Question

我正在使用PcapDorNet DLL，我试图分割我的流量：

private IEnumerable<Packet> SplitPacket(Packet packet, int numberOfFragments)
        {
            IpV4Datagram ipV4Datagram = packet.Ethernet.IpV4;
            Datagram datagram = packet.Ethernet.IpV4.Payload;
            TransportLayer transportlayer = GetTransportLayer(packet);
            if (transportlayer != null)
            {
                EthernetLayer ethernet = (EthernetLayer)packet.Ethernet.ExtractLayer();
                ILayer layer = packet.Ethernet.IpV4.ExtractLayer();
                IpV4Layer ipV4Layer = (IpV4Layer)packet.Ethernet.IpV4.ExtractLayer();
                ipV4Layer.HeaderChecksum = null;
                DateTime packetTimestamp = packet.Timestamp;                
                PayloadLayer payload = (PayloadLayer)packet.Ethernet.IpV4.Payload.ExtractLayer(); //extract the data

                int totalLength = payload.Length;
                int partialLength = totalLength / numberOfFragments; //split data into smaller segments
                partialLength = (partialLength / 8) * 8; //make sure it's divisible with 8
                if (partialLength == 0)
                    partialLength = 8;
                //(http://en.wikipedia.org/wiki/IPv4#Fragmentation_and_reassembly)
                ushort offset = 0; //send one by one

                while (offset < totalLength)
                {
                    int fragmentLength = partialLength; //get length for this fragment
                    IpV4FragmentationOptions options = IpV4FragmentationOptions.MoreFragments;

                    if (offset + fragmentLength >= totalLength) //is this the last fragment ? trim length if needed
                    {
                        options = IpV4FragmentationOptions.None;
                        fragmentLength = totalLength - offset;
                    }

                    byte[] newBuffer = ipV4Datagram.Payload.ToArray(); //copy the actual data into a new buffer
                    PayloadLayer newPayload = new PayloadLayer { Data = new Datagram(newBuffer, offset, fragmentLength) };
                    ipV4Layer.Fragmentation = new IpV4Fragmentation(options, offset); //change IP layer fragmentation options
                    transportlayer.Checksum = null;
                    yield return PacketBuilder.Build(packetTimestamp, ethernet, ipV4Layer, transportlayer, newPayload);
                    offset += (ushort)fragmentLength; //next offset
                }
            }
        }

我的输入数据包是TCP packet，输出是4个数据包：3 IPv4 packets，最后一个是TCP但是我收到的这个数据包显示在TCP层字段下这条消息：

校验和：0x5d17 [不正确，应为0xabb7（可能由＆＃34; TCP引起）校验和卸载＆＃34;？）]

使用UDP似乎每个UDP数据包也分成几个IPv4数据包，最后一个是UDP，但在UDP情况下它看起来很好。我做错了什么？

请查看原始数据包和碎片：http://www.filedropper.com/desktop_122

Answer 1

Pcap.Net site中似乎有一个非常类似的问题，对代码进行了更正。

我相信遵循这些建议有助于解决您的问题。

IP碎片导致Checksum不正确的消息

1 个答案: