Re: is skb payload missing when using NF_IP_POST_ROUTING ?

[Patrick McHardy <kaber@trash.net>]

SEBASTIEN LEGER wrote:
> Hi,
> 
> I'm trying to develop a queue handler using netfilter and
> iptables mangling to catch all data coming in and out of my network
> card. I'm using a hook on NF_IP_PRE_ROUTING and another one on
> NF_IP_POST_ROUTING. Everything is running fine but...
> 
> unfortunately,
> all skb received on NF_IP_POST_ROUTING hook seem to have a non
> initialized random payload, however packet length is correct (size is
> retrieved from IP header analysis, not from skb length field). Packet
> payload is dumped on dmesg and compared to a live tcpdump capture
> running in background: dmesg contains junk data above TCP layer instead
> of pure FTP :(((
> 
> Does it mean that skb payload is not yet
> available when NF_IP_POST_ROUTING hooks are called? my understanding
> was that post routing was called just before sending packet on the
> network device, is it wrong? if I misunderstood, how can I catch
> packets coming out of my device and access to their full content?


Most likely you're not handling non-linear skbs properly.
Look into nfnetlink_queue for an example.