Implement bungee-style IP forwarding.

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

@@ -8,6 +8,7 @@ import com.velocitypowered.proxy.data.ServerInfo;
 import com.velocitypowered.proxy.protocol.netty.MinecraftPipelineUtils;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.connection.client.ConnectedPlayer;
+import com.velocitypowered.proxy.util.UuidUtils;
 import io.netty.channel.*;
 
 public class ServerConnection {
@@ -51,12 +52,23 @@ public class ServerConnection {
                 });
     }
 
+    private String createBungeeForwardingAddress() {
+        // BungeeCord IP forwarding is simply a special injection after the "address" in the handshake,
+        // separated by \0 (the null byte). In order, you send the original host, the player's IP, their
+        // UUID (undashed), and if you are in online-mode, their login properties (retrieved from Mojang).
+        //
+        // Velocity doesn't yet support online-mode, unfortunately. That will come soon.
+        return serverInfo.getAddress().getHostString() + "\0" +
+                proxyPlayer.getRemoteAddress().getHostString() + "\0" +
+                UuidUtils.toUndashed(proxyPlayer.getUniqueId());
+    }
+
     private void startHandshake() {
         // Initiate a handshake.
         Handshake handshake = new Handshake();
         handshake.setNextStatus(2); // login
         handshake.setProtocolVersion(proxyPlayer.getConnection().getProtocolVersion());
-        handshake.setServerAddress(serverInfo.getAddress().getHostString());
+        handshake.setServerAddress(createBungeeForwardingAddress());
         handshake.setPort(serverInfo.getAddress().getPort());
         channel.write(handshake);
 

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

@@ -10,6 +10,7 @@ import net.kyori.text.TextComponent;
 import net.kyori.text.format.TextColor;
 import net.kyori.text.serializer.ComponentSerializers;
 
+import java.net.InetSocketAddress;
 import java.util.UUID;
 
 public class ConnectedPlayer {
@@ -36,6 +37,10 @@ public class ConnectedPlayer {
         return connection;
     }
 
+    public InetSocketAddress getRemoteAddress() {
+        return (InetSocketAddress) connection.getChannel().remoteAddress();
+    }
+
     public ServerConnection getConnectedServer() {
         return connectedServer;
     }
@@ -57,9 +62,7 @@ public class ConnectedPlayer {
             // The player isn't yet connected to a server - we should disconnect them.
             connection.closeWith(Disconnect.create(component));
         } else {
-            Chat chat = new Chat();
-            chat.setMessage(ComponentSerializers.JSON.serialize(component));
-            connection.write(chat);
+            connection.write(Chat.create(component));
         }
     }
 

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

@@ -1,14 +1,25 @@
 package com.velocitypowered.proxy.protocol.packets;
 
+import com.google.common.base.Preconditions;
 import com.velocitypowered.proxy.protocol.ProtocolConstants;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import io.netty.buffer.ByteBuf;
+import net.kyori.text.TextComponent;
+import net.kyori.text.serializer.ComponentSerializers;
 
 public class Chat implements MinecraftPacket {
     private String message;
     private byte position;
 
+    public Chat() {
+    }
+
+    public Chat(String message, byte position) {
+        this.message = message;
+        this.position = position;
+    }
+
     public String getMessage() {
         return message;
     }
@@ -48,4 +59,13 @@ public class Chat implements MinecraftPacket {
             buf.writeByte(position);
         }
     }
+
+    public static Chat create(TextComponent component) {
+        return create(component, (byte) 0);
+    }
+
+    public static Chat create(TextComponent component, byte pos) {
+        Preconditions.checkNotNull(component, "component");
+        return new Chat(ComponentSerializers.JSON.serialize(component), pos);
+    }
 }

src/main/java/com/velocitypowered/proxy/util/UuidUtils.java

@@ -18,6 +18,11 @@ public enum UuidUtils {
         );
     }
 
+    public static String toUndashed(final UUID uuid) {
+        Preconditions.checkNotNull(uuid, "uuid");
+        return Long.toUnsignedString(uuid.getMostSignificantBits(), 16) + Long.toUnsignedString(uuid.getLeastSignificantBits(), 16);
+    }
+
     public static UUID generateOfflinePlayerUuid(String username) {
         return UUID.nameUUIDFromBytes(("OfflinePlayer:" + username).getBytes(StandardCharsets.UTF_8));
     }

src/test/java/com/velocitypowered/proxy/util/UuidUtilsTest.java

@@ -14,12 +14,17 @@ class UuidUtilsTest {
     private static final String TEST_OFFLINE_PLAYER = "tuxed";
 
     @Test
-    void testFromUndashed() {
+    void generateOfflinePlayerUuid() {
+        assertEquals(TEST_OFFLINE_PLAYER_UUID, UuidUtils.generateOfflinePlayerUuid(TEST_OFFLINE_PLAYER), "UUIDs do not match");
+    }
+
+    @Test
+    void fromUndashed() {
         assertEquals(EXPECTED_DASHED_UUID, UuidUtils.fromUndashed(ACTUAL_UNDASHED_UUID), "UUIDs do not match");
     }
 
     @Test
-    void generateOfflinePlayerUuid() {
-        assertEquals(TEST_OFFLINE_PLAYER_UUID, UuidUtils.generateOfflinePlayerUuid(TEST_OFFLINE_PLAYER), "UUIDs do not match");
+    void toUndashed() {
+        assertEquals(ACTUAL_UNDASHED_UUID, UuidUtils.toUndashed(EXPECTED_DASHED_UUID), "UUIDs do not match");
     }
 }