Proxy Servers
A proxy server is often used with a firewall to hide the internal
network’s IP address and present a single IP address (its own) to
the outside world. A proxy server is a server that sits between a
client application, such as a web browser, and a real server. Proxy
servers prevent hackers from seeing the IP addresses of internal
machines, knowing how many machines are behind the proxy
server, or learning anything about the network configuration.
Proxy servers also provide a valuable control mechanism because
most proxy servers log all outgoing traffic. This enables network
administrators to see where employees go on the Internet. A
proxy server normally runs as software on the same machine as
your firewall.
The proxy server is configured to redirect certain traffic. For
example, incoming traffic using the HTTP protocol is usually
allowed through the proxy server but is redirected to the web
server. That means that all outgoing and incoming HTTP traffic
first goes through the proxy server. A proxy server can be
configured to redirect any traffic you want. If an e-mail server or
FTP server is on the network, all incoming and outgoing traffic for
that network will run through the proxy server.
Using a proxy server means that when a machine inside the
network visits a website, the website will only detect that the
proxy server visited it. In fact, if dozens of different machines on
the network visit a site that logs the IP addresses of incoming
connections, they will all be logged with the same IP address—
that of the proxy server.+
For the most part this sort of proxy server has been supplanted by
network address translation. However, the term proxy server is
still used, but with a different application. Now proxy servers work
with the firewall to filter things such as web content. They allow a
network administrator to block certain sites and to record all the
websites a given user visits.
This hiding of the network is a very valuable service because
knowledge of internal IP addresses can be used to execute certain
forms of attack. For example, IP spoofing is contingent upon
knowing the IP address of some internal server. Hiding those IP
