[Click] Basic question about IP header fields.

[Beyers Cronje]

Hi Josh,

The ip_len field is in network byte order, assuming you are running on an
Intel platform you need to convert it to host byte order using
ntohs(iph->ip_len)

Google on these terms: ntohs, ntohl, endianness, network byte order, host
byte order

See http://en.wikipedia.org/wiki/Endianness for an overview.

Beyers


On 10/23/07, Nikki <tuna_armadillo at hotmail.com> wrote:
>
> Hi guys,
>
> Firstly, many many thanks for the help, its been great - I've only just
> got round to putting it into practice now I've got most of the theory
> out of the way, but the code you gave me has worked a treat. I think I
> need to read up on pointers a bit more to fully understand it, but I
> think I see what its doing.
>
> So anyway, on to my new question. I tried messing around with the code,
> in my new element and it now looks like this:
>
> const click_ip *iph = p->ip_header();   // Get pointer to IP Header
>
>         if (!iph){   // Make sure it's a valid header
>         p->kill();
>         return;
>         }
>         else if (iph->ip_len <= 1000){   // Check Total Length field
>         output(0).push(p);
>         }
>         else{
>         output(1).push(p);
>         }
>
> I want to split packets through different outputs depending on packet
> size, but the code above just pushes everything through output 1. I'm
> using FromDump in my config script, and am using a dump with packets of
> varying sizes, from 40 to 1420 bytes - but they all come out of output
> 1, not output 0. Where am I going wrong?
>
> Thanks,
> Josh
>
>
> On Tue, 2007-09-11 at 01:02 +0200, Beyers Cronje wrote:
> > 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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