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Proxies?><html><head><title>Proxies (CGI Programming with Perl)</title><link href="../style/style1.css" type="text/css" rel="stylesheet" /><meta name="DC.Creator" content="Scott Guelich, Gunther Birznieks and Shishir Gundavaram" /><meta scheme="MIME" content="text/xml" name="DC.Format" /><meta content="en-US" name="DC.Language" /><meta content="O'Reilly & Associates, Inc." name="DC.Publisher" /><meta scheme="ISBN" name="DC.Source" content="1565924193L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="CGI Programming with Perl" /><meta content="Text.Monograph" name="DC.Type" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" alt="Book Home" usemap="#banner-map" border="0" /><map name="banner-map"><area alt="CGI Programming with Perl" href="index.htm" coords="0,0,466,65" shape="rect" /><area alt="Search this book" href="jobjects/fsearch.htm" coords="467,0,514,18" shape="rect" /></map><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch02_04.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm">CGI Programming with Perl</a></td><td width="172" valign="top" align="right"><a href="ch02_06.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><hr align="left" width="515" /><h2 class="sect1">2.5. Proxies</h2><p>Quite often, <a name="INDEX-434" /> <a name="INDEX-435" /><a name="INDEX-436" />web browsers do not interact <a name="INDEX-437" />directly with web servers; instead theycommunicate via a proxy. HTTP proxies are often used to reducenetwork traffic, allow access through firewalls, provide contentfiltering, etc. Proxies have their own functionality that is definedby the HTTP standard. We don't need to understand thesedetails, but we do need to recognize how they affect the HTTP requestand response cycle. You can think of a proxy as a combination of asimplified client and a server (see <a href="ch02_05.htm#ch02-93017">Figure 2-10</a>). AnHTTP client connects to a proxy with a request; in this way, it actslike a server. The proxy forwards the request to a web server andretrieves the appropriate response; in this way, it acts like aclient. Finally, it fulfills its server role by returning theresponse to the client.</p><p><a href="ch02_05.htm#ch02-93017">Figure 2-10</a> shows how an HTTP proxy affects therequest and response cycle. Note that although there is only oneproxy represented here, it's quite possible for a single HTTPtransaction to pass through many proxies.</p><p>Proxies affect us in two ways. First, they make it impossible for webservers to reliably identify the browser. Second, proxies often<a name="INDEX-438" /> <a name="INDEX-439" /> <a name="INDEX-440" /> <a name="INDEX-441" />cache content. When a client makes arequest, proxies may return a previously cached response withoutcontacting the target web server.</p><a name="ch02-93017" /><div class="figure"><img width="430" src="figs/cgi2.0210.gif" height="156" alt="Figure 2-10" /></div><h4 class="objtitle">Figure 2-10. HTTP proxies and the request/response cycle</h4><a name="ch02-99950" /><div class="sect2"><h3 class="sect2">2.5.1. Identifying Clients</h3><p>Basic HTTP requests do not contain any information that identifiesthe<a name="INDEX-442" /><a name="INDEX-443" /><a name="INDEX-444" /><a name="INDEX-445" />client. In a simple networktransaction, this is generally not an issue, because a server knowswhich client is talking to it. We can see this by analogy. If someonewalks up to you and hands you a note, you know who delivered the noteregardless of what the note says. It's apparent from thecontext.</p><p>The problem is determining who wrote the note. If the noteisn't signed, you may not know whether the person handing youthe note wrote the note or is simply delivering it. The same is truein HTTP transactions. Web servers know which system is requestinginformation from them, but they don't know whether this clientis a web browser that originated the request (i.e., the author of thenote) or if they are just a proxy (i.e., the messenger). This is nota shortcoming of proxies, because this anonymity is actually afeature of proxies integrated into firewalls. Organizations withfirewalls typically prefer that the outside world not know theaddresses of systems behind their firewall.</p><p>Thus, unless the browser passes identifying information in itsrequest to the server, there is no way to differentiate amongdifferent users on different systems since they could be connectingvia the same proxy. We'll explore how to tackle this issue in<a href="ch11_01.htm">Chapter 11, "Maintaining State"</a>.</p></div><a name="ch02-31-fm2xml" /><div class="sect2"><h3 class="sect2">2.5.2. Caching</h3><p>One of the <a name="INDEX-446" />benefits of proxies is that they makeHTTP transactions more efficient by sharing some of the work the webserver typically does. Proxies accomplish this by<a name="INDEX-447" />caching requests and responses. When aproxy receives a request, it checks its cache for a similar, previousrequest. If it finds this, and if the response is not<a name="INDEX-448" />stale (out of date), then it returnsthis cached response to the client. The proxy determines whether aresponse is stale by looking at HTTP headers of the cached response,by sending a HEAD request to the target web server to retrieve newheaders to compare against, and via its own algorithms. Regardless ofhow it determines this, if the proxy does not need to fetch a new,full response from the target web server, the proxy reduces the loadon the server and reduces network traffic between the server anditself. This can also make the transaction much faster for the user.</p><p>Because most resources on the Internet are static HTML pages andimages that do not often change, caching is very helpful. For<a name="INDEX-449" /><a name="INDEX-450" />dynamic content, however, cachingcan cause problems. CGI scripts allow us to generate dynamic content;a request to one CGI script can generate a variety of responses.Imagine a simple CGI script that returns the current time. Therequest for this CGI script looks the same each time it is called,but the response should be different each time. If a proxy caches theresponse from this CGI script and returns it for future requests, theuser would get an old copy of the page with the wrong time.</p><p>Fortunately, there are ways to indicate that the response from a webserver should not be cached. We'll explore this in the nextchapter. HTTP 1.1 also added specific guidelines for proxies thatsolved a number of problems with earlier proxies. Most currentproxies, even if they do not fully implement HTTP 1.1, have adoptedthese guidelines.</p><p>Caching is not unique to proxies. You probably know that<a name="INDEX-451" /><a name="INDEX-452" />browsers do their owncaching too. Some web pages have instructions telling users to cleartheir web browser's cache if they are having problems receivingup-to-date information. Proxies present a challenge because userscannot clear the cache of intermediate proxies (they often may noteven know they are using a proxy) <a name="INDEX-453" />as they can for their <a name="INDEX-454" /> <a name="INDEX-455" /> <a name="INDEX-456" />browser.</p></div><hr align="left" width="515" /><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch02_04.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td width="172" valign="top" align="right"><a href="ch02_06.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">2.4. Server Responses</td><td width="171" valign="top" align="center"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td width="172" valign="top" align="right">2.6. Content Negotiation</td></tr></table></div><hr align="left" width="515" /><img src="../gifs/navbar.gif" alt="Library Navigation Links" usemap="#library-map" border="0" /><p><font size="-1"><a href="copyrght.htm">Copyright © 2001</a> O'Reilly & Associates. 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