Squid (software)
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{{Infobox Software
| name = Squid
| logo = [[Image:Squid-cache logo.jpg|200px]]
| caption = Squid logo
| latest_release_version = [http://www.squid-cache.org/Versions/v3/3.0/ 3.0] / [http://www.squid-cache.org/Versions/v2/2.7 2.7]
| latest_release_date = 2008-05-20
| genre = [[web cache]], [[proxy server]]
| operating_system = [[Cross-platform]]
| license = [[GNU General Public License]]
| website = http://www.squid-cache.org/
}}
'''Squid''' is a [[proxy server]] and [[web cache]] [[daemon (computer software)|daemon]]. It has a wide variety of uses, from speeding up a [[web server]] by caching repeated requests, to caching [[World Wide Web|web]], [[Domain Name System|DNS]] and other [[computer network]] lookups for a group of people sharing network resources, to aiding security by filtering traffic. Although primarily used for [[HTTP]] and [[File Transfer Protocol|FTP]], Squid includes limited support for several other [[protocol (computing)|protocol]]s including [[Transport Layer Security|TLS]], [[Secure Sockets Layer|SSL]], [[Internet Gopher]] and [[HTTPS]].<ref>{{cite web |title = Squid FAQ: About Squid |url = http://wiki.squid-cache.org/SquidFaq/AboutSquid |accessdate = 2007-02-13 }}</ref> The development version of Squid ([http://www.squid-cache.org/Versions/v3/HEAD/ 3.1]) includes [[IPv6]] and [[Internet Content Adaptation Protocol|ICAP]] support.
Squid has been developed for many years. Early work on the program was completed at the [[University of California, San Diego]] and funded via two grants from the [[National Science Foundation]].<ref>{{ cite web |title = Squid Sponsors |url = http://www.squid-cache.org/SPONSORS.txt |accessdate = 2007-02-13 | quote = The NSF was the primary funding source for Squid development from 1996-2000. Two grants (#NCR-9616602, #NCR-9521745) received through the Advanced Networking Infrastructure and Research (ANIR) Division were administered by the University of California San Diego }}</ref> Squid is now developed almost exclusively through volunteer efforts.
Squid is primarily designed to run on [[Unix-like]] systems but it also runs on [[Microsoft Windows|Windows]]-based systems. Released under the [[GNU General Public License]], Squid is [[free software]].
== Web proxy ==
[[Caching]] is a way to store requested Internet objects (e.g. data like web pages) available via the HTTP, FTP, and Gopher protocols on a system closer to the requesting site. [[Web browser]]s can then use the local Squid cache as a [[Proxy server|proxy]] HTTP server, reducing access time as well as [[Bandwidth (computing)|bandwidth]] consumption. This is often useful for [[Internet service provider]]s to increase speed to their customers, and [[local area network|LAN]]s that share an [[Internet]] connection. Because it is also a proxy (i.e. it behaves like a client on behalf of the real client), it can provide some [[internet privacy|anonymity]] and [[security]]. However, it also can introduce significant privacy concerns as it can log a lot of data including [[Uniform Resource Locator|URL]]s requested, the exact date and time, the name and version of the requester's web browser and operating system, and the [[referer]].
A client program (e.g. browser) either has to specify explicitly the proxy server it wants to use (typical for ISP customers), or it could be using a proxy without any extra configuration: "transparent caching", in which case all outgoing HTTP requests are intercepted by Squid and all responses are cached. The latter is typically a corporate set-up (all clients are on the same LAN) and often introduces the privacy concerns mentioned above.
Squid has some features that can help [[Anonymity|anonymize]] connections, such as disabling or changing specific header fields in a [[client (computing)|client's]] HTTP requests. Whether these are set, and what they are set to do, is up to the person who controls the computer running Squid. People requesting pages through a network which transparently uses Squid will usually have no idea if this information is being logged.<ref>See the documentation for <tt>header_access</tt> and <tt>header_replace</tt> for further details.</ref>
== Reverse proxy ==
The above set-up—caching the contents of an unlimited number of [[webserver]]s for a limited number of clients—is the classical one. Another set-up is "[[reverse proxy]]" or "webserver acceleration" (using <tt>http_port 80 accel vhost</tt>). In this set-up, the cache serves an unlimited number of clients for a limited number of—or just one—web servers.
As an example, if ''slow.example.com'' is a "real" web server, and ''www.example.com'' is the Squid cache server that "accelerates" it, the first time any page is requested from ''www.example.com'', the cache server would get the actual page from ''slow.example.com'', but later requests would get the stored copy directly from the accelerator (for a configurable period, after which the stored copy would be discarded). The end result, without any action by the clients, is less traffic to the source server, meaning less CPU and memory usage, and less need for bandwidth. This does, however, mean that the source server cannot accurately report on its traffic numbers without additional configuration, as all requests would seem to have come from the reverse proxy. A way to adapt the reporting on the source server is to use the X_HTTP_FORWARDED_FOR HTTP header reported by the reverse proxy, to get the real client's IP address.
It is possible for a single Squid server to serve both as a normal and a reverse proxy simultaneously.
== Compatibility ==
Squid can run on the following [[operating system]]s:
* [[AIX operating system|AIX]]
* [[BSD/OS|BSDI]]
* [[Digital Unix]]
* [[FreeBSD]]
* [[HP-UX]]
* [[IRIX]]
* [[Linux]]
* [[Mac OS X]]
* [[NetBSD]]
* [[NeXTStep]]
* [[OpenBSD]]
* [[SCO OpenServer]]
* [[Solaris (operating system)|Solaris]]
* [[UnixWare]]
* [[Microsoft Windows| Windows]]
== See also ==
{{portal|Free software|Free Software Portal Logo.svg}}
* [[Internet Cache Protocol]]
== References ==
{{reflist}}
== External links ==
* [http://www.squid-cache.org/ Official project homepage]
* [http://meta.wikimedia.org/wiki/Wikimedia_servers List of Wikipedia servers]
* [http://squidproxy.wordpress.com/ Squid Blog]
* [https://calomel.org/squid.html Squid "how to"], [https://calomel.org/squid_ua_random.html Squid User-Agent Randomizer] and [https://calomel.org/squid_adservers.html Squid Ad-Blocker]
* [http://www.linux-faqs.com/squid.php Optimizing SQUID] and [http://linux-faqs.com/Forum/viewtopic.php?t=28 Access Control List]
* [http://www.visolve.com/squid/Squid_tutorial.php Squid Tutorial] - Tutorial for Squid
* [http://www.sweetnam.eu/index.php/Reverse_Proxy_with_Squid Squid reverse proxy] - Create a reverse proxy with Squid
* [http://www.visolve.com/squid/ Configuration Manual] - ViSolve Squid Configuration Manual Guide
* [http://www.squid-cache.org/Versions/v2/2.6/cfgman/ Configuration Manual 2.6] - Authoritative Squid 2.6 Configuration Options
* [http://www.squid-cache.org/Versions/v3/3.0/cfgman/ Configuration Manual 3.0] - Authoritative Squid 3.0 Configuration Options
* [http://www.webmasterwords.com/squid-proxy-on-solaris Solaris Setup] - Setup squid on solaris
[[Category:Free caching software]]
[[Category:Reverse proxy]]
[[Category:Gopher Clients]]
[[Category:Unix software]]
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