Minecraft Forge Handshake

This page documents the current Minecraft Forge protocol, including how to connect to servers running Minecraft Forge and what changes it makes to the vanilla protocol, for Minecraft 1.7.10, 1.8, and 1.9.

Largely based off of Pokechu22's work on getting Minecraft Console Client to connect to Forge servers.

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 (where \0 is a Wikipedia:null character) (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.

	The following information needs to be added to this page:
	Why does it send the CompleteHandshake packet to itself? How does it keep synchronized (especially near the end with the ACKs)?

• S→C: REGISTER forge's plugin channels: FML|HS, FML, FML|MP, FML, FORGE (yes, FML is there twice; technically, it does not need to be there twice but that is the way that Forge sends it and sending it that way will act more similar to forge)
• S→C: A ServerHello packet is sent on FML|HS including the player's dimension (0 if it's the first login)
• C→S: REGISTER the plugin channels: FML|HS, FML, FML|MP, FML, FORGE.
• C→S: A ClientHello packet is sent on the FML|HS channel.
• C→S: A ModList packet is sent.
• Server checks that the mod list is compatible. If incompatible, the player is disconnected with an explanatory message. Otherwise, continue.
• S→C: A ModList packet is sent.
• Client checks that the mod list is compatible. If incompatible, it disconnects. Otherwise, continue.
• C→S: A HandshakeAck packet is sent, with the phase being WAITINGSERVERDATA (2).
• S→C: A series of RegistryData packets is sent, with hasMore being true for all packets except the last.
• Client reads and stores each RegistryData, continuing reading until hasMore is false. Once hasMore is false, it checks whether it has all of the needed blocks/items. If it does not, it disconnects. Otherwise, continue.
• C→S: A HandshakeAck packet is sent with phase being WAITINGSERVERCOMPLETE (3).
• S→C: A HandshakeAck packet is sent with phase being WAITINGCACK (2).
• C→S: A HandshakeAck packet is sent with phase being PENDINGCOMPLETE (4).
• S→C: A HandshakeAck packet is sent with phase being COMPLETE (3).
• The server sends a "CompleteHandshake" packet to itself. (Not sure why)
• C→S: A HandshakeAck packet is sent with phase being COMPLETE (5).
• The client sends a "CompleteHandshake" packet to itself. (Not sure why)

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 and Dummies are

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
		Number of dummies		Varint		Number of dummies in the array below. May not be present in older versions of forge; check if there still is data remaining in the packet before continuing to read data.
		Dummies		Array of strings		Each dummy?

HandshakeAck

Confirms that the client is able to use the settings sent previously.

Channel	Bound To	Field Name	Field Type	Notes
FML|HS	Both	Discriminator	Byte	Always -1 (255) for HandshakeAck
		Phase	Byte	The current phase, which is the ordinal (0-indexed) in the FMLHandshakeClientState or FMLHandshakeServerState enums (if the server is sending it, it is in the ServerState enum, and if the client is sending it, it is the ClientState enum).

HandshakeReset

Causes the client to recomplete the entire handshake from the start. There is no payload beyond the discriminator byte.

The normal forge server does not ever use this packet, but it is used when connecting through a BungeeCord instance, specifically when transitioning from a vanilla server to a modded one or from a modded server to another modded server.

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

FML|MP and multipart packets

Forge supports splitting large clientbound plugin message packets into smaller multipart packets. This functionality does not exist serverbound. This system is available in all versions.

This system starts by sending an FML|MP packet to indicate what the actual packet is (note that this is within the plugin message itself):

Channel	Bound To	Field Name	Field Type	Notes
FML|MP	Client	Channel	String (20)	The wrapped channel
		Parts	Unsigned byte	Total number of parts (equal to ceil(dataLength / (double)(1048495))
		Length	Int	Total length of wrapped data

This is followed by a series of additional FML|MP packets which write the payload. Each packet is prefixed with the part number as an unsigned byte (starting at 0), followed by the rest of the packet containing up to 1048495 bytes of the actual message. Each packet must be sent in order.

Up to 52424750 bytes (~52 megabytes) may be sent through this system.

Differences from Forge 1.7.10

Forge for Minecraft 1.8 made some changes from the 1.7.10 version.

The most important thing to keep track of isn't (entirely) a forge change. In 1.7.10, plugin channel packets are length prefixed, while in 1.8 they are not. However, forge makes some more changes to the server to client packet (but not the client to server packet): Rather than using a short for the length, a varshort is used. Due to the way that varshorts work, this is backwards compatible, but if this is not accounted for you may receive forge packets that seem corrupted as there is an extra byte that appears seemingly randomly.

While it may seem like you can get away with not handling varshorts, on servers with lots of mods (EG FTB), this will appear.

Additionally, in 1.7.10 the registries packet is instead "ModIdData":

ModIdData

Contains numeric ID mapping for blocks and items.

	The following information needs to be added to this page:
	Again, what are blockSubstitutions and itemSubstitutions?

Channel	Bound to	Name		Type		Notes
FML|HS	Client	Discriminator		Byte		Always 3 for ModIdData
		Mapping length		VarInt		Length of the following mapping
		Mapping	Name	Array	String	Name of the block/item.
			ID		VarInt	Numeric ID of the item.
		Block substitutions length		VarInt		Length of the following array
		Block substitutions		Array of strings		Block substitutions
		Item substitutions length		VarInt		Length of the following array
		Item substitutions		Array of strings		Item substitutions