What is a proxy server?
A proxy server is an intermediary server that acts on behalf of a client. In terms of web proxy servers, your web browser makes a request to the proxy, which makes the request to the destination server on your behalf then returns the content. There are many different kinds of proxies, and different uses; they're not limited to web page content, but that is the most common use.
Let's look at a real world example. A user needs to access http://maxpowerindustries.com/apple.jpg. The user either clicks on a link, or types the URL into their web browser. The web browser connects to the proxy, and requests the URL. The proxy checks whether it already has the file (which it doesn't) and connects to the remote web server, and requests the file.
Now that the proxy server has the image, it saves it to the cache. When the web server supplied the file (apple.jpg), it also returns the Last Modified date, and the file size. The proxy keeps track of this for later, and then supplies the file to the client (the user's web browser).
If another user tries to access the same URL later, the proxy server will check its cache to see if it already has the file. If it does, it will supply it to the user. Since the proxy server is usually on the same network as the users, this happens very quickly - and the file only had to be downloaded from the internet once.
So what happens if the file changes? After the proxy identifies the file as already being cached, the proxy may check the original web server to see if the file has been changed. This doesn't happen every time - it determines the need to check based on the "refresh pattern" rules of the proxy server.*If the file has changed, it will download it again; if not, it will supply the cached version of the file.
Why use a proxy?
A proxy can be useful for many things, especially within an organisation's IT infrastructure. Features and benefits of proxies include:
- A proxy can allow access to the internet for users without giving the machine itself direct access
- Access to web pages and files can be considerably faster when using a caching proxy
- Access to content or services can be restricted (e.g. blocking pornographic web sites)
- Controlled access to the internet by blocking or limiting access for specific users
- Proxy logs can be used for auditing usage *and for billing purposes
- Save bandwidth by caching content
- Can provide anonymity for clients*
- SOCKS proxies can provide more generic proxying (whereas web proxies will usually only allow HTTP and HTTPS traffic)
ISPs are known to use proxy servers (sometimes transparently - so you don't even know you're using one) as a means to reduce bandwidth usage. Since bandwidth costs money, the use of a proxy can save a significant amount. In my experience, cache hit rates (how often the proxy already has the file and doesn't have to download it again) are around 30%.
Anonymous proxies can be used to access web content without identifying yourself. In theory, this may or may not be the case, since logging on the proxy (or various surrounding network logging) could be used to trace back to the user. Such proxies can be useful for accessing location restricted content (such as Hulu.com).
For home users with multiple computers, a proxy server can save time and bandwidth - especially for software updates. Just configure all of your machines to use the proxy, download your software update on one machine, then when you update your other machines it will already be cached. There are some caveats to be aware of. Software updates between different versions of Mac OS X will often require different files - but if you have 2 machines both running 10.6.4, you're likely to see some benefit.
Caveats
Proxies aren't always reliable. They can be configured not to check for updated files often, which may result in users not seeing up to date content. This is particularly obvious for sites with dynamic content such as digg.com or even MacTalk's forums.
A proxy can also be configured (along with some other clever system components) to manipulate content as its delivered, as is the case in this classic Wi-Fi prank.
You should also consider that proxied content can and probably is being logged. Its not difficult to determine what users have been doing all day by looking at the proxy logs.
Additionally, since the proxy facilitates all communication between the user and the server, anything transmitted via a proxy could be intercepted by a malicious user.
How to set up a proxy
If you have access to a proxy server, and you'd like to use it, you'll need to configure your computer to use it.
For Safari, the settings are stored in System Preferences under Network. Each network connection has its own proxy settings which you can access by selecting the connection name, then click Advanced, then click on the Proxies tab.
For Firefox, the settings are in the Preferences, under Advanced, on the Network tab. Click Settings to edit them.
If you don't have access to a proxy, or would prefer to set up your own, one of the most popular proxy servers is Squid. Squid is an open source, robust and highly configurable web proxy that will run on just about any platform (including Mac OS X). You can download the source and compile it yourself, or use pre-built packages, such as SquidMan.
Want to know more?
As always, Wikipedia has an excellent article which summarises the different types of proxies and their uses.
http://en.wikipedia.org/wiki/Proxy_server
This site features a list of open proxies. Use with caution.
http://www.xroxy.com/proxylist.htm
All About Proxies