Winpcap simple question - how to send packets to a specified ip/port?

Question

I read the tutorials and so, but I am not getting it. It does let you send packets, but how can you tell Winpcap where to send those packets? Is there any header I should put on the packets so it will know to which ip/port's to forward it? I mean. Let's imagine I want to send some data to my MSN, as if I had wrote something to someone on my list. I can use the sendpacket(), but it will only take the packet/byte array as argument, not specifing to which app/ip/port so send it.

Thanks

Answer 1

This is how i sent an ARP request over the wire. 1. Define structures for the protocols i.e if you want to send ARP packets you will need a structure that will contain the data link layer(Ethernet header) and network layer (ARP Header). Correspondingly if you want to send a tcp packet over IP you will need a data structure for the ethernet header, ip header and tcp header.

1. once you defined the structures, initialize an instance of the structure with values you want i.e. if you want the packet to go to all machines in the network set the destination mac value of the Ethernet header to ff:ff:ff:ff:ff:ff if you want to send the packet to machine X with(IP address 192.168.0.88) then set the destination address in the ip layer to that value.

2. Once done you will need to declare a char* array and copy all the structures to the char* array to create a byte sequence and send it over the wire.

//Just to show you what i mean by defining structures doesn't //relate to the rest of the code snippet

typedef struct IP_header 
{
    u_char      VersionNInternetHeaderLength;       // Version (4 bits) + Internet header length (4 bits)
    /*u_char        version:4;
    u_char      HeaderLength:4;*/
    u_char      Type;                               // Type of service 
    u_short     TotalLength;                        // Total length 
    u_short     Identification;                     // Identification

    u_char      rsv     : 1;
    u_char      df      : 1;
    u_char      mf      : 1;

    u_char      FragmentOffset1 : 5;
    u_char      FragmentOffset2;
    //u_short       Flags_fo;                           // Flags (3 bits) + Fragment offset (13 bits)

    u_char      TimeToLive;                         // Time to live
    u_char      Protocol;                           // Next level Protocol of the encapsulated payload
    u_short     Checksum;                           // Header checksum
    IP_address  SourceAddress;                      // Source address
    IP_address  DestinationAddress;                 // Destination address
    u_int       OptionNPadding;                     // Option + Padding

    IP_header()
    {
        mf = 0;
        rsv = 0;
        df = 0;
        FragmentOffset1 = 0;
        FragmentOffset2 = 0;

        TimeToLive = 128;

        TotalLength = sizeof(IP_header);
        Identification = 0xABCD;
        Checksum = 0xABCD;
        OptionNPadding = 0;
    }
}IP_header;


Ethernet_header EthernetHeader;// = (Ethernet_header*)malloc(sizeof(Ethernet_header));
        ARP_header ARPHeader ;//= (ARP_header*)malloc(sizeof(ARP_header));

        ARPHeader.HardwareType = htons(1);
        ARPHeader.ProtocolType = htons(0x800);
        ARPHeader.OPCODE = htons(1);

        ARPHeader.HeaderLength = 6;
        ARPHeader.ProtocolLength = 4;

        ARPHeader.SenderMAC = MY_FAKE_MAC;
        ARPHeader.SenderIP = MY_IP;

        ARPHeader.TargetMAC = MAC_address();
        ARPHeader.TargetIP = Whose;

        EthernetHeader.DestinationMAC = BROADCASTMAC;
        EthernetHeader.SourceMAC = MY_FAKE_MAC;
        EthernetHeader.EtherType = htons(0x806);

        u_char* packet = (u_char*)malloc(sizeof(EthernetHeader) + sizeof(ARPHeader));

        memcpy(packet, &EthernetHeader, sizeof(EthernetHeader));
        memcpy(packet + sizeof(EthernetHeader), &ARPHeader, sizeof(ARPHeader));

        SendPacket(packet);

Answer 2

You don't tell Winpcap where to send packets. You tell it to put a packet on the wire. The network switch will send the packet to the right destination. The TCP stack on the receiving end will send the packet to the right application/service. Obviously this means the routing information has to be in the packet itself.

To take your example, you'd need to put the IP address and TCP port of the appropriate MSN server in the packet. If you don't, your networking hardware will discard or misroute that packet.