netdev - Re: Questions about Linux kernel network programming

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Date:	Thu, 28 Aug 2008 11:12:35 -0700
From:	"James King" <t.james.king@...il.com>
To:	"Ben Hutchings" <bhutchings@...arflare.com>
Cc:	"Thiago Lacerda" <thiagotbl@...il.com>,
	"Tobias Koeck" <tobias.koeck@...il.com>, netdev@...r.kernel.org
Subject: Re: Questions about Linux kernel network programming

On Thu, Aug 28, 2008 at 10:02 AM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> Thiago Lacerda wrote:
>> BTW, anyone knows the right way of get a tcp port number in human readable form?
>> I'm doing like this:
>>
>> struct tcphdr* tcp = tcp_hdr(my_sk_buff)
>>
>> unsigned short src_port = ntohs(tcp->source)
>>
>> And it isn't working, the numbers that I get are not right.
>
> Until the packet has gone through the network protocol handler (IP), the
> transport header pointer will not be set correctly and tcp_hdr() will return
> a pointer to the start of the packet.

Thanks for the explanation Ben, I was having the same problem with a
toy netfilter module hooked in prerouting a while ago and never
bothered to find out why.  Thiago, here's how I got it working:

struct tcphdr *th = NULL, _tcph;
struct iphdr *iph = ip_hdr(skb);
if (iph && iph->protocol == IPPROTO_TCP)
th = (struct tcphdr *)(skb->data + (iph->ihl * 4));

Alternatively, I think this will work too:
th = skb_header_pointer(skb, (iph->ihl * 4), sizeof(_tcph), &_tcph);


HTH
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