Web Content Filtering with ISA Server

Blocking File Downloads by File Type
Let's consider another HTTP filtering option that most administrators will find useful: blocking file downloads based on file type (extension). From the Configure HTTP policy for rule dialog box, select the Extensions tab that Figure 5 shows.

The dialog box lets you define any number of file extensions to either allow or block, depending on how you want to create your rules. For example, if your organization's security policy indicates that users should be downloading documents only (never anything else), you can set Specify the action taken for file extensions to allow specified extensions only and populate this page with the types you want to allow (e.g., .doc, .xls, .ppt, .pdf, .rtf, .txt).

Figure 5, however, shows the opposite approach. Users behind this ISA Server can download any file type except the ones I specifically blocked: several executable content types (.exe, .pif, .scr) and .zip files. After the rule is in place, any attempt to download a file with one of the specified extensions results in an error message that explains that the HTTP filter rejected the request. . . .