Classic Protocol Extension

Negotiation

Extended clients should use magic number of 0x42 for the padding byte of the Player Identification packet. All standard/non-extended clients pad this packet with 0x00, so extended servers should be able to easily notice extended clients. All standard/non-extended servers will ignore the modified padding byte, preserving compatibility.

When the server sees that padding byte is 0x42, it should immediately reply with an ExtInfo packet, followed by zero or more ExtEntry packets, and await a response (ExtInfo packet) from the client.

After the client has sent its identification packet, it should await a response. If the server responds with an ExtInfo packet, it should parse it and any ExtEntry packets that follow. Client should then compare its locally-supported set of extensions with the list of extensions provided by the server, and find an intersection of these sets. These are the mutually-supported extensions.

At this time, client should send ExtInfo packet of its own, followed by the list of mutually-supported extensions. After receiving the last of ExtEntry packets, server should activate the mutually-supported extensions, and resume normal login procedure.

ExtInfo Packet

Packet ID	Field Name	Field Type	Example	Notes
0x10	AppName	string	MyServer	Client or server software name
	ExtensionCount	short	1	Between 0 and 32767
Total Size:	67 bytes

ExtEntry Packet

Packet ID	Field Name	Field Type	Example	Notes
0x11	ExtName	string	MyExtension	Name of a supported extension
	Version	int	1
Total Size:	69 bytes

Extensions

ClickDistance

Used to extend or restrict the distance at which client may click blocks, controlled by the server. Click range is given in player-space units (32 units per block). Upon receiving a SetClickDistance packet, client should immediately apply the change.

SetClickDistance packet

Server to Client

Packet ID	Field Name	Field Type	Example	Notes
0x20	Distance	short	160
Total Size:	3 bytes