[Snort-users] EXTERNAL_NET: any vs !$HOME_NET

Hari Sekhon hpsekhon at ...14012...
Mon Jan 1 18:41:16 EST 2007

Ok, so I should split the intrusion detection into one for external by
the borders and one or more for internal, but this still leaves me
with the same problem that I want to detect as many things as possible
as long as they aren't false alerts, again I would have to have
EXTERNAL_NET as any and then have a huge amount of normal traffic for
which I'd have to write pass rules, which is basically where I am
already at.

How many servers do you run snort on, every server, or one or two (in
which case it's not easy to see traffic to other hosts unless you have
a switch that can monitor all ports (which I don't - and don't fully
understand how you could even do this, the combined traffic from all
ports would be much more than a single port monitoring host could
handle). I've taken the web address of

I guess I would have to do a large amount of commenting out of rules
and adding a lot of pass rules for normal network activity (which I
have already had to do and looks like I will have to do much more of)

It seems that the lesser of two evils for a lot of this stuff is to
just blind myself to a lot of things and then hope what's left is


On 01/01/07, Jason Brvenik <jasonb at ...1935...> wrote:
> Hari Sekhon wrote:
> > I've currently got "var EXTERNAL_NET any" in my snort.conf and was
> > considering making it "var EXTERNAL_NET !$HOME" instead, but looking
> > at the rules files, it seems that most rules will immediately
> > disregard any suspicious traffic from your HOME_NET in this case,
> > which basically blinds you to any internal threats.
> Correct. A proper deployment will have systems monitoring external
> threats and a different system monitoring internal threats. You could
> also run multiple instances of Snort on the same machine with different
> interfaces and configurations. This is a less preferred method but often
> makes budget happier. You should be aware that bridging an external and
> internal network with _any_ device regardless of purpose has a certain
> amount of risk involved.
> >
> > I am also running snort on several servers that are not publicly
> > accessible (ie port forwards) but want to be able to see malicious or
> > suspicious traffic from all networks.
> >
> > The current problem with the EXTERNAL_NET any is that a lot of rules
> > are throwing up too many false positives and it's very difficult to go
> > around writing pass rules for every other packet that goes through the
> > network interface (I exaggerate slightly)
> You are asking too much of one system and configuration. If you needs
> are more complex and detailed, you should move to a more complex and
> detailed configuration.
> >
> > It's seems a very difficult juggling act to on the one hand stop false
> > positives and
> > on the other to not totally negate the worth of the ids by making it too loose.
> It is until you split the functions up into more manageable chunks.
> >
> > For example I have stacks of "MS Terminal server request RDP" alerts
> > coming from machines on my home net. I can see how changing the
> > EXTERNAL_NET would be a good idea to stop these unless they come from
> > outside the network, but considering that this also stops most rules
> > from matching if somebody attacks from a machine within the building
> > or any remote site connected via vpn (which are included in HOME_NET
> > and therefore excluded from EXTERNAL_NET)
> >
> > Anybody got any advise on this?
> Create external, internal, VPN, and B2B segments and then monitor each
> appropriately. Each zone has a different threat perspective and should
> be monitored with different rules and configurations.
> >
> >
> > --
> > Hari Sekhon
> >
> > -------------------------------------------------------------------------
> > Take Surveys. Earn Cash. Influence the Future of IT
> > Join SourceForge.net's Techsay panel and you'll get the chance to share your
> > opinions on IT & business topics through brief surveys - and earn cash
> > http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
> > _______________________________________________
> > Snort-users mailing list
> > Snort-users at lists.sourceforge.net
> > Go to this URL to change user options or unsubscribe:
> > https://lists.sourceforge.net/lists/listinfo/snort-users
> > Snort-users list archive:
> > http://www.geocrawler.com/redir-sf.php3?list=snort-users
> >

Hari Sekhon

More information about the Snort-users mailing list