Send custom ping data.

CustomPingData.java

package com.comphenix.example;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;

import org.bukkit.plugin.Plugin;
import org.bukkit.plugin.java.JavaPlugin;
import org.json.simple.JSONObject;
import org.json.simple.JSONValue;

import com.comphenix.protocol.PacketType;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ListenerPriority;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketEvent;
import com.comphenix.protocol.events.PacketOutputHandler;
import com.comphenix.protocol.utility.StreamSerializer;

public class CustomPingData extends JavaPlugin {    
    @Override
    public void onEnable() {
        ProtocolLibrary.getProtocolManager().addPacketListener(
          new PacketAdapter(this, PacketType.Status.Server.OUT_SERVER_INFO) {
            private StreamSerializer serializer = new StreamSerializer();

            @Override
            public void onPacketSending(PacketEvent event) {
                // There might be some way to encode data in ServerPing (Steganography?)
                // but you would be limited in how much you could send. 
                
                // It's better to modify the raw JSON string.
                event.getNetworkMarker().addOutputHandler(new PacketOutputHandler() {
                    @SuppressWarnings("unchecked")
                    @Override
                    public byte[] handle(PacketEvent event, byte[] buffer) {
                        try {
                            DataInputStream input = new DataInputStream(new ByteArrayInputStream(buffer));
                            boolean hasHeader = event.getNetworkMarker().requireOutputHeader();

                            if (hasHeader)
                                serializer.deserializeVarInt(input);
                            String json = serializer.deserializeString(new DataInputStream(input), 32767);

                            // We assume it's a JSON object
                            JSONObject parsed = (JSONObject) JSONValue.parse(json);
                            // Add our custom data here
                            parsed.put("custom_data", "This can be any JSON.");
                            
                            // Note that the maximum size is 32767 characters
                            ByteArrayOutputStream byteOutput = new ByteArrayOutputStream();
                            DataOutputStream dataOutput = new DataOutputStream(byteOutput);
                            
                            // Include the header
                            if (hasHeader)
                                serializer.serializeVarInt(dataOutput, event.getPacket().getType().getCurrentId());
                            serializer.serializeString(dataOutput, parsed.toJSONString());
                            return byteOutput.toByteArray();
                            
                        } catch (IOException e) {
                            throw new RuntimeException("Cannot modify outgoing JSON.", e);
                        }
                    }
                    
                    @Override
                    public ListenerPriority getPriority() {
                        return ListenerPriority.HIGH;
                    }
                    
                    @Override
                    public Plugin getPlugin() {
                        return ExampleMod.this;
                    }
                });
            }
        });
    }
}