Pointing Web browsers to an automatic proxy configuration file

Applicable NetCache mode

An automatic proxy configuration file can only be used to direct requests nontransparently to NetCache appliances running as Web caches.

About the automatic proxy configuration file

If you deploy transparent proxying, proxy service is available without any configuration of client Web browsers. However, if you are not using transparent proxying, you must configure client browsers so that client requests can be sent through the NetCache appliance. One way you can do this is to create an automatic proxy configuration file. An automatic proxy configuration file enables you 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. Thereafter, Web traffic is directed to the NetCache appliances identified in the file.

With an automatic proxy configuration file, you specify, in one place, the proxy configurations for your entire organization.

When to use this method

Using an automatic proxy configuration file is the better of the two nontransparent client access methods that are available to you for Web caches. The other method, configuring Web browsers to point to a single NetCache appliance, has a number of disadvantages. Pointing Web browsers to a single NetCache appliance describes the limitations of that method.

Failover possible with the automatic proxy configuration file

If you are using the automatic proxy configuration file for directing browsers to the NetCache appliance, you can include functions in the file for failover to another NetCache appliance or a third-party Web cache, the Internet, or both. Failover through an automatic proxy configuration file has the following limitations, however:

Failover through automatic proxy configuration is not very reliable. Not all browsers detect failover well. For example, a browser might not fail over at all or it might take several minutes.
You can sometimes swap out a NetCache appliance (for example, for maintenance) without affecting client requests. However, because the Web browser is failing over, problems sometimes occur, such as the browser becoming hung.
Failover does not occur if a cache is overloaded.
Note
If your organization requires URL blocking or logging to be in place at all times, do not include functions for failover to the Internet in the automatic proxy configuration file. Because NetCache provides URL blocking and logging, these functions are not invoked if the request is not sent through the NetCache appliance.

Setting up the file for request distribution

If you are using the automatic proxy configuration file for directing Web browsers to a NetCache appliance, you can include multiple NetCache appliances in the file so that requests can be distributed among the appliances. The JavaScript function then uses a hashing function to distribute requests over the NetCache appliances and any third-party Web caches you specify. Request distribution through an automatic proxy configuration file has the following limitations, however:

An automatic proxy configuration file provides browser-based traffic partitioning; therefore, how well an automatic proxy configuration file works for distributing requests varies, depending on the browser and browser version. This method usually does not present a problem with properly sized individual NetCache appliances. However, if you want more predictable results, you can combine the use of an automatic proxy configuration file with a Server Load Balancer (SLB), which provides predictable request distribution.
Request distribution is coarser than distributing requests with an L4 or L7 switch. Overload protection is not available.
The browser checks the automatic proxy configuration file at startup. Therefore, distribution of requests is not dynamic, unlike an SLB.

Example of an automatic proxy configuration file

The automatic proxy configuration file is described in detail in Appendix A, Automatic Proxy Configuration File. An example of a complete file is provided, as well as incomplete examples that highlight parts of the file. You can use the examples as a basis for creating your own file.

Advantages of using an automatic proxy configuration file

This method has the following advantages in providing client access to the NetCache appliance:

It works with all NetCache products and versions, and with third-party proxy-cache servers.
The file can be configured easily. You can copy the file shown in Examples of automatic proxy configuration files, and adapt it for your environment.
You can set up the file so that browsers can directly access Web servers inside your local domain instead of sending requests through the NetCache appliance.
Multiple NetCache appliances can be included in the automatic proxy configuration file, unless you are using a Server Load Balancer (SLB).
Initially, there is no difference in the amount of work required to configure client browsers to point directly to a NetCache appliance or to an automatic proxy configuration file. However, if you use the file, you can save reconfiguration time later because you can make changes to the file without having to reconfigure client Web browsers to match the changes.
You can set up your file so that requests are distributed over multiple NetCache appliances. Setting up the file to distribute requests based on IP address maximizes the cache hit rate.
Failover can occur to another Web cache or to the Internet, if you set up your file for this feature.

Disadvantages of using an automatic proxy configuration file

This method has the following disadvantages in providing client access to the NetCache appliance:

If users do not configure their Web browsers correctly, you can receive many calls for support.
Sophisticated users can bypass the NetCache appliance by changing the configuration in the Web browser, unless the firewall configuration prevents this reconfiguration.
With this method, Web browsers interact with the automatic proxy configuration file. Because clients use different Web browsers and different versions of Web browsers, results can be unpredictable. This disadvantage is a particular problem if you have included functions for failover, request distribution over Web caches, or both, in the file.
You cannot use the automatic proxy configuration file when deploying NetCache appliances as news caches and streaming media caches.

When this method is not recommended

Network Appliance does not recommend that dial-up ISPs use an automatic proxy configuration file because of startup problems with some Web 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's recommendation for dial-up ISPs is either to use transparent proxying or to start with an SLB and then move to the next generation of SLBs that help with partitioning. If you use an SLB instead of an automatic proxy configuration file, configure the Web browsers to point directly to the SLB.

If you are using an SLB to load balance across NetCache appliances, make sure that no routing loops exist.