S21PacketChunkData — Minecraft 1.8.9 Packet Reference

0x21 S21PacketChunkData

Sends a chunk section (16x16x16 blocks) with compressed block data, light values, and biome info.

Clientbound (Server → Client) · PLAY · net/minecraft/network/play/server/S21PacketChunkData.java

worldchunkblock

Fields

Field	Type	Description
chunkX	int	Chunk X coordinate
chunkZ	int	Chunk Z coordinate
groundUpContinuous	boolean	True if this is a fresh chunk (sends biome + skylight)
bitmask	unsigned short	Which 16-block sections are present (bit 0 = bottom section)
dataLength	VarInt	Size of compressed data
data	byte[]	Zlib-compressed chunk data (block IDs + block light + sky light + biomes)

Wire Encoding

Field	Type	Notes
Chunk X	int
Chunk Z	int
Ground-Up	bool	Full chunk vs partial update
Primary Bitmask	unsigned short	Which sections are included
Data Size	VarInt	Compressed data length
Data	byte[]	Compressed NBT-like block data

MCP References

MCPNetHandlerPlayClient.handleChunkData()

Handler Interface

HNDINetHandlerPlayClient

Notes

Each section in the bitmask contains: 4096 block IDs (2 bytes each), 2048 block light (4 bits per block), 2048 sky light (4 bits per block). For groundUp=true: 256 biome bytes at end. Data is zlib-compressed. XRay modules intercept this to replace block IDs or prevent chunk updates.

implementation Implementation Cases

XRay (Tenacity 6.0, Sigma 3.9)

XRay cancels chunk data and sends only desired blocks. Search tracks blocks in chunks. ChestESP reads chest positions.

if (event.getPacket() instanceof S21PacketChunkData) {
  S21PacketChunkData s21 = (S21PacketChunkData) event.getPacket();
  // XRay: modify block data
  // Search: scan for target blocks
}

Vanilla hook: EventReceivePacket

Search (Tenacity 6.0, Sigma 3.9)

XRay cancels chunk data and sends only desired blocks. Search tracks blocks in chunks. ChestESP reads chest positions.

if (event.getPacket() instanceof S21PacketChunkData) {
  S21PacketChunkData s21 = (S21PacketChunkData) event.getPacket();
  // XRay: modify block data
  // Search: scan for target blocks
}

Vanilla hook: EventReceivePacket

ChestESP (Tenacity 6.0, Sigma 3.9)

XRay cancels chunk data and sends only desired blocks. Search tracks blocks in chunks. ChestESP reads chest positions.

if (event.getPacket() instanceof S21PacketChunkData) {
  S21PacketChunkData s21 = (S21PacketChunkData) event.getPacket();
  // XRay: modify block data
  // Search: scan for target blocks
}

Vanilla hook: EventReceivePacket

AntiChunkBan (Tenacity 6.0, Sigma 3.9)

XRay cancels chunk data and sends only desired blocks. Search tracks blocks in chunks. ChestESP reads chest positions.

if (event.getPacket() instanceof S21PacketChunkData) {
  S21PacketChunkData s21 = (S21PacketChunkData) event.getPacket();
  // XRay: modify block data
  // Search: scan for target blocks
}

Vanilla hook: EventReceivePacket

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