netdev - Re: Linux, tcpdump and vlan

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Date:	Thu, 19 Jul 2007 14:38:20 -0700 (PDT)
From:	andrei radulescu-banu <iubica2@...oo.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Krzysztof Halasa <khc@...waw.pl>, linux-kernel@...r.kernel.org,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: Linux, tcpdump and vlan

During debugging, I noticed that dev_queue_xmit() is called twice for tx vlan frames. This results in a frame being passed twice to a packet socket bound to 'any' interface. If the packet socket is bound to a specific interface, though, it will get only one copy of the tx frame, which is good.

In more detail: suppose we're tx'ing a frame, and the route table lookup yields a vlan outgoing device eth0.2. dev_queue_xmit() is called, which calls dev_queue_xmit_nit() for dev = eth0.2 then dev->hard_start_xmit() for dev = eth0.2. 

The latter call gets into the vlan layer, which attaches the vlan id 2 (accelerated or not... in my e1000 case accelerated) then calls dev_queue_xmit() again. This time around dev_queue_xmit_nit() is called for dev = eth0, and dev->hard_start_xmit() actually calls the ethernet driver.

The net result is that dev_queue_xmit_nit() is called twice, once for dev=eth0.2 then for dev=eth0.

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