10.2. Advanced use of filters

This section contains information on using JavaScript to construct custom filters, which allows very specific filtering.

Section 10.1, “Basic use of filters” describes the basic use of filters. To use a filter, an expression is entered into the filter bar, which specifies what to filter on. The filter expression is actually interpreted in JavaScript, which allows the full power of JavaScript to be used to create a filter. The expression can take the form of a series of JavaScript statements, eg

statement-1;
statement-2;
...
statement-n
            

Each of these statements is evaluated for each network traffic datapoint found. The result used by the filter is the result of the final statement, statement-n, which must be a boolean. If the result is true, then the datapoint is passed by the filter, and added to the chart. If the result is false, then that datapoint is discarded. If the final expression is not a boolean, then an error is indicated. Note that the statements prior to the final one may have side effects, that affect the result of the final statement.

The terms that can be referenced from the filter are listed in Section 10.3, “Terms available for use in filters”. Any valid JavaScript boolean operator or function can be used to evaluate a term. This includes regular expressions, which allow more complex pattern matching than equality.

A common requirement, but one difficult to formulate in a filter, is testing if an IP address is a member of a specific subnet. To make this easier, a function is provided for this purpose: inSubnet(address, subnet, maskBits). This will return true if address is a member of subnet with a mask of length maskBits. address can be any address field, or in fact any string representing an IP address.

For example, to create a filter to retain only traffic from subnet 10.1.2.0/24, use this filter:

inSubnet(ipSource, "10.1.2.0", 24)            
            

More complex filters can be constructed; for example, if you wanted all traffic from the above subnet going to another subnet 192.168.0.0/16, then you could use:

inSubnet(ipSource, "10.1.2.0", 24) && inSubnet(ipDestination, "192.168.0.0", 16)