Isolating internal servers behind firewalls

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

Greetings list,

I'm looking for opinions on internal enterprise network firewalling. Our
environment is almost exclusively Microsoft Active Directory-based.
There are general purpose file servers, AD domain controllers, SMS
servers, Exchange servers, and MS-SQL-based datase app servers. In all
about 80+ servers for over 2500 users on about 2000 client machines, all
running Windows XP. 

How prevalent is it to segregate internal use servers away from internal
clients behind firewalls? What benefits might we gain from the practice?
What threats are we protected from?

The firewall/security group argues that servers and clients should exist
in separate security zones, and that consolidating servers behind
firewalls allows us to 
- Control which clients connect to which servers on what ports
- Centralized administration of that network access
- Centralized logging of network access
- a single point for intrusion detection and prevention measures

These benefits protect us from risk associated with internal attackers
and infected mobile devices or vendor workstations.

On the other hand, the server team counters that 

- troubleshooting problems becomes more difficult 
- firewall restrictions on which workstations can perform administration
makes general maintenance inconvenient, esp. in an emergency
- the threats we're countering are exceedingly rare
- a broken (or hacked) firewall config breaks all access to servers if
consolidated behind firewalls 

Any and all thoughts are appreciated.


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