recived byte numbers from RECVFROM()

[yangj <yangj@ailab.pku.edu.cn>]

hi:
  
  Now I am trying  to get packets using a raw_sock:

  ph_fd=socket(PF_PACKET,SOCK_RAW,htons(ETH_P_ALL));
  n=recvfrom(ph_fd,buf,bufsize*sizeof(buf),0,NULL,NULL);
  (ip *)ip_hd=(ip*)(buf+14);
   int totalLen=ip_hd->ip_len;

   the problem is :
   ip_len!=n-14.


   I checked  the buffer,find the N may be the correct bytes number 
received. is it right?
   but  why they are different?Is there any epcial meaning of ip_len??
  
   Can anyone help me?
   thank you.
  

  



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