Introduction to automatic proxy configuration

What is automatic proxy configuration?

You can use an automatic proxy configuration file to specify, in one place, the Web proxy configurations for your entire organization. Clients in your network must be configured to point to the file instead of to a specific Web cache.

The automatic proxy configuration file consists of a JavaScript function. You create the file by entering the JavaScript function in the file. Each time a client requests a URL, the JavaScript function is executed and it returns the IP address that should be used for retrieving the resource.

Note
Typically, administrators create a proxy.pac file manually. Javascript debuggers are available to help debug the javascript used to create a proxy.pac file. You can, for example, type > `javascript:' in the Netscape URL window to access a javascript debugging tool.

The ways you can customize the file include the following:

Always use a Web cache.
Use a Web cache for all requests to the Internet.
Use different Web caches for different types of requests.
Use different Web caches for URL ranges, using wildcard pattern matching.

Advantages of automatic proxy configuration

Using an automatic proxy configuration file has the following advantages:

It is inexpensive.
It is scalable.
It is relatively fail-safe.
It is easy to implement.
It works with all NetCache products and versions.
You can include failover and intelligent distribution of requests over NetCache appliances and third-party Web caches.
You can make any necessary changes to the automatic proxy configuration file without having to reconfigure client browsers to match the changes.
Because the client browsers are not configured to point to the NetCache appliances, you can swap out NetCache appliances (for example, for maintenance) without affecting client requests.

Disadvantages of automatic proxy configuration

Using an automatic proxy configuration file has the following disadvantages:

Using an automatic proxy configuration file for failover and request distribution is equivalent to having an L4 or L7 switch, but the Web browser must detect failure. Not all browsers detect failure well. For example, a browser might not fail over at all or it might take several minutes.
The system administrator must set up how request distribution occurs.
Client browsers must be configured. If users do not configure their browsers correctly, you can receive many calls for help.
If you had a Web cache previously and client browsers are already configured to point to it, these browsers must be reconfigured.
Sophisticated users can bypass the NetCache appliance by changing the configuration in the browser, unless the firewall is configured to prevent direct access to the Internet.
Failover does not occur if a Web cache is overloaded.

Achieving finer load balancing than automatic proxy configuration provides

To achieve finer load balancing and to retain control over failover from the browser, you can combine the use of the automatic proxy configuration file with a Server Load Balancer (SLB). Combining the two lets you partition the workload, and failover is predictable because the SLB's method is predictable. For more information, refer to Using a Server Load Balancer for request distribution.

When use of automatic proxy configuration is not recommended

If you are a dial-up ISP, do not use an automatic proxy configuration file because of startup problems with some browsers. With some versions of Netscape Navigator and Microsoft Internet Explorer, the browser times out before a PPP session is established. The user then receives an error message that might be confusing.

Network Appliance recommends that dial-up ISPs either use transparent proxying or start with an SLB and then move to the next generation of SLBs, which should help with partitioning.

Browsers that support automatic proxy configuration

Automatic proxy configuration is supported by both Netscape Navigator and Microsoft Internet Explorer browsers.

File location

You load the automatic proxy configuration file on a Web server or file server that is directly accessible to the clients, or on the NetCache appliance.

Caution
Before you configure client browsers, decide the name and location of the automatic proxy configuration file, then do not change them. Otherwise, client browsers must be reconfigured to match the changes.