Front cover - IBM Redbooks

136 Lotus Security Handbook
Firewall (filtering)
Internet
Firewall (filtering)
Router
Router
Internal network
Border router
DMZ network
Figure 4-5 DMZ referring to a screened subnet
In the second model, the DMZ is not completely isolated from the private
network. It logically lies between the Internet and the internal, trusted private
network. To secure this architecture, we must use more sophisticated firewall
defenses to protect the private network. So within the DMZ we employ proxy
servers and application gateways to separate the private network from the
Internet. We make the inside invisible from the outside, while nonetheless
allowing local users access to the outside world and external access to selected
services on both the DMZ and the private network. Ingress and egress through
the DMZ is carefully protected through firewall defenses. This is a simple “three
zone” security model, which has been refined to various degrees over the past
few years with respect to what functions need to be provided in the DMZ and at
its boundaries. The firewall defenses can be distributed and replicated within the
DMZ and at the boundaries to reduce potential single points of failure.
In this simple DMZ model, we have servers that are publicly available in the
DMZ, and servers that are on the internal, private network. The application
server separation is basically split into two corresponding categories: public and
private.
The three-zone model was considered adequate when the number of services or
applications made externally available was fairly limited. These services were
typically Web applications (HTTP), and possibly file transfer (FTP), often using a