[Snort-users] IDS Placement ideas for inside and outside a firewall.

David Glosser david_glosser at ...131...
Thu Apr 3 20:32:12 EST 2003

MessageI agree that an external sensor would be a Good Thing,  just that it would be the last thing I would do.  You first need plug any holes that may exist with packets allowed through your firewall, and  be perfectly comfortable with snort, have it tuned nicely, etc, before placing a sensor outside your firewall.  That guy will be receiving  alerts of a totally different order of magnitude.   I'm guessing 100 or 1000 times as many alerts than the internal sensors. (does anyone have any hard data on this?)

Now, if I had geographically dispersed sites, I may want sensors outside both firewalls, to determine if any kind of coordinated attack is happening.... 

----- Original Message ----- 
  From: Brei, Matt 
  To: brian.laing at ...8607... ; David Glosser ; FWAdmin ; snort-users at ...8192...sourceforge.net 
  Sent: Thursday, April 03, 2003 5:18 PM
  Subject: RE: [Snort-users] IDS Placement ideas for inside and outside a firewall.

  That’s exactly why I would want one outside of the firewall.  If I were to find a successful break in, I could then review logs from the external IDS and find that the same IP had done several scans or whatever that were eventually blocked by the firewall and not picked up by the internal IDS.  I would think that this would help build a better case if any type of legal action were to be taken. 


  -----Original Message-----
  From: Brian Laing [mailto:Brian.Laing at ...8609...] 
  Sent: Thursday, April 03, 2003 11:28 AM
  To: 'David Glosser'; Brei, Matt; 'FWAdmin'; snort-users at ...5870....net
  Subject: RE: [Snort-users] IDS Placement ideas for inside and outside a firewall.

  I would agree with this sort of implementation, in many of the installs I have done I will setup the external sensors to do nothing but logging and ignore the data till I see something worth looking at on one of the internal servers.  I use this data to see what else that IP has been doing or what other things have been attempted against a specific host

  Brian Laing
  Blade Software
  Cellphone: +1 650.280.2389
  Telephone: +1 650 367.9376
  eFax: +1 208.575.1374
  Blade Software - Because Real Attacks Hurt

  -----Original Message-----
  From: snort-users-admin at lists.sourceforge.net [mailto:snort-users-admin at ...2902...ists.sourceforge.net] On Behalf Of David Glosser
  Sent: Wednesday, April 02, 2003 11:10 PM
  To: Brei, Matt; FWAdmin; snort-users at lists.sourceforge.net
  Subject: Re: [Snort-users] IDS Placement ideas for inside and outside a firewall.

  If you've never set up any IDS before, I'm not sure you would want to place it outside your firewall immediately You'lll get overwhelmed with probes,scans, script kiddies etc. 

  First place the box (with the "snorting" NIC unnumbered). On the port monitoring the *internal* interface of your firewall. Let it work on all of the stuff your firewall lets through. Once you have that under control, then place another box (or another NIC on the same box) to monitor your internal servers (since breakins can come from internal users). 

  Once you have these two under control, then you can worry monitoring stuff outside the firewall,  which I believe is called *attack detection*. But do you care that much about the stuff your firewall is successfully blocking?


     I am trying to convince my company to implement IDS on our network but I have a few questions. I know I would want one on both sides of the firewall, 

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