Linux Server Security, Second Edition

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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Chapter 2: Designing Perimeter Networks

A Strong Screened-Subnet Architecture

The architecture in Figure 2-4 is therefore better: both the DMZ and the internal net-

works are protected by full-featured firewalls that are almost certainly more sophisti-

cated than routers.

The weaker screened-subnet design in Figure 2-3 is still used by some sites, but in my

opinion, it places too much trust in routers. This is problematic for several reasons.

First, routers are often under the control of a different person from the firewall, and

this person may insist that the router have a weak administrative password, weak

access-control lists, or even an attached modem so that the router’s vendor can

maintain it! Second, some routers are more hackable than well-configured comput-

ers (for example, by default, they nearly always support remote administration via

Telnet, an insecure service).

Finally, packet filtering alone is a crude and incomplete means of regulating network

traffic. Simple packet filtering seldom suffices when the stakes are high, unless per-

formed by a well-configured firewall with additional features and comprehensive log-

ging.

The architecture in Figure 2-4 is useful when very high volumes of traffic must be

supported, as it addresses a significant drawback of the three-homed ...