Re: Internal Network vs. External Network

[Peter Bates <peter.bates () ucl ac uk>]

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Hello all

On 12/09/2012 16:04, Turnbough, Bradley E. wrote:
> I have two networks behind my firewall which have a IDS
> requirement.  They are both "Internal" because they're "inside" my
> company.
>
> Snort operates on "Internal" and "External" networks.
>
> Should I consider the "internal networks" the ones that require the
> IDS, and everything outside of them to be "external networks"?

Yes - a good starting point is generally

ipvar HOME_NET [192.168.0.0/16,10.0.0.0/8]
ipvar EXTERNAL_NET !$HOME_NET

Obviously replacing the RFC1918 addresses with your actual public
address ranges.

There is an argument for

ipvar EXTERNAL_NET any

if your IDS is placed where it might see intra-network traffic - i.e.
traffic from one of your hosts to another typically indicative of
worm-like activity.

However generally the majority of unusual traffic these days seems to
be either heading from your HOME_NET to EXTERNAL_NET - or EXTERNAL_NET
attackers hitting services you may be running on HOME_NET.

- -- 
Peter Bates
Senior Computer Security Officer    Phone: +44(0)2076792049
Information Services Division       Internal Ext: 32049
University College London
London WC1E 6BT
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