Difference between revisions of "Protocol FAQ"

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People very, very often have questions regarding the Minecraft Modern [[Protocol]], so we'll try to address some of the most common ones on this document. If you're still having trouble, join us on IRC, channel #mcdevs on irc.freenode.net.
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People very, very often have questions regarding the Minecraft Modern [[Protocol]], so we'll try to address some of the most common ones on this document. If you're still having trouble, join us on IRC, channel [irc://irc.freenode.net/mcdevs #mcdevs on irc.freenode.net].
  
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===Is the protocol documentation complete?===
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== Is the protocol documentation complete? ==
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<blockquote>
 
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Depending on your definition, '''yes'''! All packet types are known and their layout documented. Some finer details are missing, but everything you need to make functional programs is present. We [[MC-Ver|have a script]] that monitors for new versions of the client and notifies us, allowing us to quickly document new packets.
 
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</blockquote>
 
  
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===What's the normal login sequence for a client?===
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Depending on your definition, ''yes''! All packet types are known and their layout documented. Some finer details are missing, but everything you need to make functional programs is present. We also collect information on the [[pre-release protocol]] changes, allowing us to quickly document new releases.
  
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See [[Authentication]] for communication with minecraft.net
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== What's the normal login sequence for a client? ==
  
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<blockquote>
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See [[Authentication]] for communication with Mojang's servers.
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The recommended login sequence looks like this, where C is the client and S is the server:<br /><br />
 
  
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#Client connects to server
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The recommended login sequence looks like this, where '''C''' is the client and '''S''' is the server:
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#'''C->S''' 0x02 handshake
 
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#'''S->C''' 0xFD encryption request - server sends its public key and 4 random bytes
 
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#Client generates symmetric key (shared secret)
 
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#Client authenticates via session.minecraft.net
 
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#Client encrypts these 4 bytes with the servers public key.
 
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#'''C->S''' 0xFC encryption response - client encrypts shared secret with server's public key and sends along with encrypted 4 bytes
 
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#Server checks that the encrypted bytes match
 
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#Server decrypts shared secret with its private key
 
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#Server checks player authenticity via session.minecraft.net
 
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#'''S->C''' 0xFC encryption response - empty payload meaning two zero length byte arrays and two zero shorts
 
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#Server enables AES stream encryption
 
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#Client enables AES stream encryption
 
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#'''C->S''' 0xCD - Payload of 0 (byte)
 
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#'''S->C''' : Either kicks (invalid login) or sends a login response
 
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#'''S->C''' : Sends chunks and entities
 
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#'''S->C''' : Sends spawn position
 
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#'''S->C''' : Sends inventory [Need to verify this since inventory changed] (beta 1.1_02: looks like Window items with type=0, then a Set slot with window id = -1 and slot = -1)
 
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#'''S->C''' : Tell the client they're ready to spawn by sending a position + look packet.
 
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#'''C->S''' : Sends a position + look packet to confirm the spawn position
 
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</blockquote>
 
  
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===I think I've done everything right, but…===
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# Client connects to server
 +
# '''C→S''': [[Protocol#Handshake|Handshake]] State=2
 +
# '''C→S''': [[Protocol#Login Start|Login Start]]
 +
# '''S→C''': [[Protocol#Encryption Request|Encryption Request]]
 +
# Client auth
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# '''C→S''': [[Protocol#Encryption Response|Encryption Response]]
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# Server auth, both enable encryption
 +
# '''S→C''': [[Protocol#Set Compression|Set Compression]] (Optional, enables compression)
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# '''S→C''': [[Protocol#Login Success|Login Success]]
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# '''S→C''': [[Protocol#Join Game|Join Game]]
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# '''S→C''': [[Protocol#Plugin Message (clientbound) |Plugin Message]]: [[Plugin channel#minecraft%3Abrand|<code>minecraft:brand</code>]] with the server's brand (Optional)
 +
# '''S→C''': [[Protocol#Server Difficulty|Server Difficulty]] (Optional)
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# '''S→C''': [[Protocol#Player Abilities (clientbound)|Player Abilities]] (Optional)
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# '''C→S''': [[Protocol#Plugin Message (serverbound)|Plugin Message]]: [[Plugin channel#minecraft%3Abrand|<code>minecraft:brand</code>]] with the client's brand (Optional)
 +
# '''C→S''': [[Protocol#Client Settings|Client Settings]]
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# '''S→C''': [[Protocol#Chunk Data|Chunk Data]] (One sent for each chunk in a square centered on the player's position)
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# '''S→C''': [[Protocol#Spawn Position|Spawn Position]] (“home” spawn, not where the client will spawn on login)
 +
# '''S→C''': [[Protocol#Player Position And Look (clientbound)|Player Position And Look]] (Required, tells the client they're ready to spawn)
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# '''C→S''': [[Protocol#Teleport Confirm|Teleport Confirm]]
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# '''C→S''': [[Protocol#Player Position And Look (serverbound)|Player Position And Look]] (to confirm the spawn position)
 +
# '''C→S''': [[Protocol#Client Status|Client Status]] (sent either before or while receiving chunks, further testing needed, server handles correctly if not sent)
 +
# '''S→C''': inventory, entities, etc
  
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====…my player isn't spawning!====
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== Offline mode ==
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<blockquote>
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If the server is in offline mode, it will not send the [[Protocol#Encryption Request|Encryption Request]] packet, and likewise, the client should not send [[Protocol#Encryption Response|Encryption Response]]. In this case, encryption is never enabled, and no authentication is performed.
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After sending the common-sense packets ([[protocol#handshake_0x02|Handshake]], [[protocol#login_request_0x01|Login]], [[protocol#player_inventory_0x05|Inventory]], [[protocol#spawn_position_0x06|Compass]] and [[protocol#map_chunk_0x33|Chunks]]) you need to finally send the player their [[protocol#player_position_look_0x0d|initial position]] for them to leave the "Loading Map" screen.
 
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</blockquote>
 
  
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'''Note that if the following steps are taken, a Minecraft client will spawn the player:'''
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Clients can tell that a server is in offline mode if the server sends a [[Protocol#Login Success|Logic Success]] without sending [[Protocol#Encryption Request|Encryption Request]].
  
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1) Do Handshake (See [[Protocol_Encryption|Protocol Encryption]])<br />
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== I think I've done everything right, but… ==
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2) Send [[protocol#spawn_position_0x06|Spawn Position]] PDU<br />
 
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3) Send [[protocol#player_position_look_0x0d|Player Position and Look]] PDU<br />
 
  
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While the above steps are sufficient for Minecraft 1.4.5, it is good form to send PDUs that inform the client about the world around the player before allowing the player to spawn.
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=== …my player isn't spawning! ===
  
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====…my client isn't receiving complete map chunks!====
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After sending the common-sense packets ([[Protocol#Handshake|Handshake]], [[Protocol#Login Start|Login Start]], [[Protocol#Window Items|inventory]], [[Protocol#Spawn Position|compass]], and [[Protocol#Chunk Data|chunks]]), you need to finally send the player their [[Protocol#Player Position And Look|initial position]] for them to leave the “Loading Map” screen.
 +
 
 +
''Note that if the following steps are taken, a Minecraft client will spawn the player:''
 +
 
 +
# Do Handshake (see [[Protocol Encryption]])
 +
# Send [[Protocol#Spawn Position|Spawn Position]] packet
 +
# Send [[Protocol#Player Position And Look|Player Position And Look]] packet
 +
 
 +
While the above steps are sufficient for Minecraft 1.4.5, it is good form to send packets that inform the client about the world around the player before allowing the player to spawn.
 +
 
 +
=== …my client isn't receiving complete map chunks! ===
 
''Main article: [[How to Write a Client]]''
 
''Main article: [[How to Write a Client]]''
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<blockquote>
 
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The standard Minecraft server sends full chunks only when your client is sending player status update packets (any of [[protocol#Player (0x0A)|Player (0x0A)]] through [[protocol#Player Position & Look (0x0D)|Player Position & Look (0x0D)]]).
 
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</blockquote>
 
  
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====…all connecting clients spasm and jerk uncontrollably!====
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The standard Minecraft server sends full chunks only when your client is sending player status update packets (any of [[Protocol#Player|Player (0x03)]] through [[Protocol#Player Position And Look|Player Position And Look (0x06)]]).
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<blockquote>
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For newer clients, your server needs to send 49 chunks ahead of time, not just one. Send a 7x7 square of chunks, centered on the connecting client's position, '''before''' spawning them.
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=== …all connecting clients spasm and jerk uncontrollably! ===
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</blockquote>
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 +
For newer clients, your server needs to send 49 chunks ahead of time, not just one. Send a 7×7 square of chunks, centered on the connecting client's position, ''before'' spawning them.
 +
 
 +
=== …the client is trying to send an invalid packet that begins with 0xFE01 ===
 +
 
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The client is attempting a [[Server_List_Ping#1.6|legacy ping]], this happens if your server did not respond to the [[Server List Ping]] properly, including if it sent malformed JSON.
 +
 
 +
=== ...the client disconnects after some time with a "Timed out" error ===
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 +
The server is expected to send a [[Protocol#Keep Alive (clientbound)|Keep Alive]] packet every second, and the client should respond with the serverbound version of that packet. If either party does not receive keep alives for some period of time, they will disconnect.
  
 
[[Category:Protocol Details]]
 
[[Category:Protocol Details]]
 
[[Category:Minecraft Modern]]
 
[[Category:Minecraft Modern]]
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 +
== How do I open/save a command block? ==
 +
 +
The process to actually open the command block window clientside is somewhat complex; the client actually uses the [[Protocol#Update Block Entity|Update Block Entity (0x09)]] packet to open it.
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 +
First, the client must have at least an {{Minecraft Wiki|Server.properties#op-permission-level|OP permission level}} of 2, or else the client will refuse to open the command block.  (The op permission level is set with the [[Protocol#Entity Status|Entity Status]] packet)
 +
 +
To actually open the command block:
 +
 +
# '''C'''→'''S''': [[Protocol#Player Block Placement|Player Block Placement (0x1C)]], with the position being the command block that was right-clicked.
 +
# '''S'''→'''C''': [[Protocol#Update Block Entity|Update Block Entity (0x09)]], with the NBT of the command block.
 +
 +
And to save it, use the [[Plugin channels#MC.7CAutoCmd|<code>MC|AutoCmd</code> plugin channel]].

Revision as of 05:45, 1 July 2019

People very, very often have questions regarding the Minecraft Modern Protocol, so we'll try to address some of the most common ones on this document. If you're still having trouble, join us on IRC, channel #mcdevs on irc.freenode.net.

Is the protocol documentation complete?

Depending on your definition, yes! All packet types are known and their layout documented. Some finer details are missing, but everything you need to make functional programs is present. We also collect information on the pre-release protocol changes, allowing us to quickly document new releases.

What's the normal login sequence for a client?

See Authentication for communication with Mojang's servers.

The recommended login sequence looks like this, where C is the client and S is the server:

  1. Client connects to server
  2. C→S: Handshake State=2
  3. C→S: Login Start
  4. S→C: Encryption Request
  5. Client auth
  6. C→S: Encryption Response
  7. Server auth, both enable encryption
  8. S→C: Set Compression (Optional, enables compression)
  9. S→C: Login Success
  10. S→C: Join Game
  11. S→C: Plugin Message: minecraft:brand with the server's brand (Optional)
  12. S→C: Server Difficulty (Optional)
  13. S→C: Player Abilities (Optional)
  14. C→S: Plugin Message: minecraft:brand with the client's brand (Optional)
  15. C→S: Client Settings
  16. S→C: Chunk Data (One sent for each chunk in a square centered on the player's position)
  17. S→C: Spawn Position (“home” spawn, not where the client will spawn on login)
  18. S→C: Player Position And Look (Required, tells the client they're ready to spawn)
  19. C→S: Teleport Confirm
  20. C→S: Player Position And Look (to confirm the spawn position)
  21. C→S: Client Status (sent either before or while receiving chunks, further testing needed, server handles correctly if not sent)
  22. S→C: inventory, entities, etc

Offline mode

If the server is in offline mode, it will not send the Encryption Request packet, and likewise, the client should not send Encryption Response. In this case, encryption is never enabled, and no authentication is performed.

Clients can tell that a server is in offline mode if the server sends a Logic Success without sending Encryption Request.

I think I've done everything right, but…

…my player isn't spawning!

After sending the common-sense packets (Handshake, Login Start, inventory, compass, and chunks), you need to finally send the player their initial position for them to leave the “Loading Map” screen.

Note that if the following steps are taken, a Minecraft client will spawn the player:

  1. Do Handshake (see Protocol Encryption)
  2. Send Spawn Position packet
  3. Send Player Position And Look packet

While the above steps are sufficient for Minecraft 1.4.5, it is good form to send packets that inform the client about the world around the player before allowing the player to spawn.

…my client isn't receiving complete map chunks!

Main article: How to Write a Client

The standard Minecraft server sends full chunks only when your client is sending player status update packets (any of Player (0x03) through Player Position And Look (0x06)).

…all connecting clients spasm and jerk uncontrollably!

For newer clients, your server needs to send 49 chunks ahead of time, not just one. Send a 7×7 square of chunks, centered on the connecting client's position, before spawning them.

…the client is trying to send an invalid packet that begins with 0xFE01

The client is attempting a legacy ping, this happens if your server did not respond to the Server List Ping properly, including if it sent malformed JSON.

...the client disconnects after some time with a "Timed out" error

The server is expected to send a Keep Alive packet every second, and the client should respond with the serverbound version of that packet. If either party does not receive keep alives for some period of time, they will disconnect.

How do I open/save a command block?

The process to actually open the command block window clientside is somewhat complex; the client actually uses the Update Block Entity (0x09) packet to open it.

First, the client must have at least an OP permission level of 2, or else the client will refuse to open the command block. (The op permission level is set with the Entity Status packet)

To actually open the command block:

  1. C→S: Player Block Placement (0x1C), with the position being the command block that was right-clicked.
  2. S→C: Update Block Entity (0x09), with the NBT of the command block.

And to save it, use the MC|AutoCmd plugin channel.

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