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<h3 class="section">1.2 Proxies and caches</h3>
<p><a name="index-proxy-6"></a><a name="index-caching-7"></a>
A proxy is a program that acts as both a client and a server. It
listens for client requests and forwards them to servers, and forwards
the servers' replies to clients.
<p>An HTTP proxy can optimise web traffic away by <dfn>caching</dfn> server
replies, storing them in memory in case they are needed again. If a
reply has been cached, a later client request may, under some
conditions, be satisfied without going to the source again.
<p>In addition to taking the shortcuts made possible by caching, proxies
can improve performance by generating better network traffic than the
client applications would do.
<p>Proxies are also useful in ways unrelated to raw performance. A proxy
can be used to contact a server that are not visible to the browser,
for example because there is a firewall in the way (see <a href="Parent-proxies.html#Parent-proxies">Parent proxies</a>), or because the client and the server use different lower
layer protocols (for example IPv4 and IPv6). Another common
application of proxies is to modify the data sent to servers and
returned to clients, for example by censoring headers that expose too
much about the client's identity (see <a href="Censoring-headers.html#Censoring-headers">Censoring headers</a>) or
removing advertisements from the data returned by the server
(see <a href="Forbidden.html#Forbidden">Forbidden</a>).
<p>Polipo is a caching HTTP proxy that was originally designed as
a <dfn>personal</dfn> proxy, i.e. a proxy that is used by a single user
or a small group of users.
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