Question

I am calculating UDP checksum using the following function (found it somewhere):

   uint16_t udp_checksum(const void *buff, size_t len, in_addr_t src_addr, in_addr_t dest_addr)
     {
             const uint16_t *buf=(const uint16_t *)buff;
             uint16_t *ip_src=(uint16_t *)&src_addr,
                      *ip_dst=(uint16_t *)&dest_addr;
             uint32_t sum;
             size_t length=len;


         // Calculate the sum                                      
         sum = 0;
         while (len > 1)
         {
                 sum += *buf++;
                 if (sum & 0x80000000)
                         sum = (sum & 0xFFFF) + (sum >> 16);
                 len -= 2;
         }

         if ( len & 1 )
                 // Add the padding if the packet length is odd         
                 sum += *((uint8_t *)buf);

         // Add the pseudo-header                                       
         sum += *(ip_src++);
         sum += *ip_src;

         sum += *(ip_dst++);
         sum += *ip_dst;

         sum += htons(IPROTO_UDP);
         sum += htons(length);

     // Add the carries                                              
         while (sum >> 16)
                 sum = (sum & 0xFFFF) + (sum >> 16);

         // Return the one's complement of sum                           
         return ( (uint16_t)(~sum)  );
 }



    int form_checksums(char * buff)
    {
      // Get IP and UDP headers
      IP_Header* ipHdr  = (IP_Header*)(buff);
      struct UDP_Header* udpHdr = (struct UDP_Header*) (buff + 4*ipHdr->ihl);

      //---- Form and fill IP checksum now--------------------------------------
      ipHdr->check = 0;
      ipHdr->check = in_cksum((unsigned short *)ipHdr, sizeof(*ipHdr));


      //---- calculate and fill udp checksum now ---
      udpHdr->checksum = 0;

      udpHdr->checksum = udp_checksum(buff + 4*ipHdr->ihl, udpHdr->length, ipHdr->saddr, ipHdr->daddr); 

      return 0;
    }

Wireshark shows that the wrong UDP checksum is calculated. I don’t see any problem in the function. What can be going wrong?

Answer 1

UDP checksum computation requires an UDP pseudo-header.

Here are some code samples from my libraries that might help:

// SmartBuffer is a stream-like buffer class
uint16_t SmartBuffer::checksum(const void* buf, size_t buflen)
{
    assert(buf);

    uint32_t r = 0;
    size_t len = buflen;

    const uint16_t* d = reinterpret_cast<const uint16_t*>(buf);

    while (len > 1)
    {
        r += *d++;
        len -= sizeof(uint16_t);
    }

    if (len)
    {
        r += *reinterpret_cast<const uint8_t*>(d);
    }

    while (r >> 16)
    {
        r = (r & 0xffff) + (r >> 16);
    }

    return static_cast<uint16_t>(~r);
}

The UDPFrame checksum computing:

uint16_t UDPFrame::computeChecksum(const ipv4_header& ih) const
{
    udp_pseudo_header uph;

    memset(&uph, 0x00, sizeof(uph));

    uph.source = ih.source;
    uph.destination = ih.destination;
    uph.mbz = 0x00;
    uph.type = ih.protocol;
    uph.length = getData()->length;

    systools::SmartBuffer tmp(sizeof(uph) + d_data.size());

    tmp.appendValue(uph);
    tmp.append(d_data); // d_data is the UDP frame payload

    return tmp.checksum();
}

Anyway, keep in mind that usually wireshark warns you that a wrong value for the checksum can be computed due to UDP checksum offload.

Perhaps your checksum function is indeed wrong but a reliable way of beeing sure is to try to receive your UDP frames.