[Snort-users] Configuring Cisco switches...

Mayers, Philip J p.mayers at ...1913...
Fri Sep 21 08:28:02 EDT 2001

Without knowing your network topology requirements, that's impossible to
answer. Generally speaking, you can only mirror what passes through the
switch - which means that if the switch sits between you and the outside
world in a tree configuration (trunk==connection to internet,
branches==connections into site) then you'll only see traffic crossing your
routing boundary.

However, if your layout is a star (connection to internet==one arm,
connections to site==other arms) then all cross-site (e.g. from one
building/distributor to another) traffic can also be mirrored - but then
you're not seeing intra-building/distributor traffic...

At that point, you have to ask yourself - by introducing a large
single-point-of-failure (the star centre) into the network, just to be
paranoid about seeing your internal traffic, haven't you just gone over the
top? Balance your business need against "cost", and if you really need to
see intra-distributor traffic, use a distributed snort system logging into a
central database with one snort box per chokepoint.


| Phil Mayers                              |
| Network & Infrastructure Group           |
| Information & Communication Technologies |
| Imperial College                         |

-----Original Message-----
From: Bryan Childs [mailto:bryan.childs at ...3120...]
Sent: 21 September 2001 15:21
To: 'Erek Adams'
Cc: 'snort-users at lists.sourceforge.net'
Subject: RE: [Snort-users] Configuring Cisco switches...

Ok - after talking to my net admin chappy - he has another question, and I
quote :

"it would be better to ask of the best way to set up an ethernet network to
optimise your chances of capturing information whilst maintaining high
performance switched networks"

and he said to ignore any smart arses that suggested going back to using
hubs :)

Well ?

Anyone got any good advice on this...

On the face of it - turning on the port mirroring on the switch sounds like
it will do the job - but will anything suffer noticeably after we've done
it? (Apart from the snort box, we're expecting that!)


> -----Original Message-----
> From: Erek Adams [mailto:erek at ...577...]
> Sent: 21 September 2001 15:15
> To: Bryan Childs
> Cc: 'snort-users at lists.sourceforge.net'
> Subject: Re: [Snort-users] Configuring Cisco switches...
> On Fri, 21 Sep 2001, Bryan Childs wrote:
> > Hi everyone - this question has probably been done to 
> death, but my google
> > searching for answers has amounted to nought - so I'm going 
> to have to ask
> > it again I'm afraid!
> It's Ok, we'll just give you lashes with a wet noodle.  ;-)
> > The network here in my building is of course suffering from 
> the recent Nimda
> > virus/worm breakout, and we're trying to track infected 
> boxes with snort.
> >
> > The entire network here is running on switched ethernet, 
> which is giving us
> > a bit of a headache. Most of the switches are dumb 3Com 
> supplied ones, but
> > we've been sensible enough (we think) to plug out snort box 
> into the Cisco
> > one which sits at the top of the network.
> >
> > The trouble is that we *still* don't seem to be able to 
> monitor attacks
> > which don't directly go for the snort box itself.
> >
> > The card is set up in promiscuous mode as it should be - 
> but we think we
> > need to do something to the switch to make sure it sees ALL 
> our internal
> > network traffic.
> >
> > Does anyone know what we might have missed? Or have any 
> suggestions at all?
> Yeppers...
> http://snort.sourcefire.com/docs/faq.html#1.8
> Now, your Cisco _should_ be able to do that.  If you don't 
> know talk with your
> local netoworking geek.  Bribe him with some wire ties or something...
> > Cheers amigos......
> Oh, you're bringing the beer?  Great!  Bring some Shinerbock.  :)
> -----
> Erek Adams
> Nifty-Type-Guy
> TheAdamsFamily.Net


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