/*
    * Copyright 2020 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    * https://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  package io.netty.handler.pcap;
  
  import io.netty.buffer.ByteBuf;
  import io.netty.buffer.ByteBufAllocator;
  import io.netty.channel.Channel;
  import io.netty.channel.ChannelDuplexHandler;
  import io.netty.channel.ChannelHandlerContext;
  import io.netty.channel.ChannelInboundHandlerAdapter;
  import io.netty.channel.ChannelPromise;
  import io.netty.channel.ServerChannel;
  import io.netty.channel.socket.DatagramChannel;
  import io.netty.channel.socket.DatagramPacket;
  import io.netty.channel.socket.ServerSocketChannel;
  import io.netty.channel.socket.SocketChannel;
  import io.netty.util.NetUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.io.Closeable;
  import java.io.IOException;
  import java.io.OutputStream;
  import java.net.Inet4Address;
  import java.net.Inet6Address;
  import java.net.InetAddress;
  import java.net.InetSocketAddress;
  import java.net.UnknownHostException;
  import java.util.concurrent.atomic.AtomicReference;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  /**
   * <p> {@link PcapWriteHandler} captures {@link ByteBuf} from {@link SocketChannel} / {@link ServerChannel}
   * or {@link DatagramPacket} and writes it into Pcap {@link OutputStream}. </p>
   *
   * <p>
   * Things to keep in mind when using {@link PcapWriteHandler} with TCP:
   *
   *    <ul>
   *        <li> Whenever {@link ChannelInboundHandlerAdapter#channelActive(ChannelHandlerContext)} is called,
   *        a fake TCP 3-way handshake (SYN, SYN+ACK, ACK) is simulated as new connection in Pcap. </li>
   *
   *        <li> Whenever {@link ChannelInboundHandlerAdapter#handlerRemoved(ChannelHandlerContext)} is called,
   *        a fake TCP 3-way handshake (FIN+ACK, FIN+ACK, ACK) is simulated as connection shutdown in Pcap.  </li>
   *
   *        <li> Whenever {@link ChannelInboundHandlerAdapter#exceptionCaught(ChannelHandlerContext, Throwable)}
   *        is called, a fake TCP RST is sent to simulate connection Reset in Pcap. </li>
   *
   *        <li> ACK is sent each time data is send / received. </li>
   *
   *        <li> Zero Length Data Packets can cause TCP Double ACK error in Wireshark. To tackle this,
   *        set {@code captureZeroByte} to {@code false}. </li>
   *    </ul>
   * </p>
   */
  public final class PcapWriteHandler extends ChannelDuplexHandler implements Closeable {
  
      /**
       * Logger for logging events
       */
      private final InternalLogger logger = InternalLoggerFactory.getInstance(PcapWriteHandler.class);
  
      /**
       * {@link PcapWriter} Instance
       */
      private PcapWriter pCapWriter;
  
      /**
       * {@link OutputStream} where we'll write Pcap data.
       */
      private final OutputStream outputStream;
  
      /**
       * {@code true} if we want to capture packets with zero bytes else {@code false}.
       */
      private final boolean captureZeroByte;
  
      /**
       * {@code true} if we want to write Pcap Global Header on initialization of
       * {@link PcapWriter} else {@code false}.
       */
      private final boolean writePcapGlobalHeader;
  
      /**
       * {@code true} if we want to synchronize on the {@link OutputStream} while writing
      * else {@code false}.
      */
     private final boolean sharedOutputStream;
 
     /**
      * TCP Sender Segment Number.
      * It'll start with 1 and keep incrementing with number of bytes read/sent.
      */
     private int sendSegmentNumber = 1;
 
     /**
      * TCP Receiver Segment Number.
      * It'll start with 1 and keep incrementing with number of bytes read/sent.
      */
     private int receiveSegmentNumber = 1;
 
     /**
      * Type of the channel this handler is registered on
      */
     private ChannelType channelType;
 
     /**
      * Address of the initiator of the connection
      */
     private InetSocketAddress initiatorAddr;
 
     /**
      * Address of the receiver of the connection
      */
     private InetSocketAddress handlerAddr;
 
     /**
      * Set to {@code true} if this handler is registered on a server pipeline
      */
     private boolean isServerPipeline;
 
     /**
      * Current of this {@link PcapWriteHandler}
      */
     private final AtomicReference<State> state = new AtomicReference<State>(State.INIT);
 
     /**
      * Create new {@link PcapWriteHandler} Instance.
      * {@code captureZeroByte} is set to {@code false} and
      * {@code writePcapGlobalHeader} is set to {@code true}.
      *
      * @param outputStream OutputStream where Pcap data will be written. Call {@link #close()} to close this
      *                     OutputStream.
      * @throws NullPointerException If {@link OutputStream} is {@code null} then we'll throw an
      *                              {@link NullPointerException}
      *
      * @deprecated Use {@link Builder} instead.
      */
     @Deprecated
     public PcapWriteHandler(OutputStream outputStream) {
         this(outputStream, false, true);
     }
 
     /**
      * Create new {@link PcapWriteHandler} Instance
      *
      * @param outputStream          OutputStream where Pcap data will be written. Call {@link #close()} to close this
      *                              OutputStream.
      * @param captureZeroByte       Set to {@code true} to enable capturing packets with empty (0 bytes) payload.
      *                              Otherwise, if set to {@code false}, empty packets will be filtered out.
      * @param writePcapGlobalHeader Set to {@code true} to write Pcap Global Header on initialization.
      *                              Otherwise, if set to {@code false}, Pcap Global Header will not be written
      *                              on initialization. This could when writing Pcap data on a existing file where
      *                              Pcap Global Header is already present.
      * @throws NullPointerException If {@link OutputStream} is {@code null} then we'll throw an
      *                              {@link NullPointerException}
      *
      * @deprecated Use {@link Builder} instead.
      */
     @Deprecated
     public PcapWriteHandler(OutputStream outputStream, boolean captureZeroByte, boolean writePcapGlobalHeader) {
         this.outputStream = checkNotNull(outputStream, "OutputStream");
         this.captureZeroByte = captureZeroByte;
         this.writePcapGlobalHeader = writePcapGlobalHeader;
         sharedOutputStream = false;
     }
 
     private PcapWriteHandler(Builder builder, OutputStream outputStream) {
         this.outputStream = outputStream;
         captureZeroByte = builder.captureZeroByte;
         sharedOutputStream = builder.sharedOutputStream;
         writePcapGlobalHeader = builder.writePcapGlobalHeader;
         channelType = builder.channelType;
         handlerAddr = builder.handlerAddr;
         initiatorAddr = builder.initiatorAddr;
         isServerPipeline = builder.isServerPipeline;
     }
 
     /**
      * Writes the Pcap Global Header to the provided {@code OutputStream}
      *
      * @param outputStream OutputStream where Pcap data will be written.
      * @throws IOException if there is an error writing to the {@code OutputStream}
      */
     public static void writeGlobalHeader(OutputStream outputStream) throws IOException {
         PcapHeaders.writeGlobalHeader(outputStream);
     }
 
     private void initializeIfNecessary(ChannelHandlerContext ctx) throws Exception {
         // If State is not 'INIT' then it means we're already initialized so then no need to initiaize again.
         if (state.get() != State.INIT) {
             return;
         }
 
         pCapWriter = new PcapWriter(this);
 
         if (channelType == null) {
             // infer channel type
             if (ctx.channel() instanceof SocketChannel) {
                 channelType = ChannelType.TCP;
 
                 // If Channel belongs to `SocketChannel` then we're handling TCP.
                 // Capture correct `localAddress` and `remoteAddress`
                 if (ctx.channel().parent() instanceof ServerSocketChannel) {
                     isServerPipeline = true;
                     initiatorAddr = (InetSocketAddress) ctx.channel().remoteAddress();
                     handlerAddr = getLocalAddress(ctx.channel(), initiatorAddr);
                 } else {
                     isServerPipeline = false;
                     handlerAddr = (InetSocketAddress) ctx.channel().remoteAddress();
                     initiatorAddr = getLocalAddress(ctx.channel(), handlerAddr);
                 }
             } else if (ctx.channel() instanceof DatagramChannel) {
                 channelType = ChannelType.UDP;
 
                 DatagramChannel datagramChannel = (DatagramChannel) ctx.channel();
 
                 // If `DatagramChannel` is connected then we can get
                 // `localAddress` and `remoteAddress` from Channel.
                 if (datagramChannel.isConnected()) {
                     handlerAddr = (InetSocketAddress) ctx.channel().remoteAddress();
                     initiatorAddr = getLocalAddress(ctx.channel(), handlerAddr);
                 }
             }
         }
 
         if (channelType == ChannelType.TCP) {
             logger.debug("Initiating Fake TCP 3-Way Handshake");
 
             ByteBuf tcpBuf = ctx.alloc().buffer();
 
             try {
                 // Write SYN with Normal Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, 0, 0,
                         initiatorAddr.getPort(), handlerAddr.getPort(), TCPPacket.TCPFlag.SYN);
                 completeTCPWrite(initiatorAddr, handlerAddr, tcpBuf, ctx.alloc(), ctx);
 
                 // Write SYN+ACK with Reversed Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, 0, 1,
                         handlerAddr.getPort(), initiatorAddr.getPort(), TCPPacket.TCPFlag.SYN, TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(handlerAddr, initiatorAddr, tcpBuf, ctx.alloc(), ctx);
 
                 // Write ACK with Normal Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, 1, 1, initiatorAddr.getPort(),
                         handlerAddr.getPort(), TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(initiatorAddr, handlerAddr, tcpBuf, ctx.alloc(), ctx);
             } finally {
                 tcpBuf.release();
             }
 
             logger.debug("Finished Fake TCP 3-Way Handshake");
         }
 
         state.set(State.WRITING);
     }
 
     @Override
     public void channelActive(ChannelHandlerContext ctx) throws Exception {
         initializeIfNecessary(ctx);
         super.channelActive(ctx);
     }
 
     @Override
     public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
         // Initialize if needed
         if (state.get() == State.INIT) {
             initializeIfNecessary(ctx);
         }
 
         // Only write if State is STARTED
         if (state.get() == State.WRITING) {
             if (channelType == ChannelType.TCP) {
                 handleTCP(ctx, msg, false);
             } else if (channelType == ChannelType.UDP) {
                 handleUDP(ctx, msg);
             } else {
                 logDiscard();
             }
         }
         super.channelRead(ctx, msg);
     }
 
     @Override
     public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
         // Initialize if needed
         if (state.get() == State.INIT) {
             initializeIfNecessary(ctx);
         }
 
         // Only write if State is STARTED
         if (state.get() == State.WRITING) {
             if (channelType == ChannelType.TCP) {
                 handleTCP(ctx, msg, true);
             } else if (channelType == ChannelType.UDP) {
                 handleUDP(ctx, msg);
             } else {
                 logDiscard();
             }
         }
         super.write(ctx, msg, promise);
     }
 
     /**
      * Handle TCP L4
      *
      * @param ctx              {@link ChannelHandlerContext} for {@link ByteBuf} allocation and
      *                         {@code fireExceptionCaught}
      * @param msg              {@link Object} must be {@link ByteBuf} else it'll be discarded
      * @param isWriteOperation Set {@code true} if we have to process packet when packets are being sent out
      *                         else set {@code false}
      */
     private void handleTCP(ChannelHandlerContext ctx, Object msg, boolean isWriteOperation) {
         if (msg instanceof ByteBuf) {
 
             // If bytes are 0 and `captureZeroByte` is false, we won't capture this.
             if (((ByteBuf) msg).readableBytes() == 0 && !captureZeroByte) {
                 logger.debug("Discarding Zero Byte TCP Packet. isWriteOperation {}", isWriteOperation);
                 return;
             }
 
             ByteBufAllocator byteBufAllocator = ctx.alloc();
             ByteBuf packet = ((ByteBuf) msg).duplicate();
             ByteBuf tcpBuf = byteBufAllocator.buffer();
             int bytes = packet.readableBytes();
 
             try {
                 if (isWriteOperation) {
                     final InetSocketAddress srcAddr;
                     final InetSocketAddress dstAddr;
                     if (isServerPipeline) {
                         srcAddr = handlerAddr;
                         dstAddr = initiatorAddr;
                     } else {
                         srcAddr = initiatorAddr;
                         dstAddr = handlerAddr;
                     }
 
                     TCPPacket.writePacket(tcpBuf, packet, sendSegmentNumber, receiveSegmentNumber, srcAddr.getPort(),
                             dstAddr.getPort(), TCPPacket.TCPFlag.ACK);
                     completeTCPWrite(srcAddr, dstAddr, tcpBuf, byteBufAllocator, ctx);
                     logTCP(true, bytes, sendSegmentNumber, receiveSegmentNumber, srcAddr, dstAddr, false);
 
                     sendSegmentNumber += bytes;
 
                     TCPPacket.writePacket(tcpBuf, null, receiveSegmentNumber, sendSegmentNumber, dstAddr.getPort(),
                             srcAddr.getPort(), TCPPacket.TCPFlag.ACK);
                     completeTCPWrite(dstAddr, srcAddr, tcpBuf, byteBufAllocator, ctx);
                     logTCP(true, bytes, sendSegmentNumber, receiveSegmentNumber, dstAddr, srcAddr, true);
                 } else {
                     final InetSocketAddress srcAddr;
                     final InetSocketAddress dstAddr;
                     if (isServerPipeline) {
                         srcAddr = initiatorAddr;
                         dstAddr = handlerAddr;
                     } else {
                         srcAddr = handlerAddr;
                         dstAddr = initiatorAddr;
                     }
 
                     TCPPacket.writePacket(tcpBuf, packet, receiveSegmentNumber, sendSegmentNumber, srcAddr.getPort(),
                             dstAddr.getPort(), TCPPacket.TCPFlag.ACK);
                     completeTCPWrite(srcAddr, dstAddr, tcpBuf, byteBufAllocator, ctx);
                     logTCP(false, bytes, receiveSegmentNumber, sendSegmentNumber, srcAddr, dstAddr, false);
 
                     receiveSegmentNumber += bytes;
 
                     TCPPacket.writePacket(tcpBuf, null, sendSegmentNumber, receiveSegmentNumber, dstAddr.getPort(),
                             srcAddr.getPort(), TCPPacket.TCPFlag.ACK);
                     completeTCPWrite(dstAddr, srcAddr, tcpBuf, byteBufAllocator, ctx);
                     logTCP(false, bytes, sendSegmentNumber, receiveSegmentNumber, dstAddr, srcAddr, true);
                 }
             } finally {
                 tcpBuf.release();
             }
         } else {
             logger.debug("Discarding Pcap Write for TCP Object: {}", msg);
         }
     }
 
     /**
      * Write TCP/IP L3 and L2 here.
      *
      * @param srcAddr          {@link InetSocketAddress} Source Address of this Packet
      * @param dstAddr          {@link InetSocketAddress} Destination Address of this Packet
      * @param tcpBuf           {@link ByteBuf} containing TCP L4 Data
      * @param byteBufAllocator {@link ByteBufAllocator} for allocating bytes for TCP/IP L3 and L2 data.
      * @param ctx              {@link ChannelHandlerContext} for {@code fireExceptionCaught}
      */
     private void completeTCPWrite(InetSocketAddress srcAddr, InetSocketAddress dstAddr, ByteBuf tcpBuf,
                                   ByteBufAllocator byteBufAllocator, ChannelHandlerContext ctx) {
 
         ByteBuf ipBuf = byteBufAllocator.buffer();
         ByteBuf ethernetBuf = byteBufAllocator.buffer();
         ByteBuf pcap = byteBufAllocator.buffer();
 
         try {
             if (srcAddr.getAddress() instanceof Inet4Address && dstAddr.getAddress() instanceof Inet4Address) {
                 IPPacket.writeTCPv4(ipBuf, tcpBuf,
                         NetUtil.ipv4AddressToInt((Inet4Address) srcAddr.getAddress()),
                         NetUtil.ipv4AddressToInt((Inet4Address) dstAddr.getAddress()));
 
                 EthernetPacket.writeIPv4(ethernetBuf, ipBuf);
             } else if (srcAddr.getAddress() instanceof Inet6Address && dstAddr.getAddress() instanceof Inet6Address) {
                 IPPacket.writeTCPv6(ipBuf, tcpBuf,
                         srcAddr.getAddress().getAddress(),
                         dstAddr.getAddress().getAddress());
 
                 EthernetPacket.writeIPv6(ethernetBuf, ipBuf);
             } else {
                 logger.error("Source and Destination IP Address versions are not same. Source Address: {}, " +
                         "Destination Address: {}", srcAddr.getAddress(), dstAddr.getAddress());
                 return;
             }
 
             // Write Packet into Pcap
             pCapWriter.writePacket(pcap, ethernetBuf);
         } catch (IOException ex) {
             logger.error("Caught Exception While Writing Packet into Pcap", ex);
             ctx.fireExceptionCaught(ex);
         } finally {
             ipBuf.release();
             ethernetBuf.release();
             pcap.release();
         }
     }
 
     /**
      * Handle UDP l4
      *
      * @param ctx {@link ChannelHandlerContext} for {@code localAddress} / {@code remoteAddress},
      *            {@link ByteBuf} allocation and {@code fireExceptionCaught}
      * @param msg {@link DatagramPacket} or {@link ByteBuf}
      */
     private void handleUDP(ChannelHandlerContext ctx, Object msg) {
         ByteBuf udpBuf = ctx.alloc().buffer();
 
         try {
             if (msg instanceof DatagramPacket) {
 
                 // If bytes are 0 and `captureZeroByte` is false, we won't capture this.
                 if (((DatagramPacket) msg).content().readableBytes() == 0 && !captureZeroByte) {
                     logger.debug("Discarding Zero Byte UDP Packet");
                     return;
                 }
 
                 DatagramPacket datagramPacket = ((DatagramPacket) msg).duplicate();
                 InetSocketAddress srcAddr = datagramPacket.sender();
                 InetSocketAddress dstAddr = datagramPacket.recipient();
 
                 // If `datagramPacket.sender()` is `null` then DatagramPacket is initialized
                 // `sender` (local) address. In this case, we'll get source address from Channel.
                 if (srcAddr == null) {
                     srcAddr = getLocalAddress(ctx.channel(), dstAddr);
                 }
 
                 logger.debug("Writing UDP Data of {} Bytes, Src Addr {}, Dst Addr {}",
                         datagramPacket.content().readableBytes(), srcAddr, dstAddr);
 
                 UDPPacket.writePacket(udpBuf, datagramPacket.content(), srcAddr.getPort(), dstAddr.getPort());
                 completeUDPWrite(srcAddr, dstAddr, udpBuf, ctx.alloc(), ctx);
             } else if (msg instanceof ByteBuf &&
                     (!(ctx.channel() instanceof DatagramChannel) || ((DatagramChannel) ctx.channel()).isConnected())) {
 
                 // If bytes are 0 and `captureZeroByte` is false, we won't capture this.
                 if (((ByteBuf) msg).readableBytes() == 0 && !captureZeroByte) {
                     logger.debug("Discarding Zero Byte UDP Packet");
                     return;
                 }
 
                 ByteBuf byteBuf = ((ByteBuf) msg).duplicate();
 
                 logger.debug("Writing UDP Data of {} Bytes, Src Addr {}, Dst Addr {}",
                         byteBuf.readableBytes(), initiatorAddr, handlerAddr);
 
                 UDPPacket.writePacket(udpBuf, byteBuf, initiatorAddr.getPort(), handlerAddr.getPort());
                 completeUDPWrite(initiatorAddr, handlerAddr, udpBuf, ctx.alloc(), ctx);
             } else {
                 logger.debug("Discarding Pcap Write for UDP Object: {}", msg);
             }
         } finally {
             udpBuf.release();
         }
     }
 
     /**
      * Write UDP/IP L3 and L2 here.
      *
      * @param srcAddr          {@link InetSocketAddress} Source Address of this Packet
      * @param dstAddr          {@link InetSocketAddress} Destination Address of this Packet
      * @param udpBuf           {@link ByteBuf} containing UDP L4 Data
      * @param byteBufAllocator {@link ByteBufAllocator} for allocating bytes for UDP/IP L3 and L2 data.
      * @param ctx              {@link ChannelHandlerContext} for {@code fireExceptionCaught}
      */
     private void completeUDPWrite(InetSocketAddress srcAddr, InetSocketAddress dstAddr, ByteBuf udpBuf,
                                   ByteBufAllocator byteBufAllocator, ChannelHandlerContext ctx) {
 
         ByteBuf ipBuf = byteBufAllocator.buffer();
         ByteBuf ethernetBuf = byteBufAllocator.buffer();
         ByteBuf pcap = byteBufAllocator.buffer();
 
         try {
             if (srcAddr.getAddress() instanceof Inet4Address && dstAddr.getAddress() instanceof Inet4Address) {
                 IPPacket.writeUDPv4(ipBuf, udpBuf,
                         NetUtil.ipv4AddressToInt((Inet4Address) srcAddr.getAddress()),
                         NetUtil.ipv4AddressToInt((Inet4Address) dstAddr.getAddress()));
 
                 EthernetPacket.writeIPv4(ethernetBuf, ipBuf);
             } else if (srcAddr.getAddress() instanceof Inet6Address && dstAddr.getAddress() instanceof Inet6Address) {
                 IPPacket.writeUDPv6(ipBuf, udpBuf,
                         srcAddr.getAddress().getAddress(),
                         dstAddr.getAddress().getAddress());
 
                 EthernetPacket.writeIPv6(ethernetBuf, ipBuf);
             } else {
                 logger.error("Source and Destination IP Address versions are not same. Source Address: {}, " +
                         "Destination Address: {}", srcAddr.getAddress(), dstAddr.getAddress());
                 return;
             }
 
             // Write Packet into Pcap
             pCapWriter.writePacket(pcap, ethernetBuf);
         } catch (IOException ex) {
             logger.error("Caught Exception While Writing Packet into Pcap", ex);
             ctx.fireExceptionCaught(ex);
         } finally {
             ipBuf.release();
             ethernetBuf.release();
             pcap.release();
         }
     }
 
     /**
      * Get the local address of a channel. If the address is a wildcard address ({@code 0.0.0.0} or {@code ::}), and
      * the address family does not match that of the {@code remote}, return the wildcard address of the {@code remote}'s
      * family instead.
      *
      * @param ch     The channel to get the local address from
      * @param remote The remote address
      * @return The fixed local address
      */
     private static InetSocketAddress getLocalAddress(Channel ch, InetSocketAddress remote) {
         InetSocketAddress local = (InetSocketAddress) ch.localAddress();
         if (remote != null && local.getAddress().isAnyLocalAddress()) {
             if (local.getAddress() instanceof Inet4Address && remote.getAddress() instanceof Inet6Address) {
                 return new InetSocketAddress(WildcardAddressHolder.wildcard6, local.getPort());
             }
             if (local.getAddress() instanceof Inet6Address && remote.getAddress() instanceof Inet4Address) {
                 return new InetSocketAddress(WildcardAddressHolder.wildcard4, local.getPort());
             }
         }
         return local;
     }
 
     @Override
     public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
 
         // If `isTCP` is true, then we'll simulate a `FIN` flow.
         if (channelType == ChannelType.TCP) {
             logger.debug("Starting Fake TCP FIN+ACK Flow to close connection");
 
             ByteBufAllocator byteBufAllocator = ctx.alloc();
             ByteBuf tcpBuf = byteBufAllocator.buffer();
 
             try {
                 // Write FIN+ACK with Normal Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, sendSegmentNumber, receiveSegmentNumber, initiatorAddr.getPort(),
                         handlerAddr.getPort(), TCPPacket.TCPFlag.FIN, TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(initiatorAddr, handlerAddr, tcpBuf, byteBufAllocator, ctx);
 
                 // Write FIN+ACK with Reversed Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, receiveSegmentNumber, sendSegmentNumber, handlerAddr.getPort(),
                         initiatorAddr.getPort(), TCPPacket.TCPFlag.FIN, TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(handlerAddr, initiatorAddr, tcpBuf, byteBufAllocator, ctx);
 
                 // Write ACK with Normal Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, sendSegmentNumber + 1, receiveSegmentNumber + 1,
                         initiatorAddr.getPort(), handlerAddr.getPort(), TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(initiatorAddr, handlerAddr, tcpBuf, byteBufAllocator, ctx);
             } finally {
                 tcpBuf.release();
             }
 
             logger.debug("Finished Fake TCP FIN+ACK Flow to close connection");
         }
 
         close();
         super.handlerRemoved(ctx);
     }
 
     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
 
         if (channelType == ChannelType.TCP) {
             ByteBuf tcpBuf = ctx.alloc().buffer();
 
             try {
                 // Write RST with Normal Source and Destination Address
                 TCPPacket.writePacket(tcpBuf, null, sendSegmentNumber, receiveSegmentNumber, initiatorAddr.getPort(),
                         handlerAddr.getPort(), TCPPacket.TCPFlag.RST, TCPPacket.TCPFlag.ACK);
                 completeTCPWrite(initiatorAddr, handlerAddr, tcpBuf, ctx.alloc(), ctx);
             } finally {
                 tcpBuf.release();
             }
 
             logger.debug("Sent Fake TCP RST to close connection");
         }
 
         close();
         ctx.fireExceptionCaught(cause);
     }
 
     /**
      * Logger for TCP
      */
     private void logTCP(boolean isWriteOperation, int bytes, int sendSegmentNumber, int receiveSegmentNumber,
                         InetSocketAddress srcAddr, InetSocketAddress dstAddr, boolean ackOnly) {
         // If `ackOnly` is `true` when we don't need to write any data so we'll not
         // log number of bytes being written and mark the operation as "TCP ACK".
         if (logger.isDebugEnabled()) {
             if (ackOnly) {
                 logger.debug("Writing TCP ACK, isWriteOperation {}, Segment Number {}, Ack Number {}, Src Addr {}, "
                         + "Dst Addr {}", isWriteOperation, sendSegmentNumber, receiveSegmentNumber, dstAddr, srcAddr);
             } else {
                 logger.debug("Writing TCP Data of {} Bytes, isWriteOperation {}, Segment Number {}, Ack Number {}, " +
                                 "Src Addr {}, Dst Addr {}", bytes, isWriteOperation, sendSegmentNumber,
                         receiveSegmentNumber, srcAddr, dstAddr);
             }
         }
     }
 
     OutputStream outputStream() {
         return outputStream;
     }
 
     boolean sharedOutputStream() {
         return sharedOutputStream;
     }
 
     boolean writePcapGlobalHeader() {
         return writePcapGlobalHeader;
     }
 
     /**
      * Returns {@code true} if the {@link PcapWriteHandler} is currently
      * writing packets to the {@link OutputStream} else returns {@code false}.
      */
     public boolean isWriting() {
         return state.get() == State.WRITING;
     }
 
     State state() {
         return state.get();
     }
 
     /**
      * Pause the {@link PcapWriteHandler} from writing packets to the {@link OutputStream}.
      */
     public void pause() {
         if (!state.compareAndSet(State.WRITING, State.PAUSED)) {
             throw new IllegalStateException("State must be 'STARTED' to pause but current state is: " + state);
         }
     }
 
     /**
      * Resume the {@link PcapWriteHandler} to writing packets to the {@link OutputStream}.
      */
     public void resume() {
         if (!state.compareAndSet(State.PAUSED, State.WRITING)) {
             throw new IllegalStateException("State must be 'PAUSED' to resume but current state is: " + state);
         }
     }
 
     void markClosed() {
         if (state.get() != State.CLOSED) {
             state.set(State.CLOSED);
         }
     }
 
     // Visible for testing only.
     PcapWriter pCapWriter() {
         return pCapWriter;
     }
 
     private void logDiscard() {
         logger.warn("Discarding pcap write because channel type is unknown. The channel this handler is registered " +
                 "on is not a SocketChannel or DatagramChannel, so the inference does not work. Please call " +
                 "forceTcpChannel or forceUdpChannel before registering the handler.");
     }
 
     @Override
     public String toString() {
         return "PcapWriteHandler{" +
                 "captureZeroByte=" + captureZeroByte +
                 ", writePcapGlobalHeader=" + writePcapGlobalHeader +
                 ", sharedOutputStream=" + sharedOutputStream +
                 ", sendSegmentNumber=" + sendSegmentNumber +
                 ", receiveSegmentNumber=" + receiveSegmentNumber +
                 ", channelType=" + channelType +
                 ", initiatorAddr=" + initiatorAddr +
                 ", handlerAddr=" + handlerAddr +
                 ", isServerPipeline=" + isServerPipeline +
                 ", state=" + state +
                 '}';
     }
 
     /**
      * <p> Close {@code PcapWriter} and {@link OutputStream}. </p>
      * <p> Note: Calling this method does not close {@link PcapWriteHandler}.
      * Only Pcap Writes are closed. </p>
      *
      * @throws IOException If {@link OutputStream#close()} throws an exception
      */
     @Override
     public void close() throws IOException {
         if (state.get() == State.CLOSED) {
             logger.debug("PcapWriterHandler is already closed");
         } else {
             pCapWriter.close();
             markClosed();
             logger.debug("PcapWriterHandler is now closed");
         }
     }
 
     private enum ChannelType {
         TCP, UDP
     }
 
     public static Builder builder() {
         return new Builder();
     }
 
     /**
      * Builder for {@link PcapWriteHandler}
      */
     public static final class Builder {
         private boolean captureZeroByte;
         private boolean sharedOutputStream;
         private boolean writePcapGlobalHeader = true;
 
         private ChannelType channelType;
         private InetSocketAddress initiatorAddr;
         private InetSocketAddress handlerAddr;
         private boolean isServerPipeline;
 
         private Builder() {
         }
 
         /**
          * Set to {@code true} to enable capturing packets with empty (0 bytes) payload. Otherwise, if set to
          * {@code false}, empty packets will be filtered out.
          *
          * @param captureZeroByte Whether to filter out empty packets.
          * @return this builder
          */
         public Builder captureZeroByte(boolean captureZeroByte) {
             this.captureZeroByte = captureZeroByte;
             return this;
         }
 
         /**
          * Set to {@code true} if multiple {@link PcapWriteHandler} instances will be
          * writing to the same {@link OutputStream} concurrently, and write locking is
          * required. Otherwise, if set to {@code false}, no locking will be done.
          * Additionally, {@link #close} will not close the underlying {@code OutputStream}.
          * Note: it is probably an error to have both {@code writePcapGlobalHeader} and
          * {@code sharedOutputStream} set to {@code true} at the same time.
          *
          * @param sharedOutputStream Whether {@link OutputStream} is shared or not
          * @return this builder
          */
         public Builder sharedOutputStream(boolean sharedOutputStream) {
             this.sharedOutputStream = sharedOutputStream;
             return this;
         }
 
         /**
          * Set to {@code true} to write Pcap Global Header on initialization. Otherwise, if set to {@code false}, Pcap
          * Global Header will not be written on initialization. This could when writing Pcap data on a existing file
          * where Pcap Global Header is already present.
          *
          * @param writePcapGlobalHeader Whether to write the pcap global header.
          * @return this builder
          */
         public Builder writePcapGlobalHeader(boolean writePcapGlobalHeader) {
             this.writePcapGlobalHeader = writePcapGlobalHeader;
             return this;
         }
 
         /**
          * Force this handler to write data as if they were TCP packets, with the given connection metadata. If this
          * method isn't called, we determine the metadata from the channel.
          *
          * @param serverAddress The address of the TCP server (handler)
          * @param clientAddress The address of the TCP client (initiator)
          * @param isServerPipeline Whether the handler is part of the server channel
          * @return this builder
          */
         public Builder forceTcpChannel(InetSocketAddress serverAddress, InetSocketAddress clientAddress,
                                        boolean isServerPipeline) {
             channelType = ChannelType.TCP;
             handlerAddr = checkNotNull(serverAddress, "serverAddress");
             initiatorAddr = checkNotNull(clientAddress, "clientAddress");
             this.isServerPipeline = isServerPipeline;
             return this;
         }
 
         /**
          * Force this handler to write data as if they were UDP packets, with the given connection metadata. If this
          * method isn't called, we determine the metadata from the channel.
          * <br>
          * Note that even if this method is called, the address information on {@link DatagramPacket} takes precedence
          * if it is present.
          *
          * @param localAddress  The address of the UDP local
          * @param remoteAddress The address of the UDP remote
          * @return this builder
          */
         public Builder forceUdpChannel(InetSocketAddress localAddress, InetSocketAddress remoteAddress) {
             channelType = ChannelType.UDP;
             handlerAddr = checkNotNull(remoteAddress, "remoteAddress");
             initiatorAddr = checkNotNull(localAddress, "localAddress");
             return this;
         }
 
         /**
          * Build the {@link PcapWriteHandler}.
          *
          * @param outputStream The output stream to write the pcap data to.
          * @return The handler.
          */
         public PcapWriteHandler build(OutputStream outputStream) {
             checkNotNull(outputStream, "outputStream");
             return new PcapWriteHandler(this, outputStream);
         }
     }
 
     private static final class WildcardAddressHolder {
         static final InetAddress wildcard4; // 0.0.0.0
         static final InetAddress wildcard6; // ::
 
         static {
             try {
                 wildcard4 = InetAddress.getByAddress(new byte[4]);
                 wildcard6 = InetAddress.getByAddress(new byte[16]);
             } catch (UnknownHostException e) {
                 // would only happen if the byte array was of incorrect size
                 throw new AssertionError(e);
             }
         }
     }
 }