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. More information on Server List 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
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.
