unsigned short checksum(unsigned short* usBuf, unsigned int nSize)

{

 unsigned long usChksum=0;
 //Calculate the checksum

 while (nSize>1)

 {

  usChksum+=*usBuf++;

  nSize-=sizeof(unsigned short);

 }
 //If we have one char left

 if (nSize)

  usChksum+=*(unsigned char*)usBuf;
 //Complete the calculations

 usChksum=(usChksum >> 16) + (usChksum & 0xffff);

 usChksum+=(usChksum >> 16);
 //Return the value (inversed)

 return (unsigned short)(~usChksum);
}

ip部分

ippacket.GetIpHeader()->check=0;

checksum((unsigned short *)ippacket.GetIpHeader(),sizeof(iphdr)); 

UDP需要加入伪首部，再计算校验和

  PseudoHeader ph;

ph.dwSrcIP = ippacket.GetIpHeader()->saddr;

ph.dwDestIP = ippacket.GetIpHeader()->daddr;

ph.cReserved = 0;

ph.cProtocol = IPPROTO_UDP;

ph.wLength = htons((WORD)ntohs( ippacket.GetUdpHeader()->uh_ulen));

ippacket.GetUdpHeader()->uh_sum=0;

ippacket.GetUdpHeader()->uh_sum=checksumudp(&ph,(unsigned short *)ippacket.GetUdpHeader(),ntohs( ippacket.GetUdpHeader()->uh_ulen));