Finish Modern forwarding v2

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

@@ -17,6 +17,7 @@
 
 package com.velocitypowered.proxy.connection.backend;
 
+import com.google.common.base.Preconditions;
 import com.velocitypowered.api.event.player.ServerLoginPluginMessageEvent;
 import com.velocitypowered.api.network.ProtocolVersion;
 import com.velocitypowered.api.proxy.crypto.IdentifiedKey;
@@ -79,9 +80,19 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
     VelocityConfiguration configuration = server.getConfiguration();
     if (configuration.getPlayerInfoForwardingMode() == PlayerInfoForwarding.MODERN
         && packet.getChannel().equals(VelocityConstants.VELOCITY_IP_FORWARDING_CHANNEL)) {
+
+      int proposedForwardingVersion = VelocityConstants.MODERN_FORWARDING_DEFAULT;
+      // Check version
+      if (packet.content().readableBytes() == 1) {
+        int requested = packet.content().readByte();
+        Preconditions.checkArgument(requested >= VelocityConstants.MODERN_FORWARDING_DEFAULT,
+                "Invalid modern forwarding version");
+        proposedForwardingVersion = Math.min(requested, VelocityConstants.MODERN_FORWARDING_WITH_KEY);
+      }
       ByteBuf forwardingData = createForwardingData(configuration.getForwardingSecret(),
-          serverConn.getPlayerRemoteAddressAsString(),
+          serverConn.getPlayerRemoteAddressAsString(), serverConn.getPlayer().getGameProfile(),
-          serverConn.getPlayer().getGameProfile(), mc.getProtocolVersion(), serverConn.getPlayer().getIdentifiedKey());
+          serverConn.getPlayer().getIdentifiedKey(), proposedForwardingVersion);
+
       LoginPluginResponse response = new LoginPluginResponse(packet.getId(), true, forwardingData);
       mc.write(response);
       informationForwarded = true;
@@ -166,20 +177,22 @@ public class LoginSessionHandler implements MinecraftSessionHandler {
   }
 
   private static ByteBuf createForwardingData(byte[] hmacSecret, String address,
-                                              GameProfile profile, ProtocolVersion version,
+                                              GameProfile profile, @Nullable IdentifiedKey playerKey,
-                                              @Nullable IdentifiedKey playerKey) {
+                                              int requestedVersion) {
     ByteBuf forwarded = Unpooled.buffer(2048);
     try {
-      int forwardingVersion = version.compareTo(ProtocolVersion.MINECRAFT_1_19) >= 0 && playerKey != null
+      int actualVersion = requestedVersion >= VelocityConstants.MODERN_FORWARDING_WITH_KEY
-              ? VelocityConstants.MODERN_FORWARDING_DEFAULT : VelocityConstants.MODERN_FORWARDING_DEFAULT;
+              ? (playerKey != null ? requestedVersion : VelocityConstants.MODERN_FORWARDING_DEFAULT) : requestedVersion;
 
-      ProtocolUtils.writeVarInt(forwarded, forwardingVersion);
+      ProtocolUtils.writeVarInt(forwarded, actualVersion);
       ProtocolUtils.writeString(forwarded, address);
       ProtocolUtils.writeUuid(forwarded, profile.getId());
       ProtocolUtils.writeString(forwarded, profile.getName());
       ProtocolUtils.writeProperties(forwarded, profile.getProperties());
 
-      if (forwardingVersion >= VelocityConstants.MODERN_FORWARDING_WITH_KEY) {
+      // This serves as additional redundancy. The key normally is stored in the
+      // login start to the server, but some setups require this.
+      if (actualVersion >= VelocityConstants.MODERN_FORWARDING_WITH_KEY) {
         ProtocolUtils.writePlayerKey(forwarded, playerKey);
       }