network programming - Java sending handshake packets to minecraft server

Question

I have been working on a java program that basically acts like Minechat(text-based app to just view chat.) I have never really worked with networking too much, so the issue is figuring out how to send packets correctly. I am currently at the position of creating the handshake with the server. After hours of research, I have come up with the following code, but it always runs into the "Failed! (Exception)" message. To me, everything looks correct, but for all I know it could be 100% wrong. If someone could point out what I'm doing wrong here, I'd really appreciate it.

For reference, feel free to use this and this.

public static void main(String[] args) throws IOException {
    host = new InetSocketAddress("162.244.165.111", 48040);
    socket = new Socket();
    System.out.println("Connecting...");
    socket.connect(host, 3000);
    System.out.println("Done!");
    System.out.println("Making streams...");
    output = new DataOutputStream(socket.getOutputStream());
    input = new DataInputStream(socket.getInputStream());
    System.out.println("Done!");
    System.out.println("Attempting handshake... "+host.getAddress().toString().substring(1));
    byte[] msg = ("47;"+host.getAddress().toString().substring(1)+";"+host.getPort()+";2;").getBytes(Charset.forName("UTF-16"));
    output.writeInt(msg.length+Integer.valueOf(0x00));
    output.writeByte(0x00);
    output.write(msg);
    output.flush();
    try {
        if (input.readByte() != 0x02)
            System.out.println("Failed!");
        else
            System.out.println("Done!");
    } catch (EOFException e) {
        System.out.println("Failed! (Exception)");
    }
}

EDIT: More research suggests I use a Byte array, but this confuses me on how to represent a string and using strings is required?

Answer 1

Looking at this page http://wiki.vg/Protocol it looks like your not writing enough data nor in the right order. You also need to be using varint which is a special type of data representation of an integer.

Relevant links to this issue:

• Handshake Protocol
• Packet format
• Server Ping Explanation and Example (which involves handshake)

---

The status ping works as follows:

C->S : Handshake State=1
C->S : Request
S->C : Response
C->S : Ping
S->C : Pong

C is client and S is server

Using the wiki and the provided code samples I modified your code to follow the entire status request.

public static void main(String [] args) throws IOException {
    String address = "162.244.165.111";
    int port = 48040;

    InetSocketAddress host = new InetSocketAddress(address, port);
    Socket socket = new Socket();
    System.out.println("Connecting...");
    socket.connect(host, 3000);
    System.out.println("Done!");
    System.out.println("Making streams...");
    DataOutputStream output = new DataOutputStream(socket.getOutputStream());
    DataInputStream input = new DataInputStream(socket.getInputStream());

    System.out.println("Done!");
    System.out.println("Attempting handshake... "+host.getAddress().toString());


    byte [] handshakeMessage = createHandshakeMessage(address, port);

    // C->S : Handshake State=1
    // send packet length and packet
    writeVarInt(output, handshakeMessage.length);
    output.write(handshakeMessage);

    // C->S : Request
    output.writeByte(0x01); //size is only 1
    output.writeByte(0x00); //packet id for ping


    // S->C : Response
    int size = readVarInt(input);
    int packetId = readVarInt(input);

    if (packetId == -1) {
        throw new IOException("Premature end of stream.");
    }

    if (packetId != 0x00) { //we want a status response
        throw new IOException("Invalid packetID");
    }
    int length = readVarInt(input); //length of json string

    if (length == -1) {
        throw new IOException("Premature end of stream.");
    }

    if (length == 0) {
        throw new IOException("Invalid string length.");
    }

    byte[] in = new byte[length];
    input.readFully(in);  //read json string
    String json = new String(in);


    // C->S : Ping
    long now = System.currentTimeMillis();
    output.writeByte(0x09); //size of packet
    output.writeByte(0x01); //0x01 for ping
    output.writeLong(now); //time!?

    // S->C : Pong
    readVarInt(input);
    packetId = readVarInt(input);
    if (packetId == -1) {
        throw new IOException("Premature end of stream.");
    }

    if (packetId != 0x01) {
        throw new IOException("Invalid packetID");
    }
    long pingtime = input.readLong(); //read response


    // print out server info
    System.out.println(json);

    System.out.println("Done!");
}

public static byte [] createHandshakeMessage(String host, int port) throws IOException {
    ByteArrayOutputStream buffer = new ByteArrayOutputStream();

    DataOutputStream handshake = new DataOutputStream(buffer);
    handshake.writeByte(0x00); //packet id for handshake
    writeVarInt(handshake, 4); //protocol version
    writeString(handshake, host, StandardCharsets.UTF_8);
    handshake.writeShort(port); //port
    writeVarInt(handshake, 1); //state (1 for handshake)

    return buffer.toByteArray();
}

public static void writeString(DataOutputStream out, String string, Charset charset) throws IOException {
    byte [] bytes = string.getBytes(charset);
    writeVarInt(out, bytes.length);
    out.write(bytes);
}

public static void writeVarInt(DataOutputStream out, int paramInt) throws IOException {
    while (true) {
        if ((paramInt & 0xFFFFFF80) == 0) {
          out.writeByte(paramInt);
          return;
        }

        out.writeByte(paramInt & 0x7F | 0x80);
        paramInt >>>= 7;
    }
}

public static int readVarInt(DataInputStream in) throws IOException {
    int i = 0;
    int j = 0;
    while (true) {
        int k = in.readByte();
        i |= (k & 0x7F) << j++ * 7;
        if (j > 5) throw new RuntimeException("VarInt too big");
        if ((k & 0x80) != 128) break;
    }
    return i;
}