After change packet IP Address using PcapDotNet DLLs also additional value except the packet IP Address changed

Question

I am using PcapDotNet DLLs in my application and while changing packet IP Address and it seems that another value is changed except the IP Address.

This is how i am change the IP Address:

private Packet ChangePacketIp(Packet packet, IpV4Address oldIpAddress, IpV4Address newIpAddress)
{
    try
    {
        EthernetLayer ethernet = (EthernetLayer)packet.Ethernet.ExtractLayer();
        IpV4Layer ipV4Layer = (IpV4Layer)packet.Ethernet.IpV4.ExtractLayer();
        IpV4Datagram ipV4Datagram = packet.Ethernet.IpV4;
        ILayer layer = ipV4Datagram.ExtractLayer();
        DateTime packetTimestamp = packet.Timestamp;
        ILayer payload = packet.Ethernet.IpV4.Payload.ExtractLayer();

        if (packet.Ethernet.IpV4.Source == oldIpAddress)
        {
            ipV4Layer.Source = newIpAddress;
            ipV4Layer.HeaderChecksum = null;
        }
        else if (packet.Ethernet.IpV4.Destination == oldIpAddress)
        {
            ipV4Layer.CurrentDestination = newIpAddress;
            ipV4Layer.HeaderChecksum = null;
        }

        return PacketBuilder.Build(packetTimestamp, ethernet, ipV4Layer, payload);
    }
    catch (Exception)
    {
        return null;
    }
}

For example, the original packets is:

http://i.stack.imgur.com/vEWuS.jpg

And the new packet after the IP Address has changed:

http://s8.postimg.org/4o78hzmyt/New_Packet.jpg

As you can see after change the IP Address from 212.25.99.74 into 80.81.82.83, another 2 bytes changes: from e6 16 into 7a d6

This is the way to change IPv4 packet or its a bug ? What is this 2 bytes ?

Answer 1

You do know that an IP packet has a checksum that has to be recalculated to match the new IP address? http://en.wikipedia.org/wiki/IPv4_header_checksum