How Internet Security and Acceleration Server Handles the Caching of Responses to Requests Received By Web Publishing

This article describes how Internet Security and Acceleration (ISA) Server handles the caching of the responses to the requests that have been received by Web publishing (reverse proxy). (Reverse proxy lets you place one or more Web servers on your internal network behind your proxy server firewall. Reverse proxy enables you to securely publish out to the Internet, which can be very useful for publishing data from other servers.)

The implementation of ISA Server reverse caching (used for incoming requests) is identical to forward caching (used for outgoing requests). An exception is that ISA Server disregards the cache-control headers of the client computer. For example, in Reverse mode, ISA Server does not follow the cache-control option with its no-cache directive for the client. The client, however, is communicating with the actual Web server so this directive does not make sense. Thus, ISA Server prevents the client from interfering with the policy of an ISA Server administrator.

There are four methods to prevent caching, either reverse or forward:

Method 1

You can configure the Web server to attach to one of the following cache-control headers:

• no-cache
• no-store
• private

ISA Server follows these directives and does not cache the object.

Method 2

You can configure the Web server to set a time-to-live (TTL) period of zero for the specified file or folder.

In this case, ISA Server caches the response from the internal Web server, but revalidates the response each time it is requested by a client (revalidate = GET + If-Modified-Since).

Method 3

You can configure the Web server to not set the explicit expiration time for the specified file or folder.

The ISA Server cache expiration policy can be configured to set a TTL period of zero for all objects that do not have an explicit expiration time. This policy can also lead to revalidation on every request.

For more details, refer to the HTTP Property dialog box in the ISA Server Administration utility (click CacheConfiguration, and then click Properties).

Method 4

This method uses Routing Rules to control caching of specific sites or URLs. To prevent caching of a specific site and path, perform the following steps:

1. Create a Destination set that, partially or fully, defines the Site and path whose caching you wish to regulate. For example:
   Destination: www.mysite.com
   Path: /somePath*
   NOTE: Do not specify "HTTP://" when you create the destination set.
2. Create a routing rule that applies to this destination set.
3. Set the caching behavior as desired in the Caching tab of the routing rule. For example, to prevent caching for this site:
   1. Locate the Properties page of your routing rule, and then click the Cache tab.
   2. Under the Cache Content dialog box, click No content will ever be cached.
4. After the wizard completes, set the order of the new rule to be above any other rules that may apply to a similar Destination Set.