checksumUDP

unsigned short checksumudp(PseudoHeader* pPseudoHeader, unsigned short* usBuf, unsigned int nSize)
{
 unsigned long usChksum = 0;
 unsigned long nPseudoHeader = sizeof(PseudoHeader);
 unsigned short* usPsBuf = (unsigned short*)pPseudoHeader;
 while( nPseudoHeader > 1 )
 {
  usChksum += *usPsBuf++;
  nPseudoHeader -= sizeof(unsigned short);
 }
 while( nSize > 1 )
 {
  usChksum += *usBuf++;
  nSize -= sizeof(unsigned short);
 }
 if( nSize )
 {
  usChksum += *(unsigned char*)usBuf;
 }
 
 usChksum=(usChksum>>16)+(usChksum&0xFFFF);
 usChksum+=(usChksum>>16);
 return (unsigned short)(~usChksum);
}

伪首:

struct PseudoHeader
{
 DWORD dwSrcIP;
 DWORD dwDestIP;
 char  cReserved;
 char  cProtocol;
 WORD  wLength;
};

wLength为udp的长度不包括伪首