Packet Protocol

Purpose

Understanding the binary packet format used by the RCON protocol for communication between client and server.

Overview

RCON uses a simple binary packet format with four fields: size, ID, type, and payload. All packets follow this structure regardless of direction (client-to-server or server-to-client).

Packet Structure

Field Size Type Description

Size

4 bytes

int32 (little-endian)

Length of ID, Type, and Payload fields combined

ID

4 bytes

int32 (little-endian)

Request/response identifier for matching

Type

4 bytes

int32 (little-endian)

Packet type (see Packet Types)

Payload

Variable

byte array

Actual data, null-terminated with \0

 
+-----------+-----------+-----------+-----------+
| Size (4)  |  ID (4)   | Type (4)  | Payload   |
+-----------+-----------+-----------+-----------+
| Little-endian integers   | Variable  |
+-----------+-----------+-----------+-----------+

Packet Types

Value Constant Description

0

SERVERDATA_AUTH_RESPONSE

Authentication response from server

2

SERVERDATA_EXECCOMMAND

Command response from server

3

SERVERDATA_AUTH

Authentication request from client

Size Field Calculation

The Size field contains the length of everything after itself:

int size = 4 (ID) + 4 (Type) + payload.length + 1 (null terminator);

For example, a password "secret" would have: - Payload: 6 bytes (s, e, c, r, e, t) - Null terminator: 1 byte - ID: 4 bytes - Type: 4 bytes - Size = 4 + 4 + 6 + 1 = 15

ID Field

The ID field matches requests with responses:

  • Client-to-server: Any positive integer (typically auto-incrementing)

  • Server-to-client: Matches the request ID, or 0 for authentication responses

// Client generates unique ID
int requestId = requestCounter.getAndIncrement();

// Server responds with same ID
// All response packets for this request use the same ID

Payload Encoding

Default: UTF-8

Standard text encoding for most commands:

Charset charset = StandardCharsets.UTF_8;
byte[] payload = command.getBytes(charset);

Color Codes: ISO-8859-1

Preserve Minecraft color codes like §c[Server]:

Charset charset = StandardCharsets.ISO_8859_1;
byte[] payload = command.getBytes(charset);

Null Termination

All payloads must end with a \0 (null) byte:

byte[] payload = command.getBytes(charset);
byte[] terminatedPayload = new byte[payload.length + 1];
System.arraycopy(payload, 0, terminatedPayload, 0, payload.length);
// terminatedPayload[payload.length] is already 0

Example Packets

Authentication Request 

Size: 12
ID: 1
Type: 3
Payload: "secret\0"

Hex (little-endian):
0C 00 00 00  // Size = 12
01 00 00 00  // ID = 1
03 00 00 00  // Type = 3 (AUTH)
73 65 63 72  65 74 00     // "secret\0"

Command Request 

Size: 15
ID: 2
Type: 3
Payload: "list\0"

Hex (little-endian):
0F 00 00 00  // Size = 15
02 00 00 00  // ID = 2
03 00 00 00  // Type = 3 (AUTH)
6C 69 73 74  00            // "list\0"

Authentication Response (Success) 

Size: 10
ID: 0
Type: 2
Payload: "\0"

Hex (little-endian):
0A 00 00 00  // Size = 10
00 00 00 00  // ID = 0 (auth response)
02 00 00 00  // Type = 2 (RESPONSE)
00                        // Empty payload (success)

Authentication Response (Failure) 

Size: 21
ID: 0
Type: 2
Payload: "Authentication failed\0"

Hex (little-endian):
15 00 00 00  // Size = 21
00 00 00 00  // ID = 0
02 00 00 00  // Type = 2 (RESPONSE)
41 75 74 68  65 6E 74 69  63 61 74 69 6F  6E 20 66 61  69 6C 65 64  00
// "Authentication failed\0"

Size Limits

Direction Limit Notes

Client → Server

1460 bytes total

Typical MTU, includes headers

Server → Client

4104 bytes total

4096 bytes payload + headers

Responses exceeding 4096 bytes are split across multiple packets. See Fragment Resolution for details.

Byte Order

Critical: All integer fields use little-endian byte order, opposite of Java’s default big-endian.

// WRONG - uses big-endian
byte[] bytes = ByteBuffer.allocate(4).putInt(value).array();

// CORRECT - uses little-endian
byte[] bytes = ByteBuffer.allocate(4)
    .order(ByteOrder.LITTLE_ENDIAN)
    .putInt(value)
    .array();

See Byte Order for more details.

See Also

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