InboundChannelLogger.java

// code used for logging
package debug;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;

import com.google.common.base.Charsets;
import com.google.common.collect.ImmutableSet;

import cpw.mods.fml.common.FMLLog;
import cpw.mods.fml.common.network.internal.FMLProxyPacket;

public class InboundChannelLogger extends ChannelInboundHandlerAdapter {

    private final String source;

    public InboundChannelLogger(String source) {
        this.source = source;
    }

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (msg instanceof FMLProxyPacket) {
            final FMLProxyPacket proxy = (FMLProxyPacket)msg;

            if (proxy.channel().equals("REGISTER")) {
                byte[] data = new byte[proxy.payload().readableBytes()];
                proxy.payload().copy().readBytes(data);
                String channels = new String(data, Charsets.UTF_8);
                String[] split = channels.split("\0");
                FMLLog.info("%s > %s (channel: %s, register: %s)", source, msg.getClass(), proxy.channel(), ImmutableSet.copyOf(split));
            } else {
                FMLLog.info("%s > %s (channel: %s)", source, msg.getClass(), proxy.channel());
            }
        } else {
            FMLLog.info("%s > %s", source, msg.getClass());
        }
        ctx.fireChannelRead(msg);
    }

}

NetworkDispatcher.java

// code used for logging

public NetworkDispatcher(NetworkManager manager)
    {
        super(Packet.class, false);
        this.manager = manager;
        this.scm = null;
        this.side = Side.CLIENT;
        this.handshakeChannel = new EmbeddedChannel(new HandshakeInjector(this),
                                                    new InboundChannelLogger("CLIENT-RAW"), new OutboundChannelLogger("CLIENT-RAW"), 
                                                    new ChannelRegistrationHandler(),
                                                    new FMLHandshakeCodec(),
                                                    new InboundChannelLogger("CLIENT-DECODED"), new OutboundChannelLogger("CLIENT-DECODED"),
                                                    new HandshakeMessageHandler<FMLHandshakeClientState>(FMLHandshakeClientState.class));
        this.handshakeChannel.attr(FML_DISPATCHER).set(this);
        this.handshakeChannel.attr(NetworkRegistry.CHANNEL_SOURCE).set(Side.SERVER);
        this.handshakeChannel.attr(NetworkRegistry.FML_CHANNEL).set("FML|HS");
        this.handshakeChannel.attr(IS_LOCAL).set(manager.isLocalChannel());
    }

    public NetworkDispatcher(NetworkManager manager, ServerConfigurationManager scm)
    {
        super(Packet.class, false);
        this.manager = manager;
        this.scm = scm;
        this.side = Side.SERVER;
        this.handshakeChannel = new EmbeddedChannel(new HandshakeInjector(this), 
                                                    new InboundChannelLogger("SERVER-RAW"), new OutboundChannelLogger("SERVER-RAW"),
                                                    new ChannelRegistrationHandler(),
                                                    new FMLHandshakeCodec(),
                                                    new InboundChannelLogger("SERVER-DECODED"), new OutboundChannelLogger("SERVER-DECODED"),
                                                    new HandshakeMessageHandler<FMLHandshakeServerState>(FMLHandshakeServerState.class));
        this.handshakeChannel.attr(FML_DISPATCHER).set(this);
        this.handshakeChannel.attr(NetworkRegistry.CHANNEL_SOURCE).set(Side.CLIENT);
        this.handshakeChannel.attr(NetworkRegistry.FML_CHANNEL).set("FML|HS");
        this.handshakeChannel.attr(IS_LOCAL).set(manager.isLocalChannel());
    }

OutboundChannelLogger.java

// code used for logging
package debug;

import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelOutboundHandlerAdapter;
import io.netty.channel.ChannelPromise;

import com.google.common.base.Charsets;
import com.google.common.collect.ImmutableSet;

import cpw.mods.fml.common.FMLLog;
import cpw.mods.fml.common.network.internal.FMLProxyPacket;

public class OutboundChannelLogger extends ChannelOutboundHandlerAdapter {

    private final String sink;

    public OutboundChannelLogger(String sink) {
        this.sink = sink;
    }

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        if (msg instanceof FMLProxyPacket) {
            final FMLProxyPacket proxy = (FMLProxyPacket)msg;

            if (proxy.channel().equals("REGISTER")) {
                byte[] data = new byte[proxy.payload().readableBytes()];
                proxy.payload().copy().readBytes(data);
                String channels = new String(data, Charsets.UTF_8);
                String[] split = channels.split("\0");
                FMLLog.info("%s < %s (channel: %s, register: %s)", sink, msg.getClass(), proxy.channel(), ImmutableSet.copyOf(split));
            } else {
                FMLLog.info("%s < %s (channel: %s)", sink, msg.getClass(), proxy.channel());
            }
        } else {
            FMLLog.info("%s < %s", sink, msg.getClass());
        }
        ctx.write(msg, promise);
    }

}