Proxy Server Functionality
In the context of firewalls, proxy servers (proxies) have a couple of primary functions:
• Act as an intermediary between hosts
• Cache data to reduce the time and external bandwidth required to service requests
Proxies act as an intermediary by literally intercepting and responding to requests
between hosts, as shown in Figure 8-1.
Figure 8-1. Communication Process Between Hosts Through a Proxy
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In this case, Server1 and Server2 are attempting to communicate with each other. The
proxy resides between the two hosts and responds to all communications and requests
between the two hosts. This ensures that the two hosts never actually communicate
directly with each other. Logically, Server1 and Server2 are communicating with each
other, even though physically the communication process is occurring through a proxy.
This function is completely transparent to the end user/system, which means that Server1
has no idea that it is not actually communicating directly with Server2 and vice versa.
Many proxies, in particular proxies that support the HTTP protocol, can also cache data,
which in turn allows the proxy to service subsequent requests for the same data from
cache, instead of needing to forward the request to the external source. This allows the
proxy to help reduce Internet bandwidth requirements, because the first request for the
data uses Internet bandwidth whereas all subsequent requests are services from the
proxy's cache. This has the additional effect of reducing the time that it takes to display
the data because many proxies are connected to the clients that use them over faster
These two elements, application filtering and functioning as a proxy, are the two
elements that really identify an application proxy firewall from other types of firewalls
such as deep packet inspecting firewalls.