linux-kernel - Re: When does Linux drop UDP packets?

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Date:	Thu, 4 Jun 2009 12:57:33 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Philipp Reh <sefi@...-f-i.de>
cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: When does Linux drop UDP packets?


On Thu, 4 Jun 2009, Steven Rostedt wrote:

> 
> Oh, btw, you don't need to strip LKML from replys (or any Cc), unless it 
> is off topic.  LKML is a public forum, and as long as things are on topic, 
> it is nice to show others the solutions that are found.
> 
> > 
> > On Thu, 4 Jun 2009, Philipp Reh wrote:
> > 
> > > I'm not really sure it is dropped but tcpdump sees the packet and
> > > recvfrom doesn't see it, so I think that the kernel might drop it
> > > somewhere when processing the IP stack logic.
> > 
> > Looking at the udp stack, selinux plays a role, and looks like it will 
> > drop a packet if it is not in the routing table.
> > 
> > Try:
> > 
> > # setenforce 0
> > 
> > and see if you start seeing packets.
> 
> BTW, I used ftrace to look into this:
> 
> Enable:
> 
> 	CONFIG_FTRACE
> 	CONFIG_FUNCTION_TRACER
> 	CONFIG_FUNCITON_GRAPH_TRACER
> 	CONFIG_DYNAMIC_FTRACE
> 
> Compile and boot:
> 
> # mount -t debugfs nodev /sys/kernel/debug
> # cd /sys/kernel/debug
> # echo udp_rcv > set_graph_function
> # echo function_graph > current_tracer
> # cat trace
> 

Could you do the above and send me (off list) the output of the trace?

-- Steve

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