RE: 2 firewalls protecting internal network

[Dan Lynch <DLynch () placer ca gov>]

> I know that there is a defence in depth idea to implement 2 firewalls,
> each from different vendor.
>
> what you think about it ? is it practical?

Whether it's practical depends in part on the complexity of the environment and of the firewall rules. But even using a 
single vendor, there are gains that can be had. Rather than running 25 interfaces on a single firewall, with 300+ 
rules, splitting the implementation can simplify the rules, and reduce the hardware performance requirement. Two $5000 
boxes might replace one $25,000 box. A 300 rule policy might be replaced by two 100 rule policies.

One firewall connects to external-facing DMZ networks and the internet, another divides internal business units with 
different security requirements from one another, segregates server networks from user networks, or test environments 
from production, etc. A separate firewall might reside in one business unit for connections to their external partners. 
One drawback is that connections that traverse multiple firewalls require a rule on each. I find this to be less of a 
problem than the overall reduction in complexity of the rules on each firewall. 

Another benefit might be to reduce the impact of maintenance downtime, and increase your flexibility in scheduling 
change windows. Consolidating everything into a single box subjects you to the lowest common denominator / most 
restrictive change policy for all connections on that box. Isolating those restrictive policies on a separate firewall 
could allow more flexibility for administering the other systems.

As always, YMMV, and probably will.

Dan Lynch, CISSP
Information Technology Analyst
County of Placer
Auburn, CA


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