Plugin channels

Plugin channels allow client mods and server plugins to communicate without cluttering up chat. This post by Dinnerbone is a good introduction and basic documentation.

Reserved channels

REGISTER

Two-way

Allows the client to register for one or more custom channels, indicating that the server should send data on those channels if the appropriate plugin is installed. Payload is a null (\00) separated list of strings.

UNREGISTER

Two-way

Allows the client to unregister from one or more custom channels, indicating that the server should stop sending data on those channels. Payload is a null-separated list of strings.

Channels internal to Minecraft

As of 1.3, Minecraft itself started using plugin channels to implement new features. These internal channel names are prefixed by MC|. They are not formally registered using the REGISTER channel. The vanilla Minecraft server will send these packets regardless, and the vanilla client will accept them.

MC|AdvCdm

Client -> Server

Adventure mode command block.

The first byte is type which is followed by either:

Type	Field Name	Field Type	Notes
0x00	X	Int
	Y	Int
	Z	Int
	Command	String

or

Type	Field Name	Field Type	Notes
0x01	Entity ID	Int
	Command	String

depending on the mode

MC|Beacon

Client to server

Two integers corresponding to the 2 effects a user wishes to have active.

MC|BEdit

Client to server

When a player edits an unsigned book.

This payload is simply a set of bytes corresponding to an ItemStack. It is serialized / deserialized exactly the same as itemstacks in packets.

MC|BSign

Client to server

When a player signs a book. This payload is simply a set of bytes corresponding to an ItemStack. It is serialized / deserialized exactly the same as itemstacks in packets.

MC|BOpen

Server to client

When a player right clicks with a signed book. This tells the client to open the book GUI. This payload is empty.

MC|ItemName

Two-way

When a player uses an anvil to name an item. The payload is just a string: the item's new name.

MC|RPack

Server to client

Remote resource packs. This contains a string representing the url to get the resource pack from.

MC|TrList

Server to client

The list of trades a villager NPC is offering.

MC|TrSel

Client to server

When a player selects a specific trade offered by a villager NPC. It contains a single int id corresponding to the selected slot int the players current (trading) inventory.

MC|PingHost

Client to server

Sent after a Server list ping in Minecraft 1.6. More information on Server List Ping#1.6. In 1.7 and above, the Request (Status, 0x00, serverbound) packet is instead sent before the ping.

MC|Brand

Two-way

Announces the server and client implementation name right after a player has logged in. For Minecraft and the standard Minecraft server, this is "vanilla" (encoded as a UTF-8 string).

Notable community plugin channels

Channels listed in this section are not Mojang-sanctioned. This is just a likely-incomplete list of channels used by mods/plugins popular within the Minecraft community.

BungeeCord

See here

FML

Two-way

Used by Forge ModLoader to negotiate required mods, among other things. [1]

ML|OpenTE

Server to client

Used by ModLoader to support custom GUI windows. Does not use the REGISTER channel.

WECUI

Two-way

Used by the server-side WorldEdit and the client-side WorldEditCUI to coordinate selections.