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Re: [OT ish] Router vs Firewall - corporate environment
From: Chris Brenton <cbrenton () chrisbrenton org>
Date: Thu, 04 Feb 2010 17:08:12 -0500

Hey Martin,

On Tue, 2010-02-02 at 13:38 +0000, martin wrote:

We're in the process of planning to split up our corporate network -
ie, a subnet for servers, one for users, one for admins etc etc.


Now a debate has started over whether we should use the router to
split up our network, or whether we should go to the extra expense of
buying a firewall to do this. 

First off, kudos on deciding to segment out the internal network based
on security zones. This will make it much easier to detect/contain
Malware if a system gets whacked.

 As I understand it, if I send a request
from subnet 1 to subnet 2 on port 80, the source port (is over 1024)
would have to be open for the reply to come back from subnet 2 to
subnet 1.  However, as firewalls are stateful, they do not require
this - I would just need to open port 80 to subnet 2.

Sort of. A stateful firewall still needs that return port opened, if
just does it for you automatically. With a static set of rules you will
need to permit all ACK traffic above 1023, all of the time. Stateful
only opens the socket when an outbound request is actually active.

So for example an nmap '-sA' scan will blow right through the router. It
should not be able to get past a good stateful firewall setup.

Apart from the greater logging capabilities, this is the only reason I
can come up with to use a firewall.

First off the logging is pretty major. Cisco routers rate limit their
logging so you never actually see all the traffic. Further, they can
generate log irregularities. For example you'll find that all of your
ICMP traffic looks like Echo-Replies unless you define rules for every
single type/code.

  Does anybody have any additional
suggestions as to why we should use a firewall ? 

More secure posture, better control of complex apps (FTP, VoIP, etc),
etec. etc. Think of all the reasons why people do not trust routers as
their only line of defense and you'll get the idea.

 Or likewise, why a firewall might not be necessary.

These days you need to plan on Malware getting past your perimeter and
take steps to mitigate/contain/detect when possible. If you are running
NAC, HIPS or app control, that will take up much of the slack.

As for a specific recommendation, can't say for sure. Don't know what
other security steps you've taken, what business you are in, how much
risk you can live with, etc. etc. Its like trying to figure out when a
puzzle piece goes without having the rest of the puzzle to look at. ;-)


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