[Bro] Troubleshooting crashes

Tritium Cat tritium.cat at gmail.com
Mon Sep 17 17:18:38 PDT 2012

On Mon, Sep 17, 2012 at 7:56 PM, Tyler T. Schoenke <
tyler.schoenke at colorado.edu> wrote:

> On 9/11/12 2:34 PM, Tritium Cat wrote:
> >     > 614589 data_before_established
> >     > 585445 possible_split_routing
> >
> >     I'm a little curious about these two.  Normally lots of these lines
> >     indicates that something is wrong with how Bro is collecting
> >     packets.  I'm interested to find out if these go away when you adapt
> >     the snap length.  Is the MTU of your network actually 9600 or did
> >     you just set that MTU for the interface to the maximum it would
> allow?
> >
> I was seeing a lot of these as well.  I am mirroring two ports, hence a
> lot of duplicate traffic.  Are you doing something similar?  When I had
> my networking engineer turn off one of the mirrored ports, I saw a 60%
> reduction in data_before_established and 66% decrease in
> possible_split_routing.  I'm comparing data between the same hour on
> Thursday and Friday, so some of that drop is related to a normal drop in
> traffic, but most is probably turning off the mirror.

No.  It was due to the traffic being 802.1Q tagged and the default hashing
algorithm for PF_RING.  The default hashing included the VLAN id and in
this network the traffic is tagged according to peering session and
direction of flow; as a result a "5-tuple" flow is really two "6-tuple"
flows so the flow ends up split among different workers.

I think the fix should be as easy as including another PF_RING environment
variable when starting bro.

The way I understand possible_split_routing means bro is missing some
packets so you should check the front-end setup.

grep -n -B5 possible_split src/TCP.cc
521-                            // We've already sent a SYN, but that
522-                            // hasn't roused the other end, yet we're
523-                            // ack'ing their data.
525-                            if ( ! Conn()->DidWeird() )
526:                                    Weird("possible_split_routing");

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