[Click] Basic question about IP header fields.

[Beyers Cronje]

Josh,

And so I inadvertently demonstrated one of the classic beginner mistakes to
watch out for :) Always either push/pull, kill or store the packet for later
use...


On 9/11/07, Eddie Kohler <kohler at cs.ucla.edu> wrote:
>
> Thanks, Beyers -- but I'd either "assert(iph)" or "if (!iph) {
> p->kill(); return; }", to avoid leaking memory.
> E
>
>
> Beyers Cronje wrote:
> > Hi Josh,
> >
> > You need to get a pointer to the IP Header. A simple example:
> >
> > const click_ip *iph = p->ip_header();   // Get pointer to IP Header
> > if (!iph)   // Make sure it's a valid header
> >   return;
> >
> > if (iph->ip_tos <= n)   // Check TOS field
> >   output(0).push(p);
> > else
> >   output(1).push(p)
> >
> > The above example expects IP packets. So you'll have to filter on IP
> packets
> > and also set the IP header annotations i.e.:
> >
> > FromDevice(eth0) -> Classifier(12/0800) -> MarkIPHeader(14) ->
> YourElement;
> >
> > Classifier to filter IP packets, and MarkIPHeader to set annotations.
> You
> > can also use CheckIPHeader instead of MarkIPHeader.
> >
> > Hope this helps.
> >
> > Beyers
> >
> > On 9/10/07, Josh Butler <tuna_armadillo at hotmail.com> wrote:
> >> Hi,
> >>
> >> This is my first question and probably won't be my last. I've been
> doing a
> >> project involving Click for a while, but owing to my very basic
> programming
> >> skills have struggled quite a lot, though I've enjoyed messing about
> with
> >> click in the process - I've been writing a bit of an idiot's guide as
> I've
> >> gone along!
> >>
> >> I've got my head round creating the router configuration scripts by
> >> combining elements and am now looking to go inside the elements and
> make
> >> some changes. I've chosen the tee element as one of the easier elements
> to
> >> look at and modify, and have a good understanding of what it does in
> push
> >> mode.
> >>
> >> I now want to modify this element in a very basic way, using a simple
> if
> >> statement if possible so that I can push packets through either the
> >> element's first or second output port depending on some criteria such
> as the
> >> type of service field within the IP header. Here's what I mean in
> >> pseudocode, if that's the correct term;
> >>
> >> if (type_of_service_field <= n){
> >>     output(0).push(p);
> >>     }
> >>     else {
> >>     output(1).push(p);
> >>     }
> >>
> >> I've looked through all the material on the click site and can't find a
> >> simple answer to the question of how to access the different fields
> within
> >> the IP header. Maybe I'm being stupid, but a bit of help would be
> awesome.
> >>
> >> Thanks,
> >> Josh
> >>
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> >>
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