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Deciding What Should Reside on the DMZ
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It also lends itself well to heterogeneous firewall environments. For example, a
packet-filtering firewall with high network throughput might be used as the “exter-
nal” firewall; an application-gateway (proxying) firewall, arguably more secure but
probably slower, might then be used as the “internal” firewall. In this way, public
web servers in the DMZ would be optimally available to the outside world, and pri-
vate systems on the inside would be most effectively isolated.
Deciding What Should Reside on the DMZ
Once you’ve decided where to put the DMZ, you need to decide precisely what’s
going to reside there. My advice is to put all publicly accessible services in the DMZ.
Too often I encounter organizations in which one or more crucial services are
“passed through” the firewall to an internal host despite an otherwise strict DMZ
policy; frequently, the exception is made for MS-Exchange or some other applica-
tion that is not necessarily designed with Internet-strength security to begin with and
hasn’t been hardened even to the extent that it could be.
But the one application passed through in this way becomes the hole in the dike: all
it takes is one buffer-overflow vulnerability in that application for an unwanted visi-
tor to gain access ...