Plugin channel

Plugin channels are implemented using packet 0xFA, allowing 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|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

Strings in this channel are UTF8 strings prefixed with a short for length.

The first string is the SubChannel followed by data depending on the value of SubChannel

Forward

Server -> Bungee

Forwards the bungeecord plugin message to the target server

Field Name	Field Type	Notes
Target	String	Server name, the value ALL sends to all servers
Channel	String	Channel name
Length	Short	Length of the following payload
Payload	Array of bytes

Connect

Server -> Bungee

Field Name	Field Type	Notes
Target	String	Server name

ConnectOther

Server -> Bungee

Field Name	Field Type	Notes
Player Name	String	Target player's name
Target	String	Server name

IP

Server -> Bungee

Request the bungeecord to send back the player's real ip

Field Name	Field Type	Notes

Bungee -> Server

Field Name	Field Type	Notes
Host	String
Port	Int

PlayerCount

Server -> Bungee

Request the bungeecord to send back number of players on the target server

Field Name	Field Type	Notes
Target	String	Target server, the value ALL means all servers

Bungee -> Server

Field Name	Field Type	Notes
Name	String
Player count	Int

PlayerList

Server -> Bungee

Request the bungeecord to send back a csv of players on the target server

Field Name	Field Type	Notes
Target	String	Target server, the value ALL means all servers

Bungee -> Server

Field Name	Field Type	Notes
Name	String
Players	String	A comma separated list of player names

GetServers

Server -> Bungee

Request the bungeecord to send back a csv of all servers

Field Name	Field Type	Notes

Bungee -> Server

Field Name	Field Type	Notes
Servers	String	A comma separated list of server names

Message

Server -> Bungee

Field Name	Field Type	Notes
Player	String
Message	String

GetServer

Server -> Bungee

Request the bungeecord to send the player's current server

Field Name	Field Type	Notes

Bungee -> Server

Field Name	Field Type	Notes
Server name	String

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.