node-minecraft-protocol: brand and Client Information are sent only on the first configuration entry — vanilla re-sends them after every server transfer

[u9g]

Title: node-minecraft-protocol: brand and Client Information are sent only on the first configuration entry — vanilla re-sends them after every server transfer

Body:

What happens

enterConfigState gates brand + settings behind a one-shot flag:

// Brand then Client Information are sent once per connection, on the first entry
// into configuration; re-entering configuration from play sends nothing else.
if (!sentClientInformation) { ... }

With Velocity modern transfers (play → configuration re-entry → play), every server the client is transferred to sees a client with no brand and no Client Information. The vanilla client sends both on every configuration phase (each world gets them again). Some servers validate the brand per world; without it the client is treated as non-vanilla (we were console-kicked "Cheating" on a network whose game servers check, while its lobbies didn't).

Fix (field-verified)

Send both on every entry into configuration (the original Hypixel ordering constraint is preserved — they still go out immediately on entry):

sentClientInformation = true
client.write('custom_payload', { channel: 'minecraft:brand', data: ... })
client.write('settings', { ... })

Fresh-account survival in a previously-kicking game world went from <60s to indefinite after this plus the pong-order and chunk-stall fixes. Single change alone is necessary but not sufficient; the three together stopped the kicks.

Patch: patches/nmp-brand-settings-every-config.patch (23 lines, src/client/play.js).

Patch

diff --git a/packages/minecraft-protocol/src/client/play.js b/packages/minecraft-protocol/src/client/play.js
index d3e3fd0..9be98ee 100644
--- a/packages/minecraft-protocol/src/client/play.js
+++ b/packages/minecraft-protocol/src/client/play.js
@@ -56,29 +56,28 @@ module.exports = function (client, options) {
         client.write('configuration_acknowledged', {})
       }
       client.state = states.CONFIGURATION
-      // Brand then Client Information are sent once per connection, on the first entry
-      // into configuration; re-entering configuration from play sends nothing else. Some
-      // servers (e.g. Hypixel) close the socket unless Client Information arrives before
-      // they send finish_configuration
-      if (!sentClientInformation) {
-        sentClientInformation = true
-        client.write('custom_payload', {
-          channel: 'minecraft:brand',
-          data: client.serializer.proto.createPacketBuffer('string', options.brand ?? 'vanilla')
-        })
-        const clientSettings = options.clientSettings || {}
-        client.write('settings', {
-          locale: clientSettings.locale ?? 'en_us',
-          viewDistance: clientSettings.viewDistance ?? 12,
-          chatFlags: clientSettings.chatFlags ?? 0,
-          chatColors: clientSettings.chatColors ?? true,
-          skinParts: clientSettings.skinParts ?? 127,
-          mainHand: clientSettings.mainHand ?? 1,
-          enableTextFiltering: clientSettings.enableTextFiltering ?? false,
-          enableServerListing: clientSettings.enableServerListing ?? true,
-          particleStatus: clientSettings.particleStatus ?? 'all'
-        })
-      }
+      // Brand then Client Information go out on EVERY entry into configuration, including
+      // re-entry after a server transfer: the vanilla client re-sends both for each world,
+      // and servers that validate the brand per world never see it again otherwise.
+      // Hypixel closes the socket unless Client Information arrives before they send
+      // finish_configuration, hence the send right here rather than later.
+      sentClientInformation = true
+      client.write('custom_payload', {
+        channel: 'minecraft:brand',
+        data: client.serializer.proto.createPacketBuffer('string', options.brand ?? 'vanilla')
+      })
+      const clientSettings = options.clientSettings || {}
+      client.write('settings', {
+        locale: clientSettings.locale ?? 'en_us',
+        viewDistance: clientSettings.viewDistance ?? 12,
+        chatFlags: clientSettings.chatFlags ?? 0,
+        chatColors: clientSettings.chatColors ?? true,
+        skinParts: clientSettings.skinParts ?? 127,
+        mainHand: clientSettings.mainHand ?? 1,
+        enableTextFiltering: clientSettings.enableTextFiltering ?? false,
+        enableServerListing: clientSettings.enableServerListing ?? true,
+        particleStatus: clientSettings.particleStatus ?? 'all'
+      })
       // The server omits the registry entries of every pack in the reply, so a pack may
       // only be listed when its data is available locally
       client.once('select_known_packs', (packet) => {

[u9g]

Not a divergence from vanilla: the vanilla client sends brand and Client Information once per connection, and re-entering configuration from play carries neither.

Client, 1.21.4 (identical in 1.20.2, 1.21.11 and 26.1):

• ClientHandshakePacketListenerImpl.handleLoginFinished: login_acknowledged → custom_payload minecraft:brand → client_information. This is the only place the client constructs a BrandPayload.
• ClientPacketListener.handleConfigurationStart: builds the new configuration listener, sends configuration_acknowledged, switches the outbound protocol. Nothing else is written.
• ClientPacketListener.broadcastClientInformation only runs from Options.save() and only when the value differs from remoteClientInformation; the play-state login packet does not trigger it.

Server, 1.21.4: ServerGamePacketListenerImpl.handleConfigurationAcknowledged carries player.clientInformation() into the cookie of the new ServerConfigurationPacketListenerImpl, so a vanilla server never expects either packet again on the same connection.

Proxies do the per-backend re-send themselves, from what the client sent during its first configuration:

• Velocity (dev/3.0.0): backend/LoginSessionHandler writes the cached ClientSettingsPacket to the backend right after LoginAcknowledged; client/ClientConfigSessionHandler.handleBackendFinishUpdate writes the cached brand before finishing the backend's configuration. Both are captured by ClientConfigSessionHandler.handle(ClientSettingsPacket) / handle(PluginMessagePacket).
• BungeeCord (master): ServerConnector and UpstreamBridge write the pending connection's brand message and user.getSettings() to each server they connect to.

So every backend behind Velocity or Bungee already sees a brand and Client Information for a client on PrismarineJS/node-minecraft-protocol#1522, and a client that re-sends both on every start_configuration is the one that differs from what those servers see from vanilla. A transfer packet is a different case: vanilla opens a new connection and sends both again, and nmp does not implement transfer, so a consumer following one creates a new client and gets a fresh handshake anyway.

Leaving #1522 as it is. The survival change reported here landed together with the pong-order (#45) and chunk-stall (#43) fixes, which are the ones that touch anticheat-visible timing.

Sources: u9g/mc-decompiled src/1.21.4/net/minecraft/client/multiplayer/{ClientHandshakePacketListenerImpl,ClientPacketListener}.java, src/1.21.4/net/minecraft/server/network/ServerGamePacketListenerImpl.java; PaperMC/Velocity dev/3.0.0; SpigotMC/BungeeCord master.