On Sunday 2010-04-11 14:56, supercodeing35271 supercodeing35271 wrote:
>Hi,i had written a kernel module with the netfilter.But i found a
>problem when i judged the value of tcphdr->syn.This is the main code:
>
>static unsigned int hook_func_in(unsigned int hooknum,
> struct sk_buff *skb,
> const struct net_device *in,
> const struct net_device *out,
> int (*okfn)(struct sk_buff *))
>{
> struct tcphdr *tcp;
> struct icmphdr *icmp;
> struct udphdr *udp;
> struct iphdr *ip;
> char msgk_to_userspace[200];
> char *pointer_mtu;
> pointer_mtu = &msgk_to_userspace[0];
>
> if(skb == NULL)
> {
> return NF_DROP;
> }
Pointless check, skb is never NULL.
> if(ip_hdr(skb))
> {
> ip = ip_hdr(skb);
>
> }
Pointless check, ip_hdr won't usually return NULL either.
> else
> {
> return NF_DROP;
> }
>
> if((skb->dev != NULL) && (in != NULL))
> {
> if(skb->dev == in)
> {
Redundant check. in is the interface.
> if(ip->protocol == IPPROTO_TCP)
> {
> tcp = tcp_hdr(skb);
>
>
> if(tcp->syn)
Must use skb_header_pointer to access parts that lie beyond skb->data.
> {
> printk("the syn flag is 1 \n");
> /*here,when i send a tcp packet from other machine which had set
>the syn flag by NMAP(command:nmap -sS myIP -p 6000),and i also used
>wireshark to confirm that the packet had set the syn flag,but the
>program didn't print "the syn flag is 1" to
>var/log/message.Otherwise,when i changed the code
>to"if(!(tcp->syn))","the syn flag is 1" could be print.Yes, it's
>opposite!Then i could not thought of why this happened........ */
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