ *  - Game Coordinator checks what type of connections the servers supports:
+ *    1) Direct connect?
+ *        - Send the client a GC_CONNECT with the peer address.
+ *        - a) Client connects, client sends CLIENT_CONNECTED to Game Coordinator.
+ *        - b) Client connect fails, client sends CLIENT_CONNECT_FAILED to Game Coordinator.
+ *  - If all fails, Game Coordinator sends GC_CONNECT_FAILED to indicate no connection is possible.
  */
 
 /** Class for handling the client side of the Game Coordinator connection. */
 class ClientNetworkCoordinatorSocketHandler : public NetworkCoordinatorSocketHandler {
 private:
 	std::chrono::steady_clock::time_point next_update; ///< When to send the next update (if server and public).
+	std::map<std::string, TCPServerConnecter *> connecter; ///< Based on tokens, the current connecters that are pending.
+	std::map<std::string, TCPServerConnecter *> connecter_pre; ///< Based on invite codes, the current connecters that are pending.
+	TCPConnecter *game_connecter = nullptr; ///< Pending connecter to the game server.
 
 protected:
 	bool Receive_GC_ERROR(Packet *p) override;
 	bool Receive_GC_REGISTER_ACK(Packet *p) override;
 	bool Receive_GC_LISTING(Packet *p) override;
+	bool Receive_GC_CONNECTING(Packet *p) override;
+	bool Receive_GC_CONNECT_FAILED(Packet *p) override;
+	bool Receive_GC_DIRECT_CONNECT(Packet *p) override;
 
 public:
 	/** The idle timeout; when to close the connection because it's idle. */
@@ -49,11 +64,18 @@ public:
 	NetworkRecvStatus CloseConnection(bool error = true) override;
 	void SendReceive();
 
+	void ConnectFailure(const std::string &token, uint8 tracking_number);
+	void ConnectSuccess(const std::string &token, SOCKET sock);
+
 	void Connect();
+	void CloseToken(const std::string &token);
+	void CloseAllTokens();
 
 	void Register();
 	void SendServerUpdate();
 	void GetListing();
+
+	void ConnectToServer(const std::string &invite_code, TCPServerConnecter *connecter);
 };
 
 extern ClientNetworkCoordinatorSocketHandler _network_coordinator_client;
diff --git a/src/network/network_gui.cpp b/src/network/network_gui.cpp
index 24884ed16..a0d2d47c8 100644
--- a/src/network/network_gui.cpp
+++ b/src/network/network_gui.cpp
@@ -751,7 +751,7 @@ public:
 				SetDParamStr(0, _settings_client.network.connect_to_ip);
 				ShowQueryString(
 					STR_JUST_RAW_STRING,
-					STR_NETWORK_SERVER_LIST_ENTER_IP,
+					STR_NETWORK_SERVER_LIST_ENTER_SERVER_ADDRESS,
 					NETWORK_HOSTNAME_PORT_LENGTH,  // maximum number of characters including '\0'
 					this, CS_ALPHANUMERAL, QSF_ACCEPT_UNCHANGED);
 				break;
@@ -1633,6 +1633,11 @@ static const NWidgetPart _nested_client_list_widgets[] = {
 					NWidget(WWT_DROPDOWN, COLOUR_GREY, WID_CL_SERVER_VISIBILITY), SetDataTip(STR_BLACK_STRING, STR_NETWORK_CLIENT_LIST_SERVER_VISIBILITY_TOOLTIP),
 				EndContainer(),
 				NWidget(NWID_HORIZONTAL), SetPIP(0, 3, 0),
+					NWidget(WWT_TEXT, COLOUR_GREY), SetMinimalTextLines(1, 0), SetDataTip(STR_NETWORK_CLIENT_LIST_SERVER_INVITE_CODE, STR_NULL),
+					NWidget(NWID_SPACER), SetMinimalSize(10, 0),
+					NWidget(WWT_TEXT, COLOUR_GREY, WID_CL_SERVER_INVITE_CODE), SetFill(1, 0), SetMinimalTextLines(1, 0), SetResize(1, 0), SetDataTip(STR_BLACK_RAW_STRING, STR_NETWORK_CLIENT_LIST_SERVER_INVITE_CODE_TOOLTIP), SetAlignment(SA_VERT_CENTER | SA_RIGHT),
+				EndContainer(),
+				NWidget(NWID_HORIZONTAL), SetPIP(0, 3, 0),
 					NWidget(WWT_TEXT, COLOUR_GREY), SetMinimalTextLines(1, 0), SetDataTip(STR_NETWORK_CLIENT_LIST_SERVER_CONNECTION_TYPE, STR_NULL),
 					NWidget(NWID_SPACER), SetMinimalSize(10, 0),
 					NWidget(WWT_TEXT, COLOUR_GREY, WID_CL_SERVER_CONNECTION_TYPE), SetFill(1, 0), SetMinimalTextLines(1, 0), SetResize(1, 0), SetDataTip(STR_BLACK_STRING, STR_NETWORK_CLIENT_LIST_SERVER_CONNECTION_TYPE_TOOLTIP), SetAlignment(SA_VERT_CENTER | SA_RIGHT),
@@ -2071,6 +2076,12 @@ public:
 				SetDParam(0, _server_visibility_dropdown[_settings_client.network.server_advertise]);
 				break;
 
+			case WID_CL_SERVER_INVITE_CODE: {
+				static std::string empty = {};
+				SetDParamStr(0, _network_server_connection_type == CONNECTION_TYPE_UNKNOWN ? empty : _network_server_invite_code);
+				break;
+			}
+
 			case WID_CL_SERVER_CONNECTION_TYPE:
 				SetDParam(0, STR_NETWORK_CLIENT_LIST_SERVER_CONNECTION_TYPE_UNKNOWN + _network_server_connection_type);
 				break;
diff --git a/src/network/network_internal.h b/src/network/network_internal.h
index 95226286c..2c6639961 100644
--- a/src/network/network_internal.h
+++ b/src/network/network_internal.h
@@ -84,6 +84,7 @@ extern uint8 _network_join_waiting;
 extern uint32 _network_join_bytes;
 extern uint32 _network_join_bytes_total;
 extern ConnectionType _network_server_connection_type;
+extern std::string _network_server_invite_code;
 
 extern uint8 _network_reconnect;
 
diff --git a/src/settings_type.h b/src/settings_type.h
index 3377bdede..79e462c3e 100644
--- a/src/settings_type.h
+++ b/src/settings_type.h
@@ -266,6 +266,8 @@ struct NetworkSettings {
 	uint16      server_port;                              ///< port the server listens on
 	uint16      server_admin_port;                        ///< port the server listens on for the admin network
 	bool        server_admin_chat;                        ///< allow private chat for the server to be distributed to the admin network
+	std::string server_invite_code;                       ///< Invite code to use when registering as server.
+	std::string server_invite_code_secret;                ///< Secret to proof we got this invite code from the Game Coordinator.
 	std::string server_name;                              ///< name of the server
 	std::string server_password;                          ///< password for joining this server
 	std::string rcon_password;                            ///< password for rconsole (server side)
diff --git a/src/table/settings/network_secrets_settings.ini b/src/table/settings/network_secrets_settings.ini
index fced9240e..4613636a8 100644
--- a/src/table/settings/network_secrets_settings.ini
+++ b/src/table/settings/network_secrets_settings.ini
@@ -74,3 +74,17 @@ type     = SLE_STR
 length   = NETWORK_SERVER_ID_LENGTH
 flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC | SF_NETWORK_ONLY
 def      = nullptr
+
+[SDTC_SSTR]
+var      = network.server_invite_code
+type     = SLE_STR
+length   = NETWORK_INVITE_CODE_LENGTH
+flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC | SF_NETWORK_ONLY
+def      = nullptr
+
+[SDTC_SSTR]
+var      = network.server_invite_code_secret
+type     = SLE_STR
+length   = NETWORK_INVITE_CODE_SECRET_LENGTH
+flags    = SF_NOT_IN_SAVE | SF_NO_NETWORK_SYNC | SF_NETWORK_ONLY
+def      = nullptr
diff --git a/src/widgets/network_widget.h b/src/widgets/network_widget.h
index a665afdb7..c8ec22e86 100644
--- a/src/widgets/network_widget.h
+++ b/src/widgets/network_widget.h
@@ -101,6 +101,7 @@ enum ClientListWidgets {
 	WID_CL_SERVER_NAME,                ///< Server name.
 	WID_CL_SERVER_NAME_EDIT,           ///< Edit button for server name.
 	WID_CL_SERVER_VISIBILITY,          ///< Server visibility.
+	WID_CL_SERVER_INVITE_CODE,         ///< Invite code for this server.
 	WID_CL_SERVER_CONNECTION_TYPE,     ///< The type of connection the Game Coordinator detected for this server.
 	WID_CL_CLIENT_NAME,                ///< Client name.
 	WID_CL_CLIENT_NAME_EDIT,           ///< Edit button for client name.