关于构造ICMP数据包

[InTheMiddle]

自己写了个程序发ICMP端口不可达的数据包，其中ICMP协议层包含了IP层字段和8个字节UDP字段，请问最后8个字节的UDP字段里的校验和是怎么算的？
是直接从某个包里拷过来的？

[yq_m]

u_short in_cksum(u_short *addr, int len)
{
register int nleft = len;
register u_short *w = addr;
register u_short answer;
register int sum = 0;

/*
* Our algorithm is simple, using a 32 bit accumulator (sum),
* we add sequential 16 bit words to it, and at the end, fold
* back all the carry bits from the top 16 bits into the lower
* 16 bits.
*/
while( nleft > 1 ) {
sum += *w++;
nleft -= 2;
}

/* mop up an odd byte, if necessary */
if( nleft == 1 ) {
u_short u = 0;

*(u_char *)(&u) = *(u_char *)w ;
sum += u;
}

/*
* add back carry outs from top 16 bits to low 16 bits
*/
sum = (sum >> 16) + (sum & 0xffff); /* add hi 16 to low 16 */
sum += (sum >> 16); /* add carry */
answer = ~sum; /* truncate to 16 bits */
return (answer);
}

[InTheMiddle]

我想请教的是最后的校验和是只算那8位UDP的数据吗？还是加UDP的伪头部？

[yq_m]

加伪IP头