Re: Snort inline extremely slow packet forwarding

[Hussein Bahaidarah <husseinb () gmail com>]

Hi Michael,

I believe a an L2 loop will form. I have tested it and found that the switch (Cisco) disabled the port: 

Jul 16 00:18:52: %ETHCNTR-3-LOOP_BACK_DETECTED: Loop-back detected on GigabitEthernet1/0/23.
Jul 16 00:18:52: %PM-4-ERR_DISABLE: loopback error detected on Gi1/0/23, putting Gi1/0/23 in err-disable state
Jul 16 00:18:53: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/23, changed state to down
Jul 16 00:18:54: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/23, changed state to down


Loopback error

A loopback error occurs when the keepalive packet is looped back to the port that sent the keepalive. The switch sends 
keepalives out all the interfaces by default. A device can loop the packets back to the source interface, which usually 
occurs because there is a logical loop in the network that the spanning tree has not blocked. The source interface 
receives the keepalive packet that it sent out, and the switch disables the interface (errdisable). This message occurs 
because the keepalive packet is looped back to the port that sent the keepalive:
%PM-4-ERR_DISABLE: loopback error detected on Gi4/1, putting Gi4/1 in
err-disable state
Keepalives are sent on all interfaces by default in Cisco IOS Software Release 12.1EA-based software. In Cisco IOS 
Software Release 12.2SE-based software and later, keepalives are not sent by default on fiber and uplink interfaces. 
For more information, refer to Cisco bug ID CSCea46385  (registered customers only) .
The suggested workaround is to disable keepalives and upgrade to Cisco IOS Software Release 12.2SE or later.



On Jul 15, 2011, at 11:24 PM, Michael Altizer wrote:

I don't believe it should cause any sort of internal loop, but I have 
never tested it.  Having additional Snort instances on the same set of 
interfaces should be fine as long as the others are not in inline mode.  
The AFPacket DAQ module cannot know about other instances running on the 
same traffic, so it would result in one copy of the packet being 
transmitted per inline copy of Snort on an interface pair.

On 07/15/2011 03:25 PM, Hussein Bahaidarah wrote:
> So, would that create an internal loop? What about if I want to sun another instance of Snort with the same pair of 
> interfaces, will it work? or a loop will take place?
>
> Regards,
>
>
> On Jul 15, 2011, at 10:14 PM, Michael Altizer wrote:
>
> Correct.  The inline mode of the AFPacket DAQ module handles all of the
> packet forwarding.  By putting those interfaces in a bridge, you are
> retransmitting every packet a second time in addition to all of the
> other overhead associated with Linux bridges.
>
> On 07/15/2011 03:05 PM, Hussein Bahaidarah wrote:
> > Yes, I am bridging them in linux. This is what assumed should be done. Do you imply that I should break the bridge? 
> > will snort do the bridging instead? Eth1 is not used and not connected to any thing.
> >
> > [root@IPS ~]# brctl show
> > bridge name     bridge id               STP enabled     interfaces
> > br0             8000.0010184d122c       no              eth3
> >                                                         eth2
> >
> > Thanks,
> >
> > On Jul 15, 2011, at 9:50 PM, Michael Altizer wrote:
> >
> > On 07/15/2011 02:41 PM, Hussein Bahaidarah wrote:
> > > Thanks Rmkml for help,
> > >
> > > I found a work around and I don't understand how and why it did work.
> > > First, let me explain my configuration:
> > > eth2 and eth3 are bridged and snort IP should run on them
> > > eth1 is not used
> > >
> > > when I use:  "snort    -N -K none -k notcp -c rules/inline -A console   --daq afpacket -i eth3:eth2   -Q"  the 
> > > slowness problem appear
> > >
> > > my work around is to use " snort    -N -K none -k notcp -c rules/inline -A console   --daq afpacket -i eth3:eth1   
> > > -Q ". This works fine though eth1 is not used!!
> > A couple questions:
> >
> > What do you mean by "eth2 and eth3 are bridged"?  You're not putting
> > them into a Linux bridge (with brctl), right?
> >
> > Why is eth1 not being used in the second scenario?


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