RE: Asymmetric routing and IDS correlation ?

["Biswas, Proneet" <pbiswas () ipolicynet com>]

The loadbalancer should work, but are the routers physically close or in different locations.

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-----Original Message-----
From: snort-users-admin () lists sourceforge net [mailto:snort-users-admin () lists sourceforge net] On Behalf Of Josh 
Berry
Sent: Tuesday, March 23, 2004 9:32 AM
To: Glenn Forbes Fleming Larratt
Cc: snort-users () lists sourceforge net
Subject: Re: [Snort-users] Asymmetric routing and IDS correlation ?

I am not sure how to do this with Snort alone, but you can use a device
that aggregates traffic such as the TopLayer IDS Balancer, feeding both
SPAN's into the Balancer and spitting them back out of one interface into
the Snort IDS sensor.

> Our border is configured with two loadshared connection between
> border and core routers, using OSPF loadsharing in its default
> (per-packet) mode of operation. Two redundant snort hosts each
> have a dedicated, unaddresses, promiscuous-mode tap on each of
> the two links.
>
> This creates the nontrivial problem that any attempt to use
> stateful features of Snort (or any other IDS) may fail because
> of asymmetric routing, thus:
>
>        border
>         |  |         +---+
>         +--U---------+   + snorthost (only one of two shown)
>         |  +---------+   +
>         |  |         +---+
>         core
>
> If, for the sake of argument, a TCP conversation, occurs:
>
>    Syn
>          Syn/Ack
>    Ack
>          {banner}
>    {query1}
>          {response1}
>    {query2}
>          {response}
>    Fin
>          Ack
>          Fin
>    Ack
>
> , and the routing and loadsharing is such that inbound traffic takes
> the left-hand link and outbound the right-hand, then neither of the
> two instances of snort on the snorthost will get enough information
> to do even minimal correlations, let alone use "flow" and "session"
> keywords.
>
> We know we could make the two links preferred/backup, rather than
> equal-value loadshare, but that throws away half our bandwidth.
>
> Question 1: Is there any way for snort to be smart enough to have
> one instance looking at both interfaces, or to share state between
> two instances?
>
> Question 2: [sort of OT for this list] is there a standards-based way
> to make two-way loadsharing "per-conversation", as it were, to obviate
> this issue?
>
> Any assistance gratefully received.
>
>       thx,-g
> --
> Glenn Forbes Fleming Larratt         The Lab Ratt (not briggs :-)
> glratt () io com                        http://www.io.com/~glratt
> There are imaginary bugs to chase in heaven.
>
>
>
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