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Can we write a network sniffer in .NET using only .NET API?

Impreuna cu un coleg de birou am fost curiosi sa vedem daca putem sa scriem un sniffer cu API de .NET, fara sa folosim alte librari.
Pe internet exista destul de multe librari care fac acest lucru. Cea mai populara am vazut ca este WinPCap. Pentru al putea folosii este nevoie sa instalam niste drivere care o sa ne permita sa accesam orice packet de date care circula pe fir.
Acuma sa vedem ce putem sa facem din .NET. Prin intermediul clasei Socket, putem sa ajungem sub layer-ul de transport si sa accesam layer-ul de retea( network). Atat .NET cat si sistemul de operate ne limiteaza accesul doar la packetele care au ca sursa sau destinatie calculatorul nostru. Nu va asteptati sa puteti accesa packete care nu va sunt destinate. Din ce am gasit pana acuma, nu este posibil sa interceptam pachetele de date dintre alte doua device-uri din reteaua noastra. Acest lucru se poate face din .NET daca ne folosim de un wrapper peste WinPCap.
Folosind direct API-ul .NET nu o sa putem sa facem un sniffer care sa intercepteze traficul din retea. Mai jos gasiti un exemplu de sniffer care intercepteaza traficul local:
Socket socket = new Socket(
AddressFamily.InterNetwork,
SocketType.Stream,
ProtocolType.IP);
socket.Bind(new IPEndPoint(IPAddress.Parse("192.168.100.105"), 0));
socket.SetSocketOption(
SocketOptionLevel.IP,
SocketOptionName.HeaderIncluded,
true);
socket.IOControl(
IOControlCode.ReceiveAll,
new byte[4] {1, 0, 0, 0},
new byte[4] {1, 0, 0, 0});
socket.BeginReceive(
packetData,
0,
packetData.Length,
SocketFlags.None,
new AsyncCallback(OnPacketIntercept), null);
In cazul in care vrem sa prindem doar packetele pe un anumit tip de protocol, putem sa ne folosim de enumeratie ProtocalType, care include atat TCP cat si UDP. Cand vrem sa facem un bind la o anumita adresa trebuie sa avem grija ca aceasta adresa fie a calculatorului nostru. In cazul in care incercati sa folositi o alta adresa veti primii o eroare care va avertizeaza ca adresa este incompatibila cu protocolul selectat.
Este destul de important sa setati ca header-ul pachetului sa fie inclus. Din header o sa vedem putin mai tarziu cum putem extrage informatii pretioase precum ip sursa, ip destinatie, protocol etc. Trebuie sa avem un sir de biti care o sa fie folositi pentru a scrie continutul pachetului. In cazul meu, packetData reprezinta acest sir de biti.
De fiecare data cand un pachet este primit sau trimis metoda OnPacketIntercept o sa fie apelata. Chiar daca continutul la packet o sa se afle in variabila packetData, avem nevoie sa stim care este lumgimea pachetului. Acest lucru il putem face in metoda OnPacketIntercept prin
 int paketLenght = mainSocket.EndReceive(ar);
Dupa acest pas putem sa incarcam sirul de biti intr-un memory stream si sa extragem date. Nu o sa descriu fiecare informatie din header. In link-ul dat mai jos puteti sa gasiti informatiile pe care le contine un IP Packet Header si locatia acestora in sirul de biti
http://en.wikipedia.org/wiki/Network_packet#Example:_IP_packets
Iar la urmatoarea adresa o sa gasiti o implementarea a unei clase care pe baza unui sir de biti extrage toate informatii din header-ul unui packet http://www.koders.com/csharp/fid779A18B94E1A0885E4569E99B6E0CB9339E1D268.aspx?s=treeview
Pentru a putea primii urmatorul packet, este nevoie sa apelam din nou socket.BeginReceive in interiorul metodei OnPacketIntercept.

Daca dorim sa interceptam pachetele care nu ne sunt destinate noua este nevoie sa ne folosim de o librarie precum WinPCap. Din ce am vazut eu pana acuma nu avem posibilitatea sa accesam in mod direct aceste packete si nici nu am aveam cum.
Daca vreti sa aflati mai multe despre socket-uri in .NET va recomand urmatoarea pagina: http://msdn.microsoft.com/en-us/library/orm-9780596527570-03-14.aspx

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