Add decompressed-bytes-per-second rate limit, update packet limiter defaults (#1786)

* Add decompressed-bytes-per-second packet limiter, update defaults
* Revert "Add compression ratio limiter"

Author: WouterGritter

proxy/src/main/java/com/velocitypowered/proxy/config/VelocityConfiguration.java

@@ -31,6 +31,7 @@
 import com.velocitypowered.proxy.config.migration.KeyAuthenticationMigration;
 import com.velocitypowered.proxy.config.migration.MiniMessageTranslationsMigration;
 import com.velocitypowered.proxy.config.migration.MotdMigration;
+import com.velocitypowered.proxy.config.migration.PacketLimiterMigration;
 import com.velocitypowered.proxy.config.migration.TransferIntegrationMigration;
 import com.velocitypowered.proxy.util.AddressUtil;
 import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
@@ -511,7 +512,8 @@ public static VelocityConfiguration read(Path path) throws IOException {
           new KeyAuthenticationMigration(),
           new MotdMigration(),
           new MiniMessageTranslationsMigration(),
-          new TransferIntegrationMigration()
+          new TransferIntegrationMigration(),
+          new PacketLimiterMigration()
       };
 
       for (final ConfigurationMigration migration : migrations) {
@@ -1004,12 +1006,13 @@ public boolean isEnabled() {
   /**
    * Configuration for packet limiting.
    *
-   * @param interval the interval in seconds to measure packets over
-   * @param pps      the maximum number of packets per second allowed
-   * @param bytes    the maximum number of bytes per second allowed
+   * @param interval                the interval in seconds to measure packets over
+   * @param pps                     the maximum number of packets per second allowed
+   * @param bytes                   the maximum number of bytes per second allowed
+   * @param bytesAfterDecompression the maximum number of decompressed bytes per second allowed
    */
-  public record PacketLimiterConfig(int interval, int pps, int bytes) {
-    public static PacketLimiterConfig DEFAULT = new PacketLimiterConfig(7, 500, -1);
+  public record PacketLimiterConfig(int interval, int pps, int bytes, int bytesAfterDecompression) {
+    public static PacketLimiterConfig DEFAULT = new PacketLimiterConfig(7, -1, -1, 5242880);
 
     /**
      * returns a PacketLimiterConfig from a config section, or the default if the section is null.
@@ -1022,7 +1025,8 @@ public static PacketLimiterConfig fromConfig(CommentedConfig config) {
         return new PacketLimiterConfig(
             config.getIntOrElse("interval", DEFAULT.interval()),
             config.getIntOrElse("packets-per-second", DEFAULT.pps()),
-            config.getIntOrElse("bytes-per-second", DEFAULT.bytes())
+            config.getIntOrElse("bytes-per-second", DEFAULT.bytes()),
+            config.getIntOrElse("decompressed-bytes-per-second", DEFAULT.bytesAfterDecompression())
         );
       } else {
         return DEFAULT;

proxy/src/main/java/com/velocitypowered/proxy/config/migration/ConfigurationMigration.java

@@ -29,7 +29,8 @@ public sealed interface ConfigurationMigration
                 KeyAuthenticationMigration,
                 MotdMigration,
                 MiniMessageTranslationsMigration,
-                TransferIntegrationMigration {
+                TransferIntegrationMigration,
+                PacketLimiterMigration {
   boolean shouldMigrate(CommentedFileConfig config);
 
   void migrate(CommentedFileConfig config, Logger logger) throws IOException;

proxy/src/main/java/com/velocitypowered/proxy/config/migration/PacketLimiterMigration.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2026 Velocity Contributors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package com.velocitypowered.proxy.config.migration;
+
+import static com.velocitypowered.proxy.config.VelocityConfiguration.PacketLimiterConfig.DEFAULT;
+
+import com.electronwill.nightconfig.core.file.CommentedFileConfig;
+import org.apache.logging.log4j.Logger;
+
+/**
+ * Configuration migration for the new [packet-limiter] section.
+ * Config version 2.7 may contain this section with only the `interval`, `packets-per-second`
+ * and `bytes-per-second` attributes. Config version 2.8 enforces these exist, adds the new
+ * `decompressed-bytes-per-second` attribute, adjusts the new default, and adds comments.
+ */
+public final class PacketLimiterMigration implements ConfigurationMigration {
+
+  @Override
+  public boolean shouldMigrate(CommentedFileConfig config) {
+    return configVersion(config) < 2.8;
+  }
+
+  @Override
+  public void migrate(CommentedFileConfig config, Logger logger) {
+    config.set("packet-limiter.interval", DEFAULT.interval());
+    config.set("packet-limiter.packets-per-second", DEFAULT.pps());
+    config.set("packet-limiter.bytes-per-second", DEFAULT.bytes());
+    config.set("packet-limiter.decompressed-bytes-per-second", DEFAULT.bytesAfterDecompression());
+
+    config.setComment("packet-limiter.interval", """
+        Size of the moving time window in seconds used to calculate average rates.
+        A larger window tolerates short bursts while still enforcing the configured limits over time.""");
+
+    config.setComment("packet-limiter.packets-per-second", """
+        Maximum average number of packets per second a client may send. -1 disables this check.""");
+
+    config.setComment("packet-limiter.bytes-per-second", """
+        Maximum average number of compressed (on-wire) bytes per second a client may send. -1 disables this check.""");
+
+    config.setComment("packet-limiter.decompressed-bytes-per-second", """
+        Maximum average number of decompressed bytes per second a client may send.
+        Protects against compression bomb attacks where small packets expand to excessive sizes after decompression.
+        -1 disables this check.""");
+
+    config.set("config-version", "2.8");
+  }
+}

proxy/src/main/java/com/velocitypowered/proxy/connection/MinecraftConnection.java

@@ -38,7 +38,9 @@
 import com.velocitypowered.proxy.connection.client.InitialLoginSessionHandler;
 import com.velocitypowered.proxy.connection.client.StatusSessionHandler;
 import com.velocitypowered.proxy.network.Connections;
+import com.velocitypowered.proxy.network.limiter.SimpleBytesPerSecondLimiter;
 import com.velocitypowered.proxy.protocol.MinecraftPacket;
+import com.velocitypowered.proxy.protocol.ProtocolUtils;
 import com.velocitypowered.proxy.protocol.StateRegistry;
 import com.velocitypowered.proxy.protocol.VelocityConnectionEvent;
 import com.velocitypowered.proxy.protocol.netty.MinecraftCipherDecoder;
@@ -571,6 +573,14 @@ public void setCompressionThreshold(int threshold) {
         channel.pipeline().addBefore(MINECRAFT_DECODER, COMPRESSION_DECODER, decoder);
         channel.pipeline().addBefore(MINECRAFT_ENCODER, COMPRESSION_ENCODER, encoder);
 
+        var packetLimiterConfig = server.getConfiguration().getPacketLimiterConfig();
+        if (minecraftDecoder.getDirection() == ProtocolUtils.Direction.SERVERBOUND
+            && packetLimiterConfig.interval() > 0
+            && packetLimiterConfig.bytesAfterDecompression() > 0) {
+          decoder.setPacketLimiter(new SimpleBytesPerSecondLimiter(
+              -1, packetLimiterConfig.bytesAfterDecompression(), packetLimiterConfig.interval()));
+        }
+
         channel.pipeline().fireUserEventTriggered(VelocityConnectionEvent.COMPRESSION_ENABLED);
       }
     }

proxy/src/main/java/com/velocitypowered/proxy/network/ServerChannelInitializer.java

@@ -80,7 +80,7 @@ protected void initChannel(final Channel ch) {
     int configuredPacketsPerSecond = packetLimiterConfig.pps();
     int configuredBytes = packetLimiterConfig.bytes();
 
-    if (configuredInterval > 0 && (configuredBytes > 0 ||  configuredPacketsPerSecond > 0)) {
+    if (configuredInterval > 0 && (configuredBytes > 0 || configuredPacketsPerSecond > 0)) {
       ch.pipeline().get(MinecraftVarintFrameDecoder.class).setPacketLimiter(
           new SimpleBytesPerSecondLimiter(configuredPacketsPerSecond, configuredBytes, configuredInterval)
       );

proxy/src/main/java/com/velocitypowered/proxy/protocol/netty/MinecraftCompressDecoder.java

@@ -22,11 +22,14 @@
 import static com.velocitypowered.proxy.protocol.util.NettyPreconditions.checkFrame;
 
 import com.velocitypowered.natives.compression.VelocityCompressor;
+import com.velocitypowered.proxy.network.limiter.PacketLimiter;
 import com.velocitypowered.proxy.protocol.ProtocolUtils;
+import com.velocitypowered.proxy.util.except.QuietDecoderException;
 import io.netty.buffer.ByteBuf;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.handler.codec.MessageToMessageDecoder;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Decompresses a Minecraft packet.
@@ -44,11 +47,12 @@ public class MinecraftCompressDecoder extends MessageToMessageDecoder<ByteBuf> {
           Boolean.getBoolean("velocity.increased-compression-cap")
                   ? HARD_MAXIMUM_UNCOMPRESSED_SIZE : SERVERBOUND_MAXIMUM_UNCOMPRESSED_SIZE;
   private static final boolean SKIP_COMPRESSION_VALIDATION = Boolean.getBoolean("velocity.skip-uncompressed-packet-size-validation");
-  private static final double MAX_COMPRESSION_RATIO = Double.parseDouble(System.getProperty("velocity.max-compression-ratio", "64"));
   private final ProtocolUtils.Direction direction;
 
   private int threshold;
   private final VelocityCompressor compressor;
+  @Nullable
+  private PacketLimiter packetLimiter;
 
   /**
    * Creates a new {@code MinecraftCompressDecoder} with the specified compression {@code threshold}.
@@ -73,10 +77,13 @@ protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) t
             + " threshold %s", actualUncompressedSize, threshold);
       }
       // This message is not compressed.
+      if (packetLimiter != null && !packetLimiter.account(in.readableBytes())) {
+        throw new QuietDecoderException("Rate limit exceeded while processing packets for %s"
+            .formatted(ctx.channel().remoteAddress()));
+      }
       out.add(in.retain());
       return;
     }
-    int length = in.readableBytes();
 
     checkFrame(claimedUncompressedSize >= threshold, "Uncompressed size %s is less than"
         + " threshold %s", claimedUncompressedSize, threshold);
@@ -88,17 +95,17 @@ protected void decode(ChannelHandlerContext ctx, ByteBuf in, List<Object> out) t
       checkFrame(claimedUncompressedSize <= SERVERBOUND_UNCOMPRESSED_CAP,
               "Uncompressed size %s exceeds hard threshold of %s", claimedUncompressedSize,
               SERVERBOUND_UNCOMPRESSED_CAP);
-      double maxCompressedAllowed = length * MAX_COMPRESSION_RATIO;
-      checkFrame(claimedUncompressedSize <= maxCompressedAllowed,
-              "Uncompressed size %s exceeds ratio threshold of %s for compressed sized %s", claimedUncompressedSize,
-              maxCompressedAllowed, length);
     }
     ByteBuf compatibleIn = ensureCompatible(ctx.alloc(), compressor, in);
     ByteBuf uncompressed = preferredBuffer(ctx.alloc(), compressor, claimedUncompressedSize);
     try {
       compressor.inflate(compatibleIn, uncompressed, claimedUncompressedSize);
       checkFrame(uncompressed.writerIndex() == claimedUncompressedSize,
               "Decompressed size %s does not match claimed uncompressed size %s", uncompressed.writerIndex(), claimedUncompressedSize);
+      if (packetLimiter != null && !packetLimiter.account(claimedUncompressedSize)) {
+        throw new QuietDecoderException("Rate limit exceeded while processing packets for %s"
+            .formatted(ctx.channel().remoteAddress()));
+      }
       out.add(uncompressed);
     } catch (Exception e) {
       uncompressed.release();
@@ -116,4 +123,8 @@ public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
   public void setThreshold(int threshold) {
     this.threshold = threshold;
   }
+
+  public void setPacketLimiter(@Nullable PacketLimiter packetLimiter) {
+    this.packetLimiter = packetLimiter;
+  }
 }

proxy/src/main/resources/default-velocity.toml

@@ -1,5 +1,5 @@
 # Config version. Do not change this
-config-version = "2.7"
+config-version = "2.8"
 
 # What port should the proxy be bound to? By default, we'll bind to all addresses on port 25565.
 bind = "0.0.0.0:25565"
@@ -75,9 +75,17 @@ sample-players-in-ping = false
 enable-player-address-logging = true
 
 [packet-limiter]
+# Size of the moving time window in seconds used to calculate average rates.
+# A larger window tolerates short bursts while still enforcing the configured limits over time.
 interval = 7
-packets-per-second = 500
+# Maximum average number of packets per second a client may send. -1 disables this check.
+packets-per-second = -1
+# Maximum average number of compressed (on-wire) bytes per second a client may send. -1 disables this check.
 bytes-per-second = -1
+# Maximum average number of decompressed bytes per second a client may send.
+# Protects against compression bomb attacks where small packets expand to excessive sizes after decompression.
+# -1 disables this check.
+decompressed-bytes-per-second = 5242880
 
 [servers]
 # Configure your servers here. Each key represents the server's name, and the value