netdev - Re: Query regarding pf

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Message-ID: <1332390522.9433.17.camel@edumazet-glaptop>
Date:	Wed, 21 Mar 2012 21:28:42 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Prashant Batra (prbatra)" <prbatra@...co.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Query regarding pf_packet sockets

On Thu, 2012-03-22 at 08:52 +0530, Prashant Batra (prbatra) wrote:
> Hi ,
> 
> I am trying to use PF_PACKET socket along with filters applied on the socket through setsockopt, SO_ATTACH_FILTER.
> Now as I create this socket, any packet coming to the kernel would be copied into the socket buffer for this socket.
> 
> So, even if I apply the filter, first few packets would always be those which do not match the filter. 
> 
> Is there a way (some socket option),  to make socket passive and active, so that I will set the socket in passive mode after creation, 
> and then set it as active, after I apply the filter, so that I only receive the filtered packet in user space.
> 
> Thanks,
> Prashant
> 
1) open PF_PACKET
2)  Install a "deny all packets" filter
3) read all packets that might have been captured right before 2)
4) Install your filter

This is what is done by libpcap




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