Re: Snort Help - Network IDS

From: Brian Whitehead (
Date: 04/15/04

  • Next message: Matt Mercer: "Re: Snort Help - Network IDS"
    Date: Wed, 14 Apr 2004 20:02:27 -0500 (CDT)
    To: <>

    > Recently I posted a question on different types of monitoring and ids
    > setups. I have decided to go with snort and have been using it on a
    > smaller network with no problem. However now, I need to move it to a
    > production network which will consist of around a 100 servers all linked
    > through 3com switches and going out through a watchgaurd firewall. I'm
    > looking for different ways to implement this without setting up another
    > single point of failure device which our firewall is. I'm not confident
    > enough yet to risk something like that. I haven't found much information
    > on packet sniffing when it comes to multiple entry points, found some
    > info on wiretap, etc. but I've always received such great help on here I
    > thought I would ask before I decided on something. Would really
    > appreciate any help, I'm in a heck of a bind right now. Thanks.
    > firewall
    > |
    > -3comswitch-servers
    > -3comswitch-servers
    > -3comswitch-servers
    > ids?
    > Jason Haith


    If you don't a single point of failure, such as using it inline between
    the firewall and switch, then you will need to setup port monitoring on
    your switches. Some switches cannot do this across stacked switches, so
    check the documentation on your switches. Also, if you are using multiple
    VLAN's you will not be able to use a single box, unless it has multiple
    NIC's to monitor more than one VLAN. Basically, the Snort box will be
    connected directly to one of the switches and the switch will be
    configured to mirror all traffic to the port that it's plugged into.
    Usually this can be configured to monitor either ingress, egress or
    traffic both ways.

    One thing to note is that the port that the NIDS is connected to cannot
    talk on the network. It can only listen. So, you will either need to
    access it physically at the console or put an additional NIC in the box to
    access it remotely. Again, with the stacked switches this will depend on
    the capabilities of the switch. Some can be managed and actually know
    that the ports on the other switches exist, while others will simple know
    that the MAC address for several machines exist through a single port. In
    the latter case, you should still be able to monitor all of the traffic in
    and out of the single port, but you won't be able to monitor traffic
    destined for the same switch if it's not directly connected. Just make
    sure that wherever you connect the NIDS that it can see all of the
    machines whose traffic you want to monitor.

    I'm sure you might be able to do some confounded setup like mirroring all
    traffic on each switch to a single port and then connect that port to the
    next switch. This would mean you would have two connections between each
    port. One that is simply mirroring the traffic and the other that is the
    actual uplink. I'm not sure this kind of setup would be a good idea
    though. You could also put multiple NIC's in the box and connect one to
    each switch. The one downfall I can see to this is that you will see some
    traffic more than once as it heads through the switches to get in and out
    of the firewall.

    Hope this helps. Sorry if I confuse you. The new Snort 2.1 book is due
    out this month if you need a good reference.

    ---------- --------
    |switch |====|IDSBOX|
    ---------- --------
    |switch |
    | switch |

    Brian W
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  • Next message: Matt Mercer: "Re: Snort Help - Network IDS"

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