Hi,
The following custom written method is called from the
kernel networking code (in dev_queue_xmit(struct
sk_buff *skb) in /net/core/dev.c):
int check_packet(struct sk_buff *skb)
{
struct iphdr *iph = skb->nh.iph;
__u8 ttl = iph->ttl;
printk("ttl = %d\n", ttl);
return 0;
}
When I run this code I get the following output:
ttl = 201
But the problem is that I _know_ that the TTL is 64.
Now to the most interesting part. I've noticed that I
get another part of the header when I would like to
get the ttl. So 201 is actual another part of the IP
header. If I decrease the pointer to the ttl in the
skb I get the correct value. Thus, I do:
__u8 ttl = *(&iph->ttl - 2);
and I get the output:
ttl = 64
The same thing is true for all other fields in the
ip-header (with some minor differences). It seems as
if the whole header is shifted 1-3 bytes. What is
happening and how do I solve this problem? I'm running
a Intel machine. It seems as if the problem is
little/big endian related.
/
h_e_w_i
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