James King wrote:

On Thu, Aug 28, 2008 at 10:02 AM, Ben Hutchings
[off-list ref] wrote:

quoted

Thiago Lacerda wrote:

quoted

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);

What makes you think ihl is valid?

Please read <http://www.cpni.gov.uk/Docs/InternetProtocol.pdf> before
trying to parse untrusted packets.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.