C - Linux - kernel module - TCP header -

i'm trying create linux kernel module, inspect incoming packets. @ moment, i'm in process of extracting tcp header of packet , reading source , destination port -> i'm getting incorrect values. have hook function:

unsigned int hook_func(unsigned int hooknum, struct sk_buff *skb,                         const struct net_device *in,                         const struct net_device *out,                         int (*okfn)(struct sk_buff *))  {     struct iphdr *ipp = (struct iphdr *)skb_network_header(skb);     struct tcphdr *hdr;     /* using filter data machine */     unsigned long ok_ip = 2396891328;      /* problem, empty network packet. stop now. */     if (!skb)         return nf_accept;      /* track packets coming 1 ip */     if (ipp->saddr != ok_ip)         return nf_accept;      /* incomming packet tcp */     if (ipp->protocol == ipproto_tcp) {         hdr = (struct tcphdr *) skb_transport_header(skb);         printk(" tcp ports: source: %d, dest: %d .\n", ntohs(hdr->source),                                                         ntohs(hdr->dest));     } } 

now, when try telnet port 21(not listening there get):

[ 4252.961912]  tcp ports: source: 17664, dest: 52 . [ 4253.453978]  tcp ports: source: 17664, dest: 52 . [ 4253.953204]  tcp ports: source: 17664, dest: 48 . 

and when telnet port 22 - ssh deamon listening there:

[ 4299.239940]  tcp ports: source: 17664, dest: 52 . [ 4299.240527]  tcp ports: source: 17664, dest: 40 . [ 4299.552566]  tcp ports: source: 17664, dest: 40 . 

as visible output i'm getting weird results, has idea problem coming from? when compile module have no errors / warnings. version of kernel(headers): 3.7.10 . not using selinux or similar.

i had same problem writing small firewall networking class found out problem having. casting tcp header wrong. try casting tcp accessing port.

here code snippet of working

struct iphdr *ip_header;       // ip header struct struct tcphdr *tcp_header;     // tcp header struct struct udphdr *udp_header;     // udp header struct struct sk_buff *sock_buff;  unsigned int sport ,              dport;   sock_buff = skb;  if (!sock_buff)     return nf_accept;  ip_header = (struct iphdr *)skb_network_header(sock_buff); if (!ip_header)     return nf_accept;   //if tcp packet if(ip_header->protocol==ipproto_tcp) {     //tcp_header = (struct tcphdr *)skb_transport_header(sock_buff); //doing cast way gave me same problem      tcp_header= (struct tcphdr *)((__u32 *)ip_header+ ip_header->ihl); //this fixed problem      sport = htons((unsigned short int) tcp_header->source); //sport has source port     dport = htons((unsigned short int) tcp_header->dest);   //dport has dest port }