UDP checksum recalculation

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  • June 6, 2006 at 11:04 pm #5029
    aty
    Participant

      Hi

      Could someone please post a code snippet of UDP checksum recalculation (something similar to the RecalculateTCPChecksum sample posted some time ago in this forum) ?

      Many thanks,
      aty

      June 7, 2006 at 9:16 am #6063
      Kon
      Participant

        VOID
        RecalculateUDPChecksum (
        PINTERMEDIATE_BUFFER pPacket
        )
        {
        udphdr_ptr pUdpHeader = NULL;
        unsigned short word16, padd = 0;
        unsigned int i, sum = 0;
        PUCHAR buff;
        DWORD dwUdpLen;

        iphdr_ptr pIpHeader = (iphdr_ptr)&pPacket->m_IBuffer[sizeof(ether_header)];

        // Sanity check
        if (pIpHeader->ip_p == IPPROTO_UDP)
        {
        pUdpHeader = (udphdr_ptr)(((PUCHAR)pIpHeader) + sizeof(DWORD)*pIpHeader->ip_hl);
        }
        else
        return;

        dwUdpLen = ntohs(pIpHeader->ip_len) – pIpHeader->ip_hl*4;//pPacket->m_Length – ((PUCHAR)(pTcpHeader) – pPacket->m_IBuffer);

        if ( (dwUdpLen/2)*2 != dwUdpLen )
        {
        padd=1;
        pPacket->m_IBuffer[dwUdpLen + pIpHeader->ip_hl*4 + sizeof(ether_header)] = 0;
        }

        buff = (PUCHAR)pUdpHeader;
        pUdpHeader->th_sum = 0;

        // make 16 bit words out of every two adjacent 8 bit words and
        // calculate the sum of all 16 vit words
        for (i=0; i< dwUdpLen+padd; i=i+2){
        word16 =((buff<<8)&0xFF00)+(buff[i+1]&0xFF);
        sum = sum + (unsigned long)word16;
        }

        // add the TCP pseudo header which contains:
        // the IP source and destination addresses,

        sum = sum + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_src.S_un.S_un_w.s_w2);
        sum = sum + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w1) + ntohs(pIpHeader->ip_dst.S_un.S_un_w.s_w2);

        // the protocol number and the length of the TCP packet
        sum = sum + IPPROTO_UDP + (unsigned short)dwUdpLen;

        // keep only the last 16 bits of the 32 bit calculated sum and add the carries
        while (sum>>16)
        sum = (sum & 0xFFFF)+(sum >> 16);

        // Take the one’s complement of sum
        sum = ~sum;

        pUdpHeader->th_sum = ntohs((unsigned short)sum);
        }

        June 7, 2006 at 9:35 am #6064
        aty
        Participant

          Thanks a lot 🙂

          aty

          p.s: It would be very useful if the checksum calculation methods (IP/TCP/UDP) will be included in future versions of the WinpkFilter DLL – add to the wish-list.

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