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
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