From 9437cc71ca1b7e3b5ee6f2130b40da55b8cd4acc Mon Sep 17 00:00:00 2001
From: Minecrell <minecrell@minecrell.net>
Date: Wed, 11 Oct 2017 18:22:50 +0200
Subject: [PATCH] Make 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 c8c5df7f95..06aefcb638 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 LOGGER = LogManager.getLogger();
     private final ServerConnection b;
+    private ByteBuf buf; // Paper
 
     public LegacyPingHandler(ServerConnection serverconnection) {
         this.b = serverconnection;
@@ -22,6 +23,16 @@ 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;
 
@@ -52,6 +63,10 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
                 this.a(channelhandlercontext, this.a(s));
                 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;
@@ -75,6 +90,7 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
                 } finally {
                     bytebuf1.release();
                 }
+                */ // Paper end - Replace with improved version below
             }
 
             bytebuf.release();
@@ -92,6 +108,90 @@ public class LegacyPingHandler extends ChannelInboundHandlerAdapter {
 
     }
 
+    // Paper start
+    private static String readLegacyString(ByteBuf buf) {
+        int size = buf.readShort() * Character.BYTES;
+        if (!buf.isReadable(size)) {
+            return null;
+        }
+
+        String result = buf.toString(buf.readerIndex(), size, StandardCharsets.UTF_16BE);
+        buf.skipBytes(size); // toString doesn't increase readerIndex automatically
+        return result;
+    }
+
+    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);
+
+        if (!buf.isReadable(Short.BYTES + Short.BYTES + Byte.BYTES + Short.BYTES + Integer.BYTES)) {
+            return;
+        }
+
+        String s = readLegacyString(buf);
+        if (s == null) {
+            return;
+        }
+
+        if (!s.equals("MC|PingHost")) {
+            removeHandler(ctx);
+            return;
+        }
+
+        if (!buf.isReadable(Short.BYTES) || !buf.isReadable(buf.readShort())) {
+            return;
+        }
+
+        MinecraftServer server = this.b.d();
+        int protocolVersion = buf.readByte();
+        String host = readLegacyString(buf);
+        if (host == null) {
+            removeHandler(ctx);
+            return;
+        }
+        int port = buf.readInt();
+
+        if (buf.isReadable()) {
+            removeHandler(ctx);
+            return;
+        }
+
+        buf.release();
+        this.buf = null;
+
+        LOGGER.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(this);
+        ctx.fireChannelRead(buf);
+    }
+
+    @Override
+    public void handlerRemoved(ChannelHandlerContext ctx) {
+        if (this.buf != null) {
+            this.buf.release();
+            this.buf = null;
+        }
+    }
+    // Paper end
+
     private void a(ChannelHandlerContext channelhandlercontext, ByteBuf bytebuf) {
         channelhandlercontext.pipeline().firstContext().writeAndFlush(bytebuf).addListener(ChannelFutureListener.CLOSE);
     }
-- 
2.21.0