Technical Writing Sample 2

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Capture Filters Overview

Capture filters are the fastest and most efficient type of filter. They detect and eliminate unwanted frames before they are passed through to the protocol parsers.

If a capture filter is created, the filtered frames never go into the capture buffer. No time is spent processing these frames and no memory resources are consumed by storing them. You can create up to four statements for each filter, and each statement can have up to four tests.

Filter tests can be any combination of the following tests:

Value-based
IP Address-based
MAC Address-based
Protocol-based

Note that if any test fails to yield either a TRUE or a FALSE, by default the result is set to FALSE. The name of the currently installed filter appears in the status bar of the main window.

Important: Filter statements and tests are processed linearly in the order that they appear in the LinkFerret Capture Filters dialog box. If the condition being tested does not evaluate to "capture the frame," no more processing occurs and the frame is discarded.

If "Statement 0" specifies "include" as its action and the tests defined for this statement evaluate to FALSE, then the frame is discarded, even if any or all of the following statements would have evaluated to include the frame. Similarly, if "Statement 0" specified "exclude" as its action and the tests defined for this statement evaluated to "TRUE," then the frame is discarded, regardless of what the results of any further filter testing may have been.

LinkFerret provides increasingly fine levels of granularity in filter testing, thereby maximizing resources. "Statement 0" should contain the coarsest level, eliminating most of the unwanted traffic from the filtering loop, and so on down the chain.

Create a Filter and a Statement

Click Filters > Manage Link Filters. The LinkFerret Capture Filters dialog box opens.
In the Filters pane, right-click LinkFerret Filters, and then click Create filter on the shortcut menu. The Filter Name dialog box opens.
Enter a name for the filter and click Okay. The system prompts you to create a statement.
Click Yes if you want to create a statement now. Click No if you want to create a statement at another time.
If you click Yes, the Filter Statement dialog box opens.
Select Include in capture or Exclude from capture, and then click Okay. The system prompts you to create a test.
Click Yes if you want to create a statement now. Click No if you want to create a statement at another time.
If you click Yes, a shortcut menu appears in the LinkFerret Capture Filters dialog box.
Click the test that you want to create on the shortcut menu.

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