netdev - Re: problem with flowi structure

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Message-ID: <AANLkTimpnqNSE21u=2qjb5VGJrn_3_--Bqp4veCNXMPE@mail.gmail.com>
Date:	Fri, 17 Sep 2010 10:46:10 +0200
From:	Nicola Padovano <nicola.padovano@...il.com>
To:	AIJAZ BAIG <aijazbaig1@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	netfilter-devel <netfilter-devel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: problem with flowi structure

Aijaz, great! Now it' clear. Thank you.
It remains a problem: the meaning of

fl.nl_u.ip4_u.saddr = 0

But, first of all:
- we know that a routing table doesn't contain the ip source
- but the routing cache contains it.

so my first question is: why routing cache has it and routing cache
not? what it its usage?
Then my second question: the meaning of fl.nl_u.ip4_u.saddr = 0

thank you very much

On Fri, Sep 17, 2010 at 7:40 AM, AIJAZ BAIG <aijazbaig1@...il.com> wrote:
> Hi there,
>
> Well..if u see the code, ull see that ip_route_output_key is a wrapper
> around ip_route_output_flow which in turn internally calls
> __ip_route_output_key. This later function tries to find a route for a
> packet first by looking up the routing cache and then if it isnt found
> there it  constructs a cache entry by calling its slow counterpart
> which is ip_route_output_slow.
>
> Now, the name tells u a lot. First ip_route_output_key tells u that
> this function 'routes' an 'output' packet. An output packet normally
> means that the packet is a locally generated packet. The routing (L3)
> protocol being used is ip (ipv4 specifically) and key tells us that we
> are specifying the key to be used to check the routing cache (since
> the cache is organized as a hash and it needs a key to retrieve an
> entry).
>
> Thus all in all, the ip_route_output_key function is used to get a
> route for an output packet either from the routing cache if an entry
> is present or create one and update the cache accordingly.
>
> Hope it helped.
>
> Regards,
> Aijaz
>
> On 9/17/10, Nicola Padovano <nicola.padovano@...il.com> wrote:
>> ok, let's step back...
>> what's the task of ip_route_output_key function ?
>>
>> --
>> Nicola Padovano
>> e-mail: nicola.padovano@...il.com
>> web: http://npadovano.altervista.org
>> --
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>



-- 
Nicola Padovano
e-mail: nicola.padovano@...il.com
web: http://npadovano.altervista.org
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