netdev - Re: raw PF_PACKET protocol selection

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Message-Id: <1191920407.6682.89.camel@gentoo-jocke.transmode.se>
Date:	Tue, 09 Oct 2007 11:00:07 +0200
From:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	'Herbert Xu' <herbert@...dor.apana.org.au>, netdev@...r.kernel.org
Subject: Re: raw PF_PACKET protocol selection

On Tue, 2007-10-09 at 12:17 +0400, Evgeniy Polyakov wrote:
> On Tue, Oct 09, 2007 at 09:51:25AM +0200, Joakim Tjernlund (joakim.tjernlund@...nsmode.se) wrote:
> > On Tue, 2007-10-09 at 11:34 +0400, Evgeniy Polyakov wrote:
> > > On Tue, Oct 09, 2007 at 09:27:38AM +0200, Joakim Tjernlund (joakim.tjernlund@...nsmode.se) wrote:
> > > > > Did you change eth_type_trans() to catch your proto?
> > > > > 
> > > > 
> > > > Just fond out something:
> > > > if I redirect my prog like so:
> > > > ./sniff > log
> > > > and press Ctrl-C after a packet has been sent to it, 
> > > > it does NOT work. I don't get ANY output in my "log" file, not
> > > > even the printf("---------\n") appears.
> > > > But if I run whithout redirect it works(at least with ETH_P_BPQ)
> > > > Anyone else see this too?
> > > 
> > > I only tested with IP and ARP packets - I can not say when packet was
> > > actually received and written to log, but it does start filling up, but
> > > maybe not immediately - it can be output buffering in shell though.
> > 
> > Did you receive many packets? Seems like when I receive just 1 or 2 pkgs
> > I get the empty log. If I strace ./sniff > log I see that recvfrom gets
> > pkgs, but there are no trace of writes. I guess this
> > is a bash(3.2_p17) or glibc(2.5.-r4) bug?
> 
> I received 1396 bytes of logs before terminated, which is 27 ARP packets, 
> so there is quite big number of packet there. 
> Your application works correctly (although you swapped source and
> destination ethernet fields) - buffered writing is not a bug,
> if you do not like it, use write(2), mmap(2) or turn buffering off as
> Herbert suggested. To get packets with your own ethernet protocol number
> you have to change eth_type_trans() function in kernel, which parses
> ethernet header and returns protocol number, under some conditions it
> will just return your number automatically, but you should check it.

I thought that flushing was done automatically when SIGINT happened
but I was apperently wrong. Sorry for the noise and thanks for your
help. I have added setvbuf calls to make it unbuffered.

 Jocke
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