Difference between revisions of "Minecraft Forge Handshake"

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(Add packet 3.)
(Revert to revision 7218 - wrong page, whoops)
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HEADING = Work in progress! |
 
HEADING = Work in progress! |
 
CONTENT =   
 
CONTENT =   
−
This page is a work in progress, and may be missing information.
+
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.
 
}}
 
}}
  
−
Documentation of the [http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2520465 World Downloader mod]'s plugin channel configuration system.
+
This page will document the changes made in the '''[https://minecraftforge.net/ Minecraft Forge] protocol'''.
  
−
See [https://github.com/Pokechu22/WorldDownloader/blob/master/src/wdl/WDLPluginChannels.java here].
+
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].
  
−
== Note ==
+
== Changes to [[Server List Ping]] ==
  
−
All types here are as specified by Java's [http://docs.oracle.com/javase/7/docs/api/java/io/DataOutputStream.html <code>DataOutputStream</code>] and can be read using a [http://docs.oracle.com/javase/7/docs/api/java/io/DataInputStream.html <code>DataInputStream</code>], unless otherwise noted.
+
When forge is installed, the [[Server List Ping]] changes with an additional <code>modinfo</code>. Example:
  
−
== <code>WDL|INIT</code> ==
+
<syntaxhighlight lang="javascript">
 +
{
 +
    "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"
 +
            }
 +
        ]
 +
    }
 +
}
 +
</syntaxhighlight>
  
−
<code>WDL|INIT</code> is sent to show that World Downloader is ready to receive new permissions.  Don't send permissions until this is received - it may still be saving from another part of the map or another sever where it has permission, and revoking permission in the middle is a Bad Thing&trade;.
+
So, to test whether forge is installed, look for the <code>modinfo</code> key and then a <code>type</code> of <code>FML</code>
  
−
{| class="wikitable"
+
The <code>modList</code> contains each installed mod's version and ID.
−
  ! Bound To
+
 
−
! Field Name
+
{{warning|The key <code>modList</code> has a capital 'L', unlike any other key in the ping result!}}
−
! Field Type
+
 
−
! Notes
+
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()].
−
|-
+
 
−
  | rowspan="2" | Server
+
== Connection to a forge server ==
−
| Mod version
+
 
−
  | UTF8 string
+
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]].
−
  | This is a byte array; the remainder of the packet is this value (there is no length specified).  In some older versions of the mod, this is not present and the length of the packet is 0.
+
 
−
  |}
+
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.
 +
 
 +
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 [[Protocol FAQ#What's the normal login sequence for a client?|normal login sequence]], right after [[Protocol#Login Success|0x02 Login Success]] is received by the client.
 +
 
 +
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)?}}
 +
 
 +
* '''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''': 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''': A ClientHello packet is sent on the <code>FML|HS</code> channel.
 +
* '''C→S''': A ModList packet is sent.
 +
* Server checks that the mod list is compatible. If incompatible, the player is [[Protocol#Disconnect|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 <code>WAITINGSERVERDATA</code> (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 <code>WAITINGSERVERCOMPLETE</code> (3).
 +
* '''S→C''': A HandshakeAck packet is sent with phase being <code>WAITINGCACK</code> (2).
 +
* '''C→S''': A HandshakeAck packet is sent with phase being <code>PENDINGCOMPLETE</code> (4).
 +
* '''S→C''': A HandshakeAck packet is sent with phase being <code>COMPLETE</code> (3).
 +
* The server sends a "CompleteHandshake" packet '''to itself'''.  (Not sure why)
 +
* '''C→S''': A HandshakeAck packet is sent with phase being <code>COMPLETE</code> (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 [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.
  
−
== <code>WDL|CONTROL</code> ==
+
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.
  
−
This channel is sent from the server to the client to specify various permissions.  It uses a 4-byte integer at the start to indicate a section.  There is no required order, but it is conventional to put them in numerical order.
+
Discriminator bytes change depending on the channel: 0 for <code>FML|HS</code> means something different from 0 for <code>FML</code>.
  
−
In all cases unless otherwise indicated, if a packet is not sent its values are treated as 'true'.
+
== <code>FML|HS</code> Packet structure ==
  
−
=== Default values ===
+
<code>FML|HS</code> is used to perform the handshake.  Discriminator bytes are found in [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeCodec.java FMLHandshakeCodec].
  
−
Specifies the default value used for permissions not sent.
+
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L33-L76 ServerHello] ===
  
−
Note that sending this packet does not actually ''change'' the value for other packets; it instead specifies an internal fallback.  Thus, this packet can be sent at any time to change this default without needing to resend other packets.
+
Starts the handshake.
  
 
{| class="wikitable"
 
{| class="wikitable"
Line 52: Line 120:
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
−
  | rowspan="2" | <code>WDL|CONTROL</code>
+
  |rowspan="6"| <code>FML|HS</code>
−
  | rowspan="2" | Client
+
  |rowspan="6"| Client
 
  | Discriminator
 
  | Discriminator
−
  | Integer
+
  | Byte
−
  | Set to 0 to indicate this packet.
+
  | Always 0 for ServerHello
 
  |-
 
  |-
−
  | Default value
+
  | FML protocol Version
−
  | Boolean
+
  | Byte
−
  | True to enable all functions not otherwise specified, false to disable them.
+
  | Determined from [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/NetworkRegistry.java#L67-L69 NetworkRegistery].  Currently <code>2</code>.
 +
|-
 +
| Override dimension
 +
| Optional Int
 +
| Only sent if protocol version is greater than 1.
 
  |}
 
  |}
  
−
=== Basic data ===
+
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L77-L95 ClientHello] ===
  
−
Sets some of the standard properties.
+
Response from the client to the ServerHello packet.
  
 
{| class="wikitable"
 
{| class="wikitable"
Line 74: Line 146:
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
−
  | rowspan="7" | <code>WDL|CONTROL</code>
+
  |rowspan="6"| <code>FML|HS</code>
−
  | rowspan="7" | Client
+
  |rowspan="6"| Server
 
  | Discriminator
 
  | Discriminator
−
  | Integer
+
  | Byte
−
  | Set to 1 to indicate this packet.
+
  | Always 1 for ClientHello.
 
  |-
 
  |-
−
  | General download enabled
+
  | FML protocol Version
−
  | Boolean
+
  | Byte
−
  | True to enable all downloading in all chunks, false to disable it.
+
  | Determined from [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/NetworkRegistry.java#L67-L69 NetworkRegistery].  Currently <code>2</code>.
 +
|}
 +
 
 +
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L96-L152 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.
 +
 
 +
{| class="wikitable"
 +
! Channel
 +
! Bound To
 +
!colspan="2"| Field Name
 +
!colspan="2"| Field Type
 +
! Notes
 
  |-
 
  |-
−
  | Save radius
+
  |rowspan="4"| <code>FML|HS</code>
−
  | Integer
+
  |rowspan="4"| Both
−
  | The distance from the player chunks can be saved from. -1 to allow all distances. This is a square distance, not a circle or a diamond.  '''Only applies if chunk caching is disabled'''.
+
  |colspan="2"| Discriminator
 +
  |colspan="2"| Byte
 +
  | Always 2 for ModList
 
  |-
 
  |-
−
  | Chunk caching enabled
+
  |colspan="2"| Number of mods
−
  | Boolean
+
  |colspan="2"| Varint
−
  | Can chunks be saved as the player moves about.  If false, then they can only save within the area indicated by save radius; if true they can save everywhere.  However, regardless of this value, if general download is disabled, they cannot download.
+
  | Number of mods below
 
  |-
 
  |-
−
  | Entity saving enabled
+
  |rowspan="2"| Mods
−
  | Boolean
+
| Mod name
−
  | True to allow the mod to save entities, false to force entities to be removed from the world.  This only applies to chunks that can be saved.
+
|rowspan="2"| Array
 +
  | String
 +
  | Name of the mod
 
  |-
 
  |-
−
  | Tile entity saving enabled
+
  | Mod version
−
  | Boolean
+
  | String
−
  | True to allow the mod to save tile entities, false to force tile entities to be removed from the world.  This only applies to chunks that can be saved.
+
  | Version of the mod
−
|-
 
−
| Container saving enabled
 
−
| Boolean
 
−
| True to allow the mod to save containers (a subset of tile entities that require manual interaction to be saved, mainly chests).  This only applies if tile entities are allowed to be saved and only applies in chunks that can be saved.
 
 
  |}
 
  |}
  
−
=== Entity track distances ===
+
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L154-L240 RegistryData] ===
  
−
Used to specify any custom entity track distances on the server, because they may change on spigot servers.  More info on why this is needed on [https://github.com/Pokechu22/WorldDownloader/wiki/Entities-GUI#what-are-track-distances the project wiki].
+
{{Need Info|Explain exactly what registries are, and figure out what Substitutions is}}
  
−
The client is free to ignore these values, and will use sane defaults based off of whether the server's brand includes "spigot" if they are not sent.  However, by default these values are used.
+
The server sends several of this packet, one for each registry.  It'll keep sending them until the hasMore value is no longer true.
  
 
{| class="wikitable"
 
{| class="wikitable"
 
  ! Channel
 
  ! Channel
 
  ! Bound To
 
  ! Bound To
−
  ! colspan="2" | Field Name
+
  !colspan="2"| Field Name
−
  ! colspan="2" | Field Type
+
  !colspan="2"| Field Type
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
−
  | rowspan="4" | <code>WDL|CONTROL</code>
+
  |rowspan="8"| <code>FML|HS</code>
−
  | rowspan="4" | Client
+
  |rowspan="8"| Both
−
  | colspan="2" | Discriminator
+
  |colspan="2"| Discriminator
−
  | colspan="2" | Integer
+
  |colspan="2"| Byte
−
  | Set to 2 to indicate this packet.
+
  | Always 3 for RegistryData.
 +
|-
 +
|colspan="2"| Has more
 +
|colspan="2"| Boolean
 +
| Marks whether another RegistryData packet will be sent after this.
 +
|-
 +
|colspan="2"| Name
 +
|colspan="2"| String
 +
| Name of the registry for this packet
 
  |-
 
  |-
−
  | colspan="2" | Number of ranges
+
  |colspan="2"| Number of ids
−
  | colspan="2" | Integer
+
  |colspan="2"| Varint
−
  | Size of the following array.
+
  | Number of ids sent below
 
  |-
 
  |-
−
  | rowspan="2" | Values
+
  |rowspan="2"| Ids
−
  | Entity name
+
  | Name
−
  | rowspan="2" | Array
+
  |rowspan="2"| Array
 
  | String
 
  | String
−
  | The name of the entity, as used in the savegame ID.
+
  | Name of the thing
 
  |-
 
  |-
−
  | Track distance
+
  | Id
−
  | Integer
+
  | Varint
−
  | The track distance of the entity, in blocks.
+
  | Numerical id of the thing
 +
|-
 +
|colspan="2"| Number of substitutions
 +
|colspan="2"| Varint
 +
| Number of substitutions sent below
 +
|-
 +
|colspan="2"| Substitutions
 +
|colspan="2"| Array of strings
 +
| Each substitution
 
  |}
 
  |}
  
−
=== Permission request info ===
+
=== [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeMessage.java#L241-L264 HandshakeAck] ===
  
−
Information used with permission requests.
+
Confirms that the client is able to use the settings sent previously.
  
 
{| class="wikitable"
 
{| class="wikitable"
 
  ! Channel
 
  ! Channel
−
  ! Bound to
+
  ! Bound To
 
  ! Field Name
 
  ! Field Name
 
  ! Field Type
 
  ! Field Type
 
  ! Notes
 
  ! Notes
 
  |-
 
  |-
−
  | rowspan="3" | <code>WDL|CONTROL</code>
+
  |rowspan="6"| <code>FML|HS</code>
−
  | rowspan="3" | Client
+
  |rowspan="6"| Both
 
  | Discriminator
 
  | Discriminator
−
  | Integer
+
  | Byte
−
  | Set to 3 to indicate this packet.
+
  | Always -1 (255) for HandshakeAck
 
  |-
 
  |-
−
  | Requests enabled
+
  | Phase
−
  | Boolean
+
  | Byte
−
  | Whether requests work on this server.  In '''all cases''' this should be true unless you have an out of date plugin... in which case you probably wouldn't be sending this anyways.
+
  | The current phase, which is the ordinal (0-indexed) in the [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeClientState.java FMLHandshakeClientState] or [https://github.com/MinecraftForge/MinecraftForge/blob/ebe9b6d4cbc4a5281c386994f1fbda04df5d2e1f/src/main/java/net/minecraftforge/fml/common/network/handshake/FMLHandshakeServerState.java 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).
 +
  |}
 +
 
 +
=== [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. Does not appear to be sent.  There is no payload beyond the discriminator byte.
 +
 
 +
{| class="wikitable"
 +
! Channel
 +
! Bound To
 +
! Field Name
 +
! Field Type
 +
! Notes
 
  |-
 
  |-
−
  | Request message
+
  |rowspan="6"| <code>FML|HS</code>
−
  | String
+
|rowspan="6"| Client
−
  | A message displayed to players on the permissions and permissions request screen. You can use this to display rules relating to use of the mod, or explain how you want it to be used, or what info to include in the request, etc.
+
  | Discriminator
 +
  | Byte
 +
  | Always -2 (254) for HandshakeReset
 
  |}
 
  |}
  
−
== <code>WDL|REQUEST</code> ==
+
== 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.
  
−
Used for permission requests.
+
{{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.
 +
|-
 +
| 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
 +
|}

Revision as of 20:01, 25 January 2016

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.

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 (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).
  • 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 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

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

Huh.png 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

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.
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
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