I need to code system, when client connects to server, send packet and handle it on server side. 

1 minute ago, diesieben07 said:

https://mcforge.readthedocs.io/en/latest/networking/simpleimpl/

Im currently using this, but nothing happens

package me.mrfunny.nextlevelauth;

import com.google.gson.Gson;
import net.minecraft.client.Minecraft;
import net.minecraft.entity.player.PlayerEntity;
import net.minecraft.entity.player.ServerPlayerEntity;
import net.minecraft.network.PacketBuffer;
import net.minecraft.server.MinecraftServer;
import net.minecraft.util.ResourceLocation;
import net.minecraft.util.text.TranslationTextComponent;
import net.minecraftforge.api.distmarker.Dist;
import net.minecraftforge.api.distmarker.OnlyIn;
import net.minecraftforge.common.MinecraftForge;
import net.minecraftforge.event.entity.EntityJoinWorldEvent;
import net.minecraftforge.event.entity.player.PlayerEvent;
import net.minecraftforge.eventbus.api.SubscribeEvent;
import net.minecraftforge.fml.common.Mod;
import net.minecraftforge.fml.network.NetworkEvent;
import net.minecraftforge.fml.network.NetworkRegistry;
import net.minecraftforge.fml.network.simple.SimpleChannel;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import java.io.File;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Scanner;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.Supplier;

// The value here should match an entry in the META-INF/mods.toml file
@Mod("nextlevelauth")
public class ExampleMod {
    // Directly reference a log4j logger.
    private static final Logger LOGGER = LogManager.getLogger();
    private static final String PROTOCOL_VERSION = "1";
    public static final SimpleChannel INSTANCE = NetworkRegistry.newSimpleChannel(
            new ResourceLocation("nextlevelauth", "authmain"),
            () -> PROTOCOL_VERSION,
            PROTOCOL_VERSION::equals,
            PROTOCOL_VERSION::equals
    );
    private int id = 0;
    public ExampleMod() {
        MinecraftForge.EVENT_BUS.register(ExampleMod.class);
        try{
            INSTANCE.registerMessage(id, AuthMessage.class, new BiConsumer<AuthMessage, PacketBuffer>() {
                @Override
                public void accept(AuthMessage authMessage, PacketBuffer packetBuffer) {
                    id++;
                    packetBuffer.writeString(authMessage.token);
                }
            }, new Function<PacketBuffer, AuthMessage>() {
                @Override
                public AuthMessage apply(PacketBuffer packetBuffer) {
                    LOGGER.info("logged in1");
                    return new AuthMessage(packetBuffer.readString());
                }
            }, new BiConsumer<AuthMessage, Supplier<NetworkEvent.Context>>() {
                @Override
                public void accept(AuthMessage authMessage, Supplier<NetworkEvent.Context> contextSupplier) {
                    LOGGER.info("logged in2");
                    handle(authMessage, contextSupplier);
                }
            });
        } catch (Exception exception){
            exception.printStackTrace();
        }

    }

    public static void handle(AuthMessage msg, Supplier<NetworkEvent.Context> ctx) {
        ctx.get().enqueueWork(() -> {
            ServerPlayerEntity sender = ctx.get().getSender(); // the client that sent this packet
            try{
                URL url = new URL("https://nextlevel.su/api/v1/user/");
                HttpURLConnection connection = (HttpURLConnection) url.openConnection();
                connection.setRequestMethod("GET");
                connection.addRequestProperty("User-Agent", "PostmanRuntime/7.26.8");
                connection.addRequestProperty("Authorization", "Bearer " + msg.token);
                connection.addRequestProperty("Content-Type", "application/json");
                connection.addRequestProperty("Accept", "application/json");
                connection.connect();
                int responseCode = connection.getResponseCode();
                if(responseCode != 200){
                    connection.disconnect();
                    sender.disconnect();
                    return;
                }
                ctx.get().setPacketHandled(true);
                connection.disconnect();
            } catch (Exception exception){
                exception.printStackTrace();
                ctx.get().getSender().disconnect();
            }
        });
    }
    @OnlyIn(Dist.CLIENT)
    @SubscribeEvent
    public static void onJoin(PlayerEvent.PlayerLoggedInEvent event){
        try {
            LOGGER.info("connected");
        } catch (Exception exception) {
            exception.printStackTrace();
        }
    }
    public static String readFile(String path, Charset encoding) {
        byte[] encoded = new byte[0];
        try {
            encoded = Files.readAllBytes(Paths.get(path));
        } catch (IOException exception) {
            exception.printStackTrace();
        }
        return new String(encoded, encoding).replace("\n", "").replace("\r", "");
    }
}

My AuthMessage class
 

package me.mrfunny.nextlevelauth;

public class AuthMessage {
    public String token;
    public AuthMessage(String token){
        this.token = token;
    }
}

 

Because I have modded server and with custom launcher. I have JWT token and on login I need send to server this token and on server i need to validate this token.

4 minutes ago, diesieben07 said:

Do not make custom launchers with custom authentication.

Why? I just need this.

Just now, diesieben07 said:

No, you don't.

Make a whitelist. Tada, problem solved.

I need to install mods and other things in 1 click with site and others features for server. Can you just leave example for send and handle or not?

Just now, diesieben07 said:

No.

Custom launchers are not supported. Use an installer approach to create a profile in the vanilla launcher.

How about forge hacks and another. For example launcher checks hash, automatical update from server and another things.

Just now, diesieben07 said:

Automatic download and execution of code is a terrible idea.

What are "forge hacks"?

Cheats that works through forge

1 minute ago, diesieben07 said:

So you think having a custom launcher will prevent people from installing cheats.

You are naive, my friend.

Not only for that. can you not ask unnecessary questions?