The Cache node in the ISA Server console, shown in Figure 3.31, is where content caching can be enabled and configured on an ISA Server. Although not enabled by default in the ISA Console, enabling caching can improve network performance and response time by saving copies of images, text, and other data that clients download from web and FTP sites on the Internet and making them available to the next client that requests information from that particular site.
Figure 3.31. Viewing the ISA Console Cache node.
This section contains a high-level description of the settings available in the ISA Server Console under the Cache node. Further information on deploying ISA Server for its content caching capabilities can be found in Chapter 8.
It is not immediately evident how to enable caching, in that it is disabled by default when ISA is deployed. Caching is enabled when physical drive space is made available to the caching service. To perform this action, follow these steps:
Unlike most other changes made in the ISA Console, configuring cache drives is one of the changes that requires a restart of the firewall service, as noted in the preceding procedure.
Understanding Cache Rules
Caching behavior by ISA is made granular and more configurable through the addition of specific caching rules. Each caching rule allows for specific types of content to be processed in different ways, depending on the needs of the administrator.
By default, when caching is enabled, a default cache rule is put into place that caches objects based on default settings. Additional caching rules can be configured by clicking on the Create a Cache Rule link in the Tasks tab. Each rule created can contain the following customizations:
Just as with firewall rules, caching rules are applied in order, from top to bottom, until a match is made. Through the creation of multiple caching rules, fine-grained control over the caching settings of the clients can be achieved.
Examining Content Download Jobs
The final set of options available under the Cache node revolve around the capability of the ISA caching engine to automatically download content based on a defined schedule. This can be useful if specific websites need to be always up to date and quickly available to internal clients.
Content Download Jobs can be enabled and configured via the Content Download Jobs tab in the Central Details pane of the Cache node. When configuring this setting up via the Schedule a Content Download Job link in the Tasks tab, two changes must be made to the configuration. These changes, shown in the dialog box in Figure 3.33, are to allow the Local Host to listen for web proxy requests via a rule, and enabling a special system policy rule. After these settings are automatically configured, specific content download jobs can be created.
Figure 3.33. Enabling content download jobs.
Content download jobs can be scheduled weekly, daily, hourly, or only once, as needed. They also can be configured to browse and download the content of only a single URL page on the Internet, or to follow a certain number of links "deep" from the page that is being accessed.
Care should be taken to not configure content download jobs to be too aggressive because they can consume exponential amounts of bandwidth, depending on the depth of the links that will be followed. For example, a simple page with five links on it, and five links on its sub-pages, would access only six total pages if the content download job were to be configured to scour pages one link deep. If the job were changed to two links deep, however, a total of 31 pages would need to be accessed. This could pose a serious drain on the Internet bandwidth available if not configured properly.