Difference between revisions of "Minecraft Forge Handshake"

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(→‎Forge handshake: Document the handshake.)
(add 1.7.10 ModIdData mapping name discriminator bytes)
 
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{{Box|
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This page documents the current '''[https://minecraftforge.net/ 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.
BORDER = #FFCC00|
 
BACKGROUND = #FFFF00|
 
WIDTH = 100%|
 
ICON = |
 
HEADING = Work in progress! |
 
CONTENT = 
 
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 '''[https://minecraftforge.net/ Minecraft Forge] protocol'''.
+
Largely based off of [https://github.com/ORelio/Minecraft-Console-Client/pull/100 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 mod information.  Example for 1.13.2:
 +
 
 +
<syntaxhighlight lang="javascript">
 +
{
 +
  "description": {
 +
    "text": "A Minecraft Server"
 +
  },
 +
  "players": {
 +
    "max": 20,
 +
    "online": 0
 +
  },
 +
  "version": {
 +
    "name": "1.13.2",
 +
    "protocol": 404
 +
  },
 +
  "forgeData": {
 +
    "channels": [
 +
      {
 +
        "res": "ironchest:main_channel",
 +
        "version": "1",
 +
        "required": false
 +
      }
 +
    ],
 +
    "mods": [
 +
      {
 +
        "modId": "forge",
 +
        "modmarker": "ANY"
 +
      },
 +
      {
 +
        "modId": "ironchest",
 +
        "modmarker": "1.13.2-8.0.2"
 +
      }
 +
    ],
 +
    "fmlNetworkVersion": 2
 +
  }
 +
}
 +
</syntaxhighlight>
 +
 
 +
{{warning|The key <code>modId</code> has a capital 'I', unlike <code>modid</code> pre-1.13}}
  
Largely based on my own attempts to document the protocol and allow connection to forge servers [https://github.com/ORelio/Minecraft-Console-Client/pull/100 here].
 
  
== Changes to [[Server List Ping]] ==
+
===Pre-1.13===
  
When forge is installed, the [[Server List Ping]] changes with an additional <code>modinfo</code>. Example:
+
1.12.2 and earlier use a different JSON structure:
  
 
<syntaxhighlight lang="javascript">
 
<syntaxhighlight lang="javascript">
Line 58: Line 91:
 
{{warning|The key <code>modList</code> has a capital 'L', unlike any other key in the ping result!}}
 
{{warning|The key <code>modList</code> has a capital 'L', unlike any other key in the ping result!}}
  
This is injected in [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/patches/minecraft/net/minecraft/network/ServerStatusResponse.java.patch#L45 ServerStatusResponse] by calling [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/internal/FMLNetworkHandler.java#L192-L206 FMLNetworkHandler.enhanceStatusQuery()].
+
This is injected in [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/patches/minecraft/net/minecraft/network/ServerStatusResponse.java.patch#L45 ServerStatusResponse] by calling [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/internal/FMLNetworkHandler.java#L197-L211 FMLNetworkHandler.enhanceStatusQuery()].
  
 
== Connection to a forge server ==
 
== Connection to a forge server ==
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Attempting to connect to a forge server without making any changes results in getting immediately disconnected with the message "<code>This server requires FML/Forge to be installed. Contact your server admin for more details.</code>".  This message is displayed in [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/FMLCommonHandler.java#L653-L661 FMLCommonHandler] and is [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/patches/minecraft/net/minecraft/network/handshake/client/C00Handshake.java.patch#L35 injected] in a modified [[Protocol#Handshake|0x00 Handshake packet]].
 
Attempting to connect to a forge server without making any changes results in getting immediately disconnected with the message "<code>This server requires FML/Forge to be installed. Contact your server admin for more details.</code>".  This message is displayed in [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/FMLCommonHandler.java#L653-L661 FMLCommonHandler] and is [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/patches/minecraft/net/minecraft/network/handshake/client/C00Handshake.java.patch#L35 injected] in a modified [[Protocol#Handshake|0x00 Handshake packet]].
  
If <code>\0FML\0</code> 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.
+
If <code>\0FML\0</code> is added to the end of the server's address (where <code>\0</code> 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.
 
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.
Line 74: Line 107:
 
In addition to the [[Debugging|normal packet debugging]] for the server, if <code>-Dfml.debugNetworkHandshake=true</code> is passed to the forge server, some more information about the current handshake status will be outputted.
 
In addition to the [[Debugging|normal packet debugging]] for the server, if <code>-Dfml.debugNetworkHandshake=true</code> is passed to the forge server, some more information about the current handshake status will be outputted.
  
{{Need info|Why does it send the CompleteHandshake packet to itself?  How does it keep synchronized (especially near the end with the ACKs)?}}
+
{{Need Info|Why does it send the CompleteHandshake packet to itself?  How does it keep synchronized (especially near the end with the ACKs)?}}
  
* '''S→C''': [[Plugin channel#REGISTER|<code>REGISTER</code>]] forge's plugin channels: FML|HS, FML, FML|MP, FML, FORGE (yes, FML is there twice).
+
* '''S→C''': [[Plugin channel#REGISTER|<code>REGISTER</code>]] 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 <code>FML|HS</code> including the player's dimension (0 if it's the first login)
 
* '''S→C''': A ServerHello packet is sent on <code>FML|HS</code> including the player's dimension (0 if it's the first login)
 
* '''C→S''': <code>REGISTER</code> the plugin channels: FML|HS, FML, FML|MP, FML, FORGE.
 
* '''C→S''': <code>REGISTER</code> the plugin channels: FML|HS, FML, FML|MP, FML, FORGE.
Line 99: Line 132:
 
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 [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/ByteBufUtils.java#L58-L89 ByteBufUtils].
 
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 [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/ByteBufUtils.java#L58-L89 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 [[Plugin channel#MC&#x7c;AdvCdm|<code>MC|AdvCdm</code> 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.
+
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 [https://wiki.vg/index.php?title=Plugin_channels&oldid=14089#MC.7CAdvCmd <code>MC|AdvCdm</code> 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 [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/FMLIndexedMessageToMessageCodec.java FMLIndexedMessageToMessageCodec] to automatically switch the value.
 
In the same way, forge uses the first byte of the plugin message to specify which packet to use.  It uses [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/FMLIndexedMessageToMessageCodec.java FMLIndexedMessageToMessageCodec] to automatically switch the value.
Line 120: Line 153:
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
  |rowspan="6"| <code>FML|HS</code>
+
  |rowspan="3"| <code>FML|HS</code>
  |rowspan="6"| Client
+
  |rowspan="3"| Client
 
  | Discriminator
 
  | Discriminator
 
  | Byte
 
  | Byte
Line 146: Line 179:
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
  |rowspan="6"| <code>FML|HS</code>
+
  |rowspan="2"| <code>FML|HS</code>
  |rowspan="6"| Server
+
  |rowspan="2"| Server
 
  | Discriminator
 
  | Discriminator
 
  | Byte
 
  | Byte
Line 189: Line 222:
 
  |}
 
  |}
  
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L154-L240 RegistryData] ===
+
=== [https://github.com/MinecraftForge/MinecraftForge/blob/6e90348dc546164b21735aefd83ed0bac0d8283c/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L154-L240 RegistryData] ===
  
{{Need Info|Explain exactly what registries are, and figure out what Substitutions is}}
+
{{Need Info|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.
 
The server sends several of this packet, one for each registry.  It'll keep sending them until the hasMore value is no longer true.
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  ! Notes
 
  ! Notes
 
  |-
 
  |-
  |rowspan="8"| <code>FML|HS</code>
+
  |rowspan="10"| <code>FML|HS</code>
  |rowspan="8"| Both
+
  |rowspan="10"| Both
 
  |colspan="2"| Discriminator
 
  |colspan="2"| Discriminator
 
  |colspan="2"| Byte
 
  |colspan="2"| Byte
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  |colspan="2"| Array of strings
 
  |colspan="2"| Array of strings
 
  | Each substitution
 
  | Each substitution
 +
|-
 +
|colspan="2"| Number of dummies
 +
|colspan="2"| 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.
 +
|-
 +
|colspan="2"| Dummies
 +
|colspan="2"| Array of strings
 +
| Each dummy?
 
  |}
 
  |}
  
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  ! Notes
 
  ! Notes
 
  |-
 
  |-
  |rowspan="6"| <code>FML|HS</code>
+
  |rowspan="2"| <code>FML|HS</code>
  |rowspan="6"| Both
+
  |rowspan="2"| Both
 
  | Discriminator
 
  | Discriminator
 
  | Byte
 
  | Byte
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=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L265-L267 HandshakeReset] ===
 
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L265-L267 HandshakeReset] ===
  
{{Need Info|Is this ever actually sent?}}
+
Causes the client to recomplete the entire handshake from the start.  There is no payload beyond the discriminator byte.
  
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.
+
The normal forge server does not ever use this packet, but it is used [https://github.com/SpigotMC/BungeeCord/blob/aaddc9fcfdcada8d19ab115f2dcc8aed413a9206/proxy/src/main/java/net/md_5/bungee/ServerConnector.java#L126-L138 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.
  
 
{| class="wikitable"
 
{| class="wikitable"
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  ! Notes
 
  ! Notes
 
  |-
 
  |-
  |rowspan="6"| <code>FML|HS</code>
+
  |rowspan="1"| <code>FML|HS</code>
  |rowspan="6"| Client
+
  |rowspan="1"| Client
 
  | Discriminator
 
  | Discriminator
 
  | Byte
 
  | Byte
 
  | Always -2 (254) for HandshakeReset
 
  | Always -2 (254) for HandshakeReset
 +
|}
 +
 +
== <code>FML|MP</code> and multipart packets ==
 +
 +
Forge supports splitting large clientbound plugin message packets into smaller multipart packets.  This functionality does not <!-- appear to --> exist serverbound. <!-- which seems odd, given that the clientbound packet has a max size of 1 megabyte while the serverbound one is much smaller --> This system is available in all versions. <!-- or at least in 1.7.10 and above -->
 +
 +
This system starts by [https://github.com/MinecraftForge/MinecraftForge/blob/16bfd8cef1d12ee9ca0de1122addaf9916767ae9/src/main/java/net/minecraftforge/fml/common/network/internal/FMLProxyPacket.java#L164-L167 sending] an <code>FML|MP</code> packet to indicate what the actual packet is (note that this is within the plugin message itself):
 +
 +
{| class="wikitable"
 +
! Channel
 +
! Bound To
 +
! Field Name
 +
! Field Type
 +
! Notes
 +
|-
 +
|rowspan="3"| <code>FML|MP</code>
 +
|rowspan="3"| Client
 +
| Channel
 +
| String (20)
 +
| The wrapped channel
 +
|-
 +
| Parts
 +
| Unsigned byte
 +
| Total number of parts (equal to <code>ceil(dataLength / (double)(1048495))</code>
 +
|-
 +
| Length
 +
| Int
 +
| Total length of wrapped data
 +
|}
 +
 +
This is [https://github.com/MinecraftForge/MinecraftForge/blob/16bfd8cef1d12ee9ca0de1122addaf9916767ae9/src/main/java/net/minecraftforge/fml/common/network/internal/FMLProxyPacket.java#L173-L178 followed] by a series of additional <code>FML|MP</code> 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 <!-- (0x100000 - 0x50 - 1) * 255 - the -1 is different from what FML actually checks, but matches what it seems to encode --> 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, [http://wiki.vg/index.php?title=Protocol&oldid=6003#Plugin_Message_2 plugin channel packets] are length prefixed, while in 1.8 they are not.  However, forge makes [https://github.com/MinecraftForge/MinecraftForge/blob/605457deecba2144d69113d3c9ce6589021f542b/fml/patches/minecraft/net/minecraft/network/play/server/S3FPacketCustomPayload.java.patch 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.
 +
 +
{{Warning|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":
 +
 +
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L154-L240 ModIdData] ===
 +
 +
Contains numeric ID mapping for blocks and items.
 +
 +
{{Need Info|Again, what are blockSubstitutions and itemSubstitutions?}}
 +
 +
{| class="wikitable"
 +
! Channel
 +
! Bound to
 +
! colspan="2"|Name
 +
! colspan="2"|Type
 +
! Notes
 +
|-
 +
| rowspan="8"|<code>FML|HS</code>
 +
| rowspan="8"|Client
 +
| colspan="2"|Discriminator
 +
| colspan="2"|Byte
 +
| Always 3 for ModIdData
 +
|-
 +
| colspan="2"|Mapping length
 +
| colspan="2"|VarInt
 +
| Length of the following mapping
 +
|-
 +
| rowspan="2"|Mapping
 +
| Name
 +
| rowspan="2"|Array
 +
| String
 +
| Name of the block/item. Prefixed with a [https://github.com/MinecraftForge/MinecraftForge/blob/1.7.10/fml/src/main/java/cpw/mods/fml/common/registry/GameData.java#L763-L766 discriminator byte], 01 = block, 02 = item.
 +
|-
 +
| ID
 +
| VarInt
 +
| Numeric ID of the item.
 +
|-
 +
| colspan="2"|Block substitutions length
 +
| colspan="2"|VarInt
 +
| Length of the following array
 +
|-
 +
| colspan="2"|Block substitutions
 +
| colspan="2"|Array of strings
 +
| Block substitutions
 +
|-
 +
| colspan="2"|Item substitutions length
 +
| colspan="2"|VarInt
 +
| Length of the following array
 +
|-
 +
| colspan="2"|Item substitutions
 +
| colspan="2"|Array of strings
 +
| Item substitutions
 
  |}
 
  |}

Latest revision as of 16:19, 15 May 2019

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 mod information. Example for 1.13.2:

{
  "description": {
    "text": "A Minecraft Server"
  },
  "players": {
    "max": 20,
    "online": 0
  },
  "version": {
    "name": "1.13.2",
    "protocol": 404
  },
  "forgeData": {
    "channels": [
      {
        "res": "ironchest:main_channel",
        "version": "1",
        "required": false
      }
    ],
    "mods": [
      {
        "modId": "forge",
        "modmarker": "ANY"
      },
      {
        "modId": "ironchest",
        "modmarker": "1.13.2-8.0.2"
      }
    ],
    "fmlNetworkVersion": 2
  }
}

Warning.png The key modId has a capital 'I', unlike modid pre-1.13


Pre-1.13

1.12.2 and earlier use a different JSON structure:

{
    "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.

Warning.png 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.

Huh.png 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

Huh.png 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.

Warning.png 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.

Huh.png 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. Prefixed with a discriminator byte, 01 = block, 02 = 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