Implement legacy forwarding BungeeGuard handshake support

proxy/src/main/java/com/velocitypowered/proxy/config/PlayerInfoForwarding.java

@@ -3,5 +3,6 @@ package com.velocitypowered.proxy.config;
 public enum PlayerInfoForwarding {
   NONE,
   LEGACY,
+  BUNGEEGUARD,
   MODERN
 }

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -61,6 +61,9 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
       "            proxy and will have offline-mode UUIDs.",
       "- \"legacy\": Forward player IPs and UUIDs in a BungeeCord-compatible format. Use this if",
       "            you run servers using Minecraft 1.12 or lower.",
+      "- \"bungeeguard\": Forward player IPs and UUIDs in a format supported by the BungeeGuard",
+      "            plugin. Use this if you run servers using Minecraft 1.12 or lower, and are",
+      "            unable to implement network level firewalling (on a shared host).",
       "- \"modern\": Forward player IPs and UUIDs as part of the login process using Velocity's ",
       "            native forwarding. Only applicable for Minecraft 1.13 or higher."
   })
@@ -185,6 +188,7 @@ public class VelocityConfiguration extends AnnotatedConfig implements ProxyConfi
             + "from the proxy and will have offline-mode UUIDs.");
         break;
       case MODERN:
+      case BUNGEEGUARD:
         if (forwardingSecret == null || forwardingSecret.length == 0) {
           logger.error("You don't have a forwarding secret set. This is required for security.");
           valid = false;

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

@@ -13,10 +13,12 @@ import static com.velocitypowered.proxy.network.Connections.MINECRAFT_ENCODER;
 import static com.velocitypowered.proxy.network.Connections.READ_TIMEOUT;
 
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.api.proxy.ServerConnection;
 import com.velocitypowered.api.proxy.messages.ChannelIdentifier;
 import com.velocitypowered.api.proxy.server.ServerInfo;
+import com.velocitypowered.api.util.GameProfile.Property;
 import com.velocitypowered.proxy.VelocityServer;
 import com.velocitypowered.proxy.config.PlayerInfoForwarding;
 import com.velocitypowered.proxy.connection.ConnectionTypes;
@@ -41,8 +43,11 @@ import io.netty.channel.ChannelFutureListener;
 import io.netty.channel.ChannelInitializer;
 import io.netty.handler.flow.FlowControlHandler;
 import io.netty.handler.timeout.ReadTimeoutHandler;
+import java.nio.charset.StandardCharsets;
+import java.util.List;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.TimeUnit;
+import java.util.function.UnaryOperator;
 import org.checkerframework.checker.nullness.qual.Nullable;
 
 public class VelocityServerConnection implements MinecraftConnectionAssociation, ServerConnection {
@@ -106,7 +111,7 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     return result;
   }
 
-  private String createLegacyForwardingAddress() {
+  private String createLegacyForwardingAddress(UnaryOperator<List<Property>> propertiesTransform) {
     // 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 (from Mojang).
@@ -117,10 +122,24 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
         .append('\0')
         .append(proxyPlayer.getGameProfile().getUndashedId())
         .append('\0');
-    GSON.toJson(proxyPlayer.getGameProfile().getProperties(), data);
+    GSON.toJson(propertiesTransform.apply(proxyPlayer.getGameProfile().getProperties()), data);
     return data.toString();
   }
 
+  private String createLegacyForwardingAddress() {
+    return createLegacyForwardingAddress(UnaryOperator.identity());
+  }
+
+  private String createBungeeGuardForwardingAddress(byte[] forwardingSecret) {
+    // Append forwarding secret as a BungeeGuard token.
+    Property property = new Property("bungeeguard-token",
+        new String(forwardingSecret, StandardCharsets.UTF_8), "");
+    return createLegacyForwardingAddress(properties -> ImmutableList.<Property>builder()
+        .addAll(properties)
+        .add(property)
+        .build());
+  }
+
   private void startHandshake() {
     final MinecraftConnection mc = ensureConnected();
     PlayerInfoForwarding forwardingMode = server.getConfiguration().getPlayerInfoForwardingMode();
@@ -132,6 +151,9 @@ public class VelocityServerConnection implements MinecraftConnectionAssociation,
     handshake.setProtocolVersion(protocolVersion);
     if (forwardingMode == PlayerInfoForwarding.LEGACY) {
       handshake.setServerAddress(createLegacyForwardingAddress());
+    } else if (forwardingMode == PlayerInfoForwarding.BUNGEEGUARD) {
+      byte[] secret = server.getConfiguration().getForwardingSecret();
+      handshake.setServerAddress(createBungeeGuardForwardingAddress(secret));
     } else if (proxyPlayer.getConnection().getType() == ConnectionTypes.LEGACY_FORGE) {
       handshake.setServerAddress(handshake.getServerAddress() + HANDSHAKE_HOSTNAME_TOKEN);
     } else {