[Bug] Error with a connection below 1.7 occurs

[Finoway]

Hello, I've just found on NanoLimbo version 1.5.2 that if a client lower than 1.7 connects (tested in 1.6.4) it causes an error in the console and the client remains on "Logging in". Would it be possible to refuse the connection directly? As for 1.7/1.7.1, where it replies "Unsupported client version" or not to answer the client ?

What's more, if you read nanolimbo's README, you might think that it supports versions 1.7-1.7.1, but this is not the case.

Here's the error in the NanoLimbo console:

[09:39:26] [ERROR]: Unhandled exception: 
java.lang.NullPointerException: Cannot read field "serverBound" because "state" is null
        at ua.nanit.limbo.connection.pipeline.PacketDecoder.updateState(PacketDecoder.java:68)
        at ua.nanit.limbo.connection.ClientConnection.updateState(ClientConnection.java:225)
        at ua.nanit.limbo.connection.PacketHandler.handle(PacketHandler.java:45)
        at ua.nanit.limbo.protocol.packets.PacketHandshake.handle(PacketHandshake.java:70)
        at ua.nanit.limbo.connection.ClientConnection.handlePacket(ClientConnection.java:117)
        at ua.nanit.limbo.connection.ClientConnection.channelRead(ClientConnection.java:112)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
        at ua.nanit.limbo.connection.pipeline.PacketDecoder.decode(PacketDecoder.java:57)
        at ua.nanit.limbo.connection.pipeline.PacketDecoder.decode(PacketDecoder.java:31)
        at io.netty.handler.codec.MessageToMessageDecoder.channelRead(MessageToMessageDecoder.java:88)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
        at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:346)
        at io.netty.handler.codec.ByteToMessageDecoder.fireChannelRead(ByteToMessageDecoder.java:333)
        at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:454)
        at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:290)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:444)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
        at io.netty.handler.timeout.IdleStateHandler.channelRead(IdleStateHandler.java:286)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:442)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
        at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:412)
        at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:440)
        at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:420)
        at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919)
        at io.netty.channel.epoll.AbstractEpollStreamChannel$EpollStreamUnsafe.epollInReady(AbstractEpollStreamChannel.java:800)
        at io.netty.channel.epoll.EpollEventLoop.processReady(EpollEventLoop.java:499)
        at io.netty.channel.epoll.EpollEventLoop.run(EpollEventLoop.java:397)
        at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:997)
        at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74)
        at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30)
        at java.base/java.lang.Thread.run(Thread.java:833)

[Nan1t]

Yes, it supports 1.7.2+ as the first full version (1.7 and 1.7.1 are pre-release). 1.6.4 and below use different protocol format, so we can't just check its protocol version number and say that this version is unsupported. You should prevent it on the proxy server side.