ICMP报头分析问题！急！

wangxiaojie100 · 发表于 2006-7-1 10:13:11

在ping程序的ICMP报头解析过程中：void CPing:

ecodeICMPHeader(char *buf, int bytes, SOCKADDR_IN *from)
{
IpHeader    *iphdr = NULL;
IcmpHeader    *icmphdr = NULL;
unsigned short  iphdrlen;
DWORD          tick;
static int icmpcount = 0;

iphdr = (IpHeader *)buf;
// Number of 32-bit words * 4 = bytes
iphdrlen = iphdr->h_len * 4;
tick = GetTickCount();

if ((iphdrlen == MAX_IP_HDR_SIZE) && (!icmpcount))
      DecodeIPOptions(buf, bytes);

CString temp;
if (bytes  < iphdrlen + ICMP_MIN)
{
temp.Format("Too few bytes from %s \r\n",inet_ntoa(from->sin_addr));
m_dlg->m_result+=temp;
m_dlg->SetDlgItemText(IDC_EDIT2,m_dlg->m_result);
}
icmphdr = (IcmpHeader*)(buf + iphdrlen);
这里 iphdrlen = iphdr->h_len * 4;和  icmphdr = (IcmpHeader*)(buf + iphdrlen);
这两条语句是什么意思啊？
我看了很长时间也不明白，请各位高手指教！

ValorZ · 发表于 2006-7-2 17:50:55

iphdrlen = iphdr->h_len * 4 是因为IP头中IP头长度中所包含的数据的是以32位为一个单位
就是说iphdr->h_len=1，则是32位，即为4个字节。
所以，要获得IP头长度的字节数，则需*4
后面一个我不知道

wangxiaojie100 · 发表于 2006-7-4 19:52:00

呵呵，先要谢谢楼上说的，但我还是没有明白。IP头是这样定义的
typedef struct _IPHEADER {
      unsigned char  header_len:4;
      unsigned char  version:4;
      unsigned char  tos;          // type of service
      unsigned short total_len;    // length of the packet
      unsigned short ident;       // unique identifier
      unsigned short flags;
      unsigned char  ttl;
      unsigned char  proto;       // protocol ( IP , TCP, UDP etc)
      unsigned short checksum;
      unsigned int sourceIP;
      unsigned int destIP;

}IPHEADER;
  unsigned char 这个不是占一个字节吗？
怎么又是32位，四个字节呢？