Implement Velocity native IP forwarding for 1.13+

No other server implements this (as of yet). We plan to get support for
this into at least Paper and Sponge.

src/main/java/com/velocitypowered/proxy/connection/VelocityConstants.java

@@ -0,0 +1,9 @@
+package com.velocitypowered.proxy.connection;
+
+public class VelocityConstants {
+    private VelocityConstants() {
+        throw new AssertionError();
+    }
+
+    public static final String VELOCITY_IP_FORWARDING_CHANNEL = "velocity:player_info";
+}

src/main/java/com/velocitypowered/proxy/connection/backend/LoginSessionHandler.java

@@ -3,14 +3,12 @@ package com.velocitypowered.proxy.connection.backend;
 import com.velocitypowered.proxy.connection.client.ClientPlaySessionHandler;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.StateRegistry;
-import com.velocitypowered.proxy.protocol.packets.Disconnect;
-import com.velocitypowered.proxy.protocol.packets.EncryptionRequest;
-import com.velocitypowered.proxy.protocol.packets.ServerLoginSuccess;
+import com.velocitypowered.proxy.protocol.packets.*;
 import com.velocitypowered.proxy.connection.MinecraftSessionHandler;
-import com.velocitypowered.proxy.protocol.packets.SetCompression;
 
 public class LoginSessionHandler implements MinecraftSessionHandler {
     private final ServerConnection connection;
+    private int forwardingPacketId = -1;
 
     public LoginSessionHandler(ServerConnection connection) {
         this.connection = connection;
@@ -20,20 +18,21 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     public void handle(MinecraftPacket packet) {
         if (packet instanceof EncryptionRequest) {
             throw new IllegalStateException("Backend server is online-mode!");
-        }
-
-        if (packet instanceof Disconnect) {
+        } else if (packet instanceof LoginPluginResponse) {
+            LoginPluginResponse lpr = (LoginPluginResponse) packet;
+            if (lpr.getId() == forwardingPacketId) {
+                if (!lpr.isSuccess()) {
+                    throw new IllegalStateException("Unable to forward player information to server! Is it configured properly?");
+                }
+            }
+        } else if (packet instanceof Disconnect) {
             Disconnect disconnect = (Disconnect) packet;
             connection.disconnect();
             connection.getProxyPlayer().handleConnectionException(connection.getServerInfo(), disconnect);
-        }
-
-        if (packet instanceof SetCompression) {
+        } else if (packet instanceof SetCompression) {
             SetCompression sc = (SetCompression) packet;
             connection.getChannel().setCompressionThreshold(sc.getThreshold());
-        }
-
-        if (packet instanceof ServerLoginSuccess) {
+        } else if (packet instanceof ServerLoginSuccess) {
             // The player has been logged on to the backend server.
             connection.getChannel().setState(StateRegistry.PLAY);
             ServerConnection existingConnection = connection.getProxyPlayer().getConnectedServer();
@@ -53,4 +52,12 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     public void exception(Throwable throwable) {
         connection.getProxyPlayer().handleConnectionException(connection.getServerInfo(), throwable);
     }
+
+    public int getForwardingPacketId() {
+        return forwardingPacketId;
+    }
+
+    public void setForwardingPacketId(int forwardingPacketId) {
+        this.forwardingPacketId = forwardingPacketId;
+    }
 }

src/main/java/com/velocitypowered/proxy/connection/backend/ServerConnection.java

@@ -2,23 +2,28 @@ package com.velocitypowered.proxy.connection.backend;
 
 import com.velocitypowered.proxy.config.IPForwardingMode;
 import com.velocitypowered.proxy.connection.MinecraftConnectionAssociation;
+import com.velocitypowered.proxy.connection.VelocityConstants;
+import com.velocitypowered.proxy.data.GameProfile;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import com.velocitypowered.proxy.protocol.netty.MinecraftDecoder;
 import com.velocitypowered.proxy.protocol.netty.MinecraftEncoder;
 import com.velocitypowered.proxy.protocol.netty.MinecraftVarintFrameDecoder;
 import com.velocitypowered.proxy.protocol.netty.MinecraftVarintLengthEncoder;
 import com.velocitypowered.proxy.protocol.packets.Handshake;
+import com.velocitypowered.proxy.protocol.packets.LoginPluginMessage;
 import com.velocitypowered.proxy.protocol.packets.ServerLogin;
 import com.velocitypowered.proxy.connection.MinecraftConnection;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.data.ServerInfo;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
 import io.netty.channel.*;
 import io.netty.handler.timeout.ReadTimeoutHandler;
 
-import java.util.List;
-import java.util.UUID;
+import java.util.concurrent.ThreadLocalRandom;
 import java.util.concurrent.TimeUnit;
 
 import static com.velocitypowered.network.Connections.FRAME_DECODER;
@@ -99,9 +104,29 @@ public class ServerConnection implements MinecraftConnectionAssociation {
         handshake.setPort(serverInfo.getAddress().getPort());
         channel.write(handshake);
 
-        channel.setProtocolVersion(proxyPlayer.getConnection().getProtocolVersion());
+        int protocolVersion = proxyPlayer.getConnection().getProtocolVersion();
+        channel.setProtocolVersion(protocolVersion);
         channel.setState(StateRegistry.LOGIN);
 
+        // 1.13 stuff
+        if (protocolVersion >= ProtocolConstants.MINECRAFT_1_13) {
+            if (VelocityServer.getServer().getConfiguration().getIpForwardingMode() == IPForwardingMode.MODERN) {
+                // Velocity's IP forwarding includes the player's IP address and their game profile.
+                GameProfile profile = proxyPlayer.getProfile();
+                ByteBuf buf = createForwardingData(proxyPlayer.getRemoteAddress().getHostString(), profile);
+
+                // Send the message on
+                LoginPluginMessage forwarding = new LoginPluginMessage();
+                forwarding.setId(ThreadLocalRandom.current().nextInt());
+                forwarding.setChannel(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL);
+                forwarding.setData(buf);
+
+                LoginSessionHandler lsh = (LoginSessionHandler) channel.getSessionHandler();
+                lsh.setForwardingPacketId(forwarding.getId());
+                channel.write(forwarding);
+            }
+        }
+
         // Login
         ServerLogin login = new ServerLogin();
         login.setUsername(proxyPlayer.getUsername());
@@ -129,4 +154,24 @@ public class ServerConnection implements MinecraftConnectionAssociation {
     public String toString() {
         return "[server connection] " + proxyPlayer.getProfile().getName() + " -> " + serverInfo.getName();
     }
+
+    private static ByteBuf createForwardingData(String address, GameProfile profile) {
+        ByteBuf buf = Unpooled.buffer();
+        ProtocolUtils.writeString(buf, address);
+        ProtocolUtils.writeString(buf, profile.getName());
+        ProtocolUtils.writeString(buf, profile.idAsUuid().toString());
+        ProtocolUtils.writeVarInt(buf, profile.getProperties().size());
+        for (GameProfile.Property property : profile.getProperties()) {
+            ProtocolUtils.writeString(buf, property.getName());
+            ProtocolUtils.writeString(buf, property.getValue());
+            String signature = property.getSignature();
+            if (signature != null) {
+                buf.writeBoolean(true);
+                ProtocolUtils.writeString(buf, signature);
+            } else {
+                buf.writeBoolean(false);
+            }
+        }
+        return buf;
+    }
 }

src/main/java/com/velocitypowered/proxy/connection/client/LoginSessionHandler.java

@@ -1,6 +1,7 @@
 package com.velocitypowered.proxy.connection.client;
 
 import com.google.common.base.Preconditions;
+import com.velocitypowered.proxy.connection.VelocityConstants;
 import com.velocitypowered.proxy.data.GameProfile;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.StateRegistry;
@@ -12,6 +13,7 @@ import com.velocitypowered.proxy.connection.backend.ServerConnection;
 import com.velocitypowered.proxy.data.ServerInfo;
 import com.velocitypowered.proxy.util.EncryptionUtils;
 import com.velocitypowered.proxy.util.UuidUtils;
+import io.netty.buffer.Unpooled;
 import net.kyori.text.TextComponent;
 import net.kyori.text.format.TextColor;
 import org.apache.logging.log4j.LogManager;
@@ -27,7 +29,6 @@ import java.util.concurrent.ThreadLocalRandom;
 
 public class LoginSessionHandler implements MinecraftSessionHandler {
     private static final Logger logger = LogManager.getLogger(LoginSessionHandler.class);
-
     private static final String MOJANG_SERVER_AUTH_URL =
             "https://sessionserver.mojang.com/session/minecraft/hasJoined?username=%s&serverId=%s&ip=%s";
 
@@ -41,7 +42,22 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
 
     @Override
     public void handle(MinecraftPacket packet) throws Exception {
-        if (packet instanceof ServerLogin) {
+        if (packet instanceof LoginPluginMessage) {
+            LoginPluginMessage lpm = (LoginPluginMessage) packet;
+            if (lpm.getChannel().equals(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL)) {
+                // Uh oh, someone's trying to run Velocity behind Velocity. We don't want that happening.
+                inbound.closeWith(Disconnect.create(
+                        TextComponent.of("Running Velocity behind Velocity isn't supported.", TextColor.RED)
+                ));
+            } else {
+                // We don't know what this message is.
+                LoginPluginResponse response = new LoginPluginResponse();
+                response.setId(lpm.getId());
+                response.setSuccess(false);
+                response.setData(Unpooled.EMPTY_BUFFER);
+                inbound.write(response);
+            }
+        } else if (packet instanceof ServerLogin) {
             this.login = (ServerLogin) packet;
 
             if (VelocityServer.getServer().getConfiguration().isOnlineMode()) {
@@ -53,9 +69,7 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
                 // Offline-mode, don't try to request encryption.
                 handleSuccessfulLogin(GameProfile.forOfflinePlayer(login.getUsername()));
             }
-        }
-
-        if (packet instanceof EncryptionResponse) {
+        } else if (packet instanceof EncryptionResponse) {
             KeyPair serverKeyPair = VelocityServer.getServer().getServerKeyPair();
             EncryptionResponse response = (EncryptionResponse) packet;
             byte[] decryptedVerifyToken = EncryptionUtils.decryptRsa(serverKeyPair, response.getVerifyToken());

src/main/java/com/velocitypowered/proxy/protocol/StateRegistry.java

@@ -108,6 +108,8 @@ public enum StateRegistry {
                     genericMappings(0x00));
             SERVERBOUND.register(EncryptionResponse.class, EncryptionResponse::new,
                     genericMappings(0x01));
+            SERVERBOUND.register(LoginPluginMessage.class, LoginPluginMessage::new,
+                    map(0x02, MINECRAFT_1_13));
 
             CLIENTBOUND.register(Disconnect.class, Disconnect::new,
                     genericMappings(0x00));
@@ -117,6 +119,8 @@ public enum StateRegistry {
                     genericMappings(0x02));
             CLIENTBOUND.register(SetCompression.class, SetCompression::new,
                     genericMappings(0x03));
+            CLIENTBOUND.register(LoginPluginResponse.class, LoginPluginResponse::new,
+                    map(0x04, MINECRAFT_1_13));
         }
     };
 

src/main/java/com/velocitypowered/proxy/protocol/packets/LoginPluginMessage.java

@@ -0,0 +1,64 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+
+public class LoginPluginMessage implements MinecraftPacket {
+    private int id;
+    private String channel;
+    private ByteBuf data;
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public String getChannel() {
+        return channel;
+    }
+
+    public void setChannel(String channel) {
+        this.channel = channel;
+    }
+
+    public ByteBuf getData() {
+        return data;
+    }
+
+    public void setData(ByteBuf data) {
+        this.data = data;
+    }
+
+    @Override
+    public String toString() {
+        return "LoginPluginMessage{" +
+                "id=" + id +
+                ", channel='" + channel + '\'' +
+                ", data=" + data +
+                '}';
+    }
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        this.id = ProtocolUtils.readVarInt(buf);
+        this.channel = ProtocolUtils.readString(buf);
+        if (buf.isReadable()) {
+            this.data = buf.readRetainedSlice(buf.readableBytes());
+        } else {
+            this.data = Unpooled.EMPTY_BUFFER;
+        }
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        ProtocolUtils.writeVarInt(buf, id);
+        ProtocolUtils.writeString(buf, channel);
+        buf.writeBytes(data);
+    }
+}

src/main/java/com/velocitypowered/proxy/protocol/packets/LoginPluginResponse.java

@@ -0,0 +1,64 @@
+package com.velocitypowered.proxy.protocol.packets;
+
+import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolConstants;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.Unpooled;
+
+public class LoginPluginResponse implements MinecraftPacket {
+    private int id;
+    private boolean success;
+    private ByteBuf data;
+
+    public int getId() {
+        return id;
+    }
+
+    public void setId(int id) {
+        this.id = id;
+    }
+
+    public boolean isSuccess() {
+        return success;
+    }
+
+    public void setSuccess(boolean success) {
+        this.success = success;
+    }
+
+    public ByteBuf getData() {
+        return data;
+    }
+
+    public void setData(ByteBuf data) {
+        this.data = data;
+    }
+
+    @Override
+    public String toString() {
+        return "LoginPluginResponse{" +
+                "id=" + id +
+                ", success=" + success +
+                ", data=" + data +
+                '}';
+    }
+
+    @Override
+    public void decode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        this.id = ProtocolUtils.readVarInt(buf);
+        this.success = buf.readBoolean();
+        if (buf.isReadable()) {
+            this.data = buf.readRetainedSlice(buf.readableBytes());
+        } else {
+            this.data = Unpooled.EMPTY_BUFFER;
+        }
+    }
+
+    @Override
+    public void encode(ByteBuf buf, ProtocolConstants.Direction direction, int protocolVersion) {
+        ProtocolUtils.writeVarInt(buf, id);
+        buf.writeBoolean(success);
+        buf.writeBytes(data);
+    }
+}