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A Network Sniffer in C#

, 11 Feb 2008 CPOL
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A simple network sniffer which can parse IP, TCP, UDP, and DNS packets.
Sample Image - CSNetworkSniffer.jpg


In this article, I will discuss the working of a simple network sniffer which can parse IP, TCP, UDP, and DNS packets.

Capturing the Packets

// For sniffing the socket to capture the packets 
// has to be a raw socket, with the address family
// being of type internetwork, and protocol being IP
mainSocket = newSocket(AddressFamily.InterNetwork, SocketType.Raw,

// Bind the socket to the selected IP address

// Set the socket options
mainSocket.SetSocketOption(SocketOptionLevel.IP,  //Applies only to IP packets
                           SocketOptionName.HeaderIncluded, //Set the include header
                           true);                           //option to true

byte[] byTrue = newbyte[4]{1, 0, 0, 0};
byte[] byOut = newbyte[4];

//Socket.IOControl is analogous to the WSAIoctl method of Winsock 2
mainSocket.IOControl(IOControlCode.ReceiveAll,  //SIO_RCVALL of Winsock
                     byTrue, byOut);

//Start receiving the packets asynchronously
mainSocket.BeginReceive(byteData, 0, byteData.Length, SocketFlags.None,
                        newAsyncCallback(OnReceive), null);

For capturing the packets, we use a raw socket and bind it to the IP address. After setting the proper options for the socket, we then call the IOControl method on it. Notice that IOControl is analogous to the Winsock2WSAIoctl method. The IOControlCode.ReceiveAll implies that all incoming and outgoing packets on the particular interface be captured.

The second parameter passed to IOControl with IOControlCode.ReceiveAll should be TRUE so an array byTrue is created and passed to it (thanks to Leonid Molochniy for this). Next we start receiving all packets asynchronously.

Analysing the Packets

The IP datagram encapsulates the TCP and UDP packets. This further contains the data sent by the application layer protocols such as DNS, HTTP, FTP, SMTP, SIP, etc. Thus a TCP packet is received inside the IP datagram, like this:

| IP header | TCP header |        Data        |  

So the first thing that we need to do is to parse the IP header. The stripped version of the IP header class is shown below, the comments describe the things as they happen.

public classIPHeader 
  //IP Header fields 
  private byte byVersionAndHeaderLength; // Eight bits for version and header 
                                         // length 
  private byte byDifferentiatedServices; // Eight bits for differentiated 
                                         // services
  private ushort usTotalLength;          // Sixteen bits for total length 
  private ushort usIdentification;       // Sixteen bits for identification
  private ushort usFlagsAndOffset;       // Eight bits for flags and frag. 
                                         // offset 
  private byte byTTL;                    // Eight bits for TTL (Time To Live) 
  private byte byProtocol;               // Eight bits for the underlying 
                                         // protocol 
  private short sChecksum;               // Sixteen bits for checksum of the 
                                         //  header 
  private uint uiSourceIPAddress;        // Thirty two bit source IP Address 
  private uint uiDestinationIPAddress;   // Thirty two bit destination IP Address 

  //End IP Header fields   
  private byte byHeaderLength;             //Header length 
  private byte[] byIPData = new byte[4096]; //Data carried by the datagram
  public IPHeader(byte[] byBuffer, int nReceived)
    //Create MemoryStream out of the received bytes
    MemoryStream memoryStream = newMemoryStream(byBuffer, 0, nReceived);
    //Next we create a BinaryReader out of the MemoryStream
    BinaryReader binaryReader = newBinaryReader(memoryStream);

    //The first eight bits of the IP header contain the version and
    //header length so we read them
    byVersionAndHeaderLength = binaryReader.ReadByte();

    //The next eight bits contain the Differentiated services
    byDifferentiatedServices = binaryReader.ReadByte();
    //Next eight bits hold the total length of the datagram
    usTotalLength = 
             (ushort) IPAddress.NetworkToHostOrder(binaryReader.ReadInt16());

    //Next sixteen have the identification bytes
    usIdentification = 

    //Next sixteen bits contain the flags and fragmentation offset
    usFlagsAndOffset = 

    //Next eight bits have the TTL value
    byTTL = binaryReader.ReadByte();

    //Next eight represent the protocol encapsulated in the datagram
    byProtocol = binaryReader.ReadByte();

    //Next sixteen bits contain the checksum of the header
    sChecksum = IPAddress.NetworkToHostOrder(binaryReader.ReadInt16());

    //Next thirty two bits have the source IP address
    uiSourceIPAddress = (uint)(binaryReader.ReadInt32());

    //Next thirty two hold the destination IP address
    uiDestinationIPAddress = (uint)(binaryReader.ReadInt32());

    //Now we calculate the header length
    byHeaderLength = byVersionAndHeaderLength;

    //The last four bits of the version and header length field contain the
    //header length, we perform some simple binary arithmetic operations to
    //extract them
    byHeaderLength <<= 4;
    byHeaderLength >>= 4;

    //Multiply by four to get the exact header length
    byHeaderLength *= 4;

    //Copy the data carried by the datagram into another array so that
    //according to the protocol being carried in the IP datagram
               byHeaderLength, //start copying from the end of the header
               byIPData, 0, usTotalLength - byHeaderLength);
    catch (Exception ex)
        MessageBox.Show(ex.Message, "MJsniff", MessageBoxButtons.OK, 
//Please see the attached codes for the properties…

Firstly, the class contains the data members corresponding to the fields of the IP header. Kindly see RFC 791 for a detailed explanation of the IP header and its fields. The constructor of the class takes the bytes received and creates a MemoryStream on the received bytes and then creates a BinaryReader to read the data from the MemoryStream byte-by-byte. Also note that the data received from the network is in big-endian form so we use the IPAddress.NetworkToHostOrder to correct the byte ordering. This has to be done for all non-byte data members.

The TCP, UDP headers are also parsed in an identical fashion, the only difference is that they are read from the point where the IP header ends. As an example of parsing the application layer protocols, the attached project can detect DNS packets as well.



  • 9th February, 2008: Fixed the code to capture both incoming and outgoing packets. Thanks to Darren_vms for pointing this in the comments.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Hitesh Sharma

United States United States
No Biography provided

Comments and Discussions

QuestionHow about ARP? PinprofessionalASM29935-Sep-14 4:59 
AnswerRe: How about ARP? PinmemberMiguelAntonio30-Sep-14 15:49 
GeneralMy vote of 4 PinmemberHerre Kuijpers30-Jul-14 11:36 
Questionempty zip source file PinmemberMember 1040682325-Jul-14 17:55 
Questionthe attempted opeartion is not supported for the type of object refranced PinmemberSir JeSon18-Jun-14 17:08 
GeneralMy vote of 5 PinmemberMember 1068660424-Mar-14 6:14 
QuestionDouble packets PinmemberPSU Mike7-Feb-14 2:11 
GeneralMy vote of 1 Pinmemberapplemeteor8-Dec-13 19:27 
Questionhow to tell income or outgoing PinmemberHuisheng Chen6-Jun-13 5:30 
QuestionTrying to sniff on a local network PinmemberHammett199322-Apr-13 11:11 
QuestionStart Sniff while Cable Disconnected? PinmemberAli Fakoor6-Feb-13 20:31 
QuestionProblem with VISTA Pinmemberdot net tech20-Dec-12 22:15 
QuestionNetwork problems PinmemberAJMAL SHAHZAD10-Dec-12 19:45 
QuestionHow can decode package PinmemberBilgisoft30-Oct-12 11:00 
QuestionSniffing out ALL wifi devices? Pinmemberpwsmietan18-Jul-12 0:23 
QuestionHow do we find the MAC address of the IP we sniffed? Pinmemberpwsmietan17-Jul-12 13:53 
QuestionTrying to filter only all UDP pakets on a single port from any sources PinmemberSharePointerrr2-Mar-12 6:01 
QuestionProblem with mainSocket.IOControl Pinmemberbyval26-Feb-12 6:35 
Hi, I can't start the app, I', constantly receiving error message on
mainSocket.IOControl(IOControlCode.ReceiveAll,byTrue, byOut) - invalid argument.
Also, if I want to look for trafic at port 6555, than I would set the socket at that port, am I right?
QuestionProblem... Pinmemberbyval26-Feb-12 6:35 
QuestionCaptures Only inward traffic PinmembersaneehaAamir13-Feb-12 19:47 
AnswerRe: Captures Only inward traffic PinmemberMember 372970625-Feb-13 1:04 
GeneralWorks perfect. Vote 5. Pinmemberlboy2-Feb-12 19:51 
QuestionHow I can get the process PID of the current packet?? Pinmemberterrinfo10-Jan-12 4:34 
AnswerRe: How I can get the process PID of the current packet?? PinmemberMember 372970625-Feb-13 1:06 
QuestionThis doesnt work correctly...source and destination IPs mixed up Pinmembernukenuts7-Dec-11 12:20 
Bugno capturing incoming packets with Windows 7 PinmemberDirex0077-Nov-11 23:43 
GeneralRe: no capturing incoming packets with Windows 7 Pinmember0Pointer18-Jul-12 21:28 
GeneralRe: no capturing incoming packets with Windows 7 PinmemberMember 372970625-Feb-13 1:09 
QuestionTrying to convert this to web forms PinmemberKasey Krehbiel13-Oct-11 4:46 
Question5 and you earned it PinmemberDamian 229-Sep-11 0:25 
QuestionPackets of other nodes PinmemberMehrdad Seyrafi18-Jun-11 23:46 
GeneralGeneral question Pinmemberhanskarl200024-May-11 11:35 
GeneralCapture To a Variable PinmemberMember 776618027-Apr-11 14:30 
Questionmac address PinmemberMember 77196069-Mar-11 21:18 
QuestionHow to capture URL Pinmembereg_Anubhava23-Feb-11 23:45 
Questionintialization of some fields to 555 in tcp header? Pinmemberashborg20-Feb-11 8:52 
Questionconfigure to listen to SMTP Pinmembersbeesu11-Feb-11 12:56 
GeneralCouple of points [modified] Pinmemberpeaco34410-Nov-10 2:19 
GeneralAn invalid argument was supplied Pinmemberedsfunsite28-Oct-10 2:16 
GeneralRe: An invalid argument was supplied Pinmemberpeaco34422-Nov-10 4:44 
Questionwinpcap Pinmemberngok9-Oct-10 13:18 
GeneralAn attempt was made to access the socket in a way forbidden by its access permissions Pinmemberars.029-Oct-10 8:57 
AnswerRe: An attempt was made to access the socket in a way forbidden by its access permissions PinmemberJonathan Laloz15-Oct-10 11:29 
GeneralRe: An attempt was made to access the socket in a way forbidden by its access permissions Pinmemberars.0220-Oct-10 19:16 
GeneralRe: An attempt was made to access the socket in a way forbidden by its access permissions Pinmemberars.0220-Oct-10 19:17 
GeneralRe: An attempt was made to access the socket in a way forbidden by its access permissions Pinmemberpriyashivasamy6-Dec-10 22:21 
QuestionHow to capture incoming packets PinmemberGr3Ger24-Aug-10 6:07 
Generalproblem with working more time Pinmembersinan balcin23-Aug-10 23:37 
GeneralRe: problem with working more time Pinmembersinan balcin23-Aug-10 23:48 
Generalhi i have aproblem in the code Pinmemberaws227-Aug-10 12:45 

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