From e5e2cd4ea0e1734564f236f55af9b40950da47db Mon Sep 17 00:00:00 2001
From: Minecrell <minecrell@minecrell.net>
Date: Wed, 11 Oct 2017 18:22:50 +0200
Subject: [PATCH] Make the legacy ping handler more reliable

The Minecraft server often fails to respond to old ("legacy") pings
from old Minecraft versions using the protocol used before the switch
to Netty in Minecraft 1.7.

Due to packet fragmentation[1], we might not have all needed bytes
available when the LegacyPingHandler is called. In this case, it will
run into an error, remove the handler and continue using the modern
protocol.

This is unlikely to happen for the first two revisions of the legacy
ping protocol (used in Minecraft 1.5.x and older) since the request
consists of only one or two bytes, but happens frequently for the
last/third revision introduced in Minecraft 1.6.

It has much larger, variable packet sizes due to the inclusion of
the virtual host (the hostname/port used to connect to the server).

The solution[2] is simple: If we find more than two matching bytes,
we buffer the remaining bytes until we have enough to fully read and
respond to the request.

[1]: https://netty.io/wiki/user-guide-for-4.x.html#wiki-h3-11
[2]: https://netty.io/wiki/user-guide-for-4.x.html#wiki-h4-13

diff --git a/src/main/java/net/minecraft/server/LegacyPingHandler.java b/src/main/java/net/minecraft/server/LegacyPingHandler.java
index 4c1a0181..f084a653 100644
--- a/src/main/java/net/minecraft/server/LegacyPingHandler.java
+++ b/src/main/java/net/minecraft/server/LegacyPingHandler.java
@@ -14,6 +14,7 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
 
     private static final Logger a = LogManager.getLogger();
     private final ServerConnection b;
+    private ByteBuf buf; // Paper
 
     public LegacyPingHandler(ServerConnection serverconnection) {
         this.b = serverconnection;
@@ -21,7 +22,16 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
 
     public void channelRead(ChannelHandlerContext channelhandlercontext, Object object) throws Exception {
         ByteBuf bytebuf = (ByteBuf) object;
-
+        // Paper start - Make legacy ping handler more reliable
+        if (this.buf != null) {
+            try {
+                readLegacy1_6(channelhandlercontext, bytebuf);
+            } finally {
+                bytebuf.release();
+            }
+            return;
+        }
+        // Paper end
         bytebuf.markReaderIndex();
         boolean flag = true;
 
@@ -50,6 +60,10 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
                     break;
 
                 default:
+                    // Paper start - Replace with improved version below
+                    if (bytebuf.readUnsignedByte() != 0x01 || bytebuf.readUnsignedByte() != 0xFA) return;
+                    readLegacy1_6(channelhandlercontext, bytebuf);
+                    /*
                     boolean flag1 = bytebuf.readUnsignedByte() == 1;
 
                     flag1 &= bytebuf.readUnsignedByte() == 250;
@@ -73,6 +87,8 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
                     } finally {
                         bytebuf1.release();
                     }
+                    */
+                    // Paper end
                 }
 
                 bytebuf.release();
@@ -92,6 +108,74 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
 
     }
 
+    // Paper start
+    private void readLegacy1_6(ChannelHandlerContext ctx, ByteBuf part) {
+        ByteBuf buf = this.buf;
+
+        if (buf == null) {
+            this.buf = buf = ctx.alloc().buffer();
+            buf.markReaderIndex();
+        } else {
+            buf.resetReaderIndex();
+        }
+
+        buf.writeBytes(part);
+
+        // Short + Short + Byte + Short + Int
+        if (!buf.isReadable(2 + 2 + 1 + 2 + 4)) {
+            return;
+        }
+
+        short length = buf.readShort();
+        if (!buf.isReadable(length * 2)) {
+            return;
+        }
+
+        if (!buf.readBytes(length * 2).toString(StandardCharsets.UTF_16BE).equals("MC|PingHost")) {
+            removeHandler(ctx);
+            return;
+        }
+
+        if (!buf.isReadable(2)) {
+            return;
+        }
+
+        length = buf.readShort();
+        if (!buf.isReadable(length)) {
+            return;
+        }
+
+        MinecraftServer server = this.b.d();
+        int protocolVersion = buf.readByte();
+        length = buf.readShort();
+        String host = buf.readBytes(length * 2).toString(StandardCharsets.UTF_16BE);
+        int port = buf.readInt();
+
+        if (buf.isReadable()) {
+            removeHandler(ctx);
+            return;
+        }
+
+        buf.release();
+        this.buf = null;
+
+        a.debug("Ping: (1.6) from {}", ctx.channel().remoteAddress());
+
+        String response = String.format("\u00a71\u0000%d\u0000%s\u0000%s\u0000%d\u0000%d",
+                Byte.MAX_VALUE, server.getVersion(), server.getMotd(), server.getPlayerCount(), server.getMaxPlayers());
+        this.a(ctx, this.a(response));
+    }
+
+    private void removeHandler(ChannelHandlerContext ctx) {
+        ByteBuf buf = this.buf;
+        this.buf = null;
+
+        buf.resetReaderIndex();
+        ctx.pipeline().remove("legacy_query");
+        ctx.fireChannelRead(buf);
+    }
+    // Paper end
+
     private void a(ChannelHandlerContext channelhandlercontext, ByteBuf bytebuf) {
         channelhandlercontext.pipeline().firstContext().writeAndFlush(bytebuf).addListener(ChannelFutureListener.CLOSE);
     }
-- 
2.14.3