[Click] Basic question about IP header fields.

[Eddie Kohler]

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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