Minecraft Forge Handshake

Work in progress! This page is a work in progress, and may be missing a lot of information or may contain incorrect information. Feel free to edit it to add your own information.

This page will document the changes made in the Minecraft Forge protocol.

Largely based on my own attempts to document the protocol and allow connection to forge servers here.

Changes to Server List Ping

When forge is installed, the Server List Ping changes with an additional modinfo. Example:

{
    "description": "A Minecraft Server",
    "players": {
        "max": 20,
        "online": 0
    },
    "version": {
        "name": "1.8",
        "protocol": 47
    },
    "modinfo": {
        "type": "FML",
        "modList": [
            {
                "modid": "mcp",
                "version": "9.05"
            },
            {
                "modid": "FML",
                "version": "8.0.99.99"
            },
            {
                "modid": "Forge",
                "version": "11.14.3.1512"
            },
            {
                "modid": "rpcraft",
                "version": "Beta 1.3 - 1.8.0"
            }
        ]
    }
}

So, to test whether forge is installed, look for the modinfo key and then a type of FML

The modList contains each installed mod's version and ID.

The key modList has a capital 'L', unlike any other key in the ping result!

This is injected in ServerStatusResponse by calling FMLNetworkHandler.enhanceStatusQuery().

Connection to a forge server

Attempting to connect to a forge server without making any changes results in getting immediately disconnected with the message "This server requires FML/Forge to be installed. Contact your server admin for more details.". This message is displayed in FMLCommonHandler and is injected in a modified 0x00 Handshake packet.

If \0FML\0 is added to the end of the server's address (again, in the 0x00 handshake packet), the server will not immediately reject you. However, you still will not complete the connection.

This occurs because Minecraft Forge has an additional set of handshakes (for determining mods and block IDs), that must be completed before the server allows logging in.

Forge handshake

Forge's handshake occurs in the middle of the normal login sequence, right after 0x02 Login Success is received by the client.

In addition to the normal packet debugging for the server, if -Dfml.debugNetworkHandshake=true is passed to the forge server, some more information about the current handshake status will be outputted.

Definitions

Minecraft Forge uses an additional type not covered in data types for some things -- a varshort. This is essentially the same as a regular short, except that if the top byte (what is normally sign) is set, it is followed by an additional byte. This allows forge to retain backwards compatibility but extend the length of certain numbers -- the varshort is only used in places where, in vanilla Minecraft, the sign bit would not have been set. It is implemented in ByteBufUtils.

Discriminator bytes are the way that Minecraft Forge allows sending multiple messages on one plugin channel. They aren't part of the original specification, but a similar concept is used with the MC|AdvCdm plugin channel (used for command blocks). That packet decides whether a block location is being sent or an entity ID (minecart command blocks) is being sent based off of the value of the first byte in the packet.

In the same way, forge uses the first byte of the plugin message to specify which packet to use. It uses FMLIndexedMessageToMessageCodec to automatically switch the value.

Discriminator bytes change depending on the channel: 0 for FML|HS means something different from 0 for FML.

FML|HS Packet structure

FML|HS is used to perform the handshake. Discriminator bytes are found in FMLHandshakeCodec.

ServerHello

Starts the handshake.

Channel	Bound To	Field Name	Field Type	Notes
FML|HS	Client	Discriminator	Byte	Always 0 for ServerHello
		FML protocol Version	Byte	Determined from NetworkRegistery. Currently 2.
		Override dimension	Optional Int	Only sent if protocol version is greater than 1.

ClientHello

Response from the client to the ServerHello packet.

Channel	Bound To	Field Name	Field Type	Notes
FML|HS	Server	Discriminator	Byte	Always 1 for ClientHello.
		FML protocol Version	Byte	Determined from NetworkRegistery. Currently 2.

ModList

Contains a list of all mods installed on the server or client. Sent from the client to the server first, and then the server responds with its mod list. The server's mod list matches the one sent in the ping.

Channel	Bound To	Field Name		Field Type		Notes
FML|HS	Both	Discriminator		Byte		Always 2 for ModList
		Number of mods		Varint		Number of mods below
		Mods	Mod name	Array	String	Name of the mod
			Mod version		String	Version of the mod

RegistryData

	The following information needs to be added to this page:
	Explain exactly what registries are, and figure out what Substitutions is

The server sends several of this packet, one for each registry. It'll keep sending them until the hasMore value is no longer true.

Channel	Bound To	Field Name		Field Type		Notes
FML|HS	Both	Discriminator		Byte		Always 3 for RegistryData.
		Has more		Boolean		Marks whether another RegistryData packet will be sent after this.
		Name		String		Name of the registry for this packet
		Number of ids		Varint		Number of ids sent below
		Ids	Name	Array	String	Name of the thing
			Id		Varint	Numerical id of the thing
		Number of substitutions		Varint		Number of substitutions sent below
		Substitutions		Array of strings		Each substitution

HandshakeReset

	The following information needs to be added to this page:
	Is this ever actually sent?

Causes the client to recomplete the entire handshake from the start. Does not appear to be sent. There is no payload beyond the discriminator byte.

Channel	Bound To	Field Name	Field Type	Notes
FML|HS	Client	Discriminator	Byte	Always -2 (254) for HandshakeReset