Thank you for your reply, but I cant use netfilter hooks for another reasons.
On Mon, Jun 3, 2013 at 5:36 AM, Silviu Popescu
<silviupopescu1990@xxxxxxxxx> wrote:
> On Sun, Jun 2, 2013 at 6:44 PM, Felipe Dias <felipediassss@xxxxxxxxx> wrote:
>> Hi, someone might try to tell me what I'm doing wrong, please?
>>
>> I have a simple module and I registered one packet handler, in init function:
>> ---
>> pseudo_proto.type = htons(ETH_P_ALL);
>> pseudo_proto.dev = NULL;
>> pseudo_proto.func = packet_handler;
>> dev_add_pack(&pseudo_proto);
>> ---
>>
>> My packet_handler, is a simple function just to print in dmesg info
>> about the packet, its check if protocol is TCP or UDP before.
>>
>> The problem is with TCP headers. I'm trying to get tcp ports with:
>>
>> ntohs(tcp_hdr(skb)->dest);
>>
>> But always come strange ports numbers.
>>
>> My question is: Are the TCP headers filled at this stage ?
>> Or I have to register another packet_handler with dev_add_pack() after
>> TCP code have done the heavy work?
>
> 1. I'm not sure if you should be converting form network to host order
> when using the TCP destination port. Try both ways, see what works.
>
> 2. Your concern regarding to filled TCP headers is a good hunch. You
> would be better off using a netfilter hook (actually two) for this
> job. You would need a netfilter hook that intercepts outgoing packets
> originated from you machine (which have the TCP header set) and
> another hook for outgoing packets (which have the TCP header set a
> little bit later).
>
> The way you would do this is by defining two netfilter hook
> structures similar to:
> static struct nf_hook_ops my_nfho = {
> .owner = THIS_MODULE,
> .hook = my_nf_hookfn,
> .hooknum = NF_INET_LOCAL_IN,
> .pf = PF_INET,
> .priority = NF_IP_PRI_FIRST
> };
> The hooknum field tells netfilter where to intercept packets.
> NF_INET_LOCAL_IN means incoming packets, NF_INET_LOCAL_OUT means
> outgoing.
>
> The hook field tells netfilter what function to run on intercepted
> packets. It should have a header similar to:
> static unsigned int my_nf_hookfn(unsigned int hooknum, struct sk_buff
> *skb, const struct net_device *in, const struct net_device *out, int
> (*okfn)(struct sk_buff *))
> The parameters will be populated by netfilter, but I think you are
> only interested in hooknum and skb. hooknum will be NF_INET_LOCAL_IN
> or similar, so you could use the same function for both filters.
>
> You also have to register/unregister the hooks using
> nf_register_hook()/nf_unregister_hook(). Browse the kernel source or
> google to find out exactly how to glue these elements together.
>
> Good luck!
> Silviu Popescu
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