6.5. The “Filter Expression” dialog box
When you are accustomed to Wireshark’s filtering system and know what labels you
wish to use in your filters it can be very quick to simply type a filter string.
However if you are new to Wireshark or are working with a slightly unfamiliar
protocol it can be very confusing to try to figure out what to type. The
“Filter Expression” dialog box helps with this.
The “Filter Expression” dialog box is an excellent way to learn how to write
Wireshark display filter strings.
Figure 6.7. The “Filter Expression” dialog box
When you first bring up the Filter Expression dialog box you are shown a tree
of field names, organized by protocol, and a box for selecting a relation.
- Field Name
Select a protocol field from the protocol field tree. Every protocol with
filterable fields is listed at the top level. (You can search for a particular
protocol entry by entering the first few letters of the protocol name). By
expanding a protocol name you can get a list of the field names available for
filtering for that protocol.
Select a relation from the list of available relation. The is present is a
unary relation which is true if the selected field is present in a packet. All
other listed relations are binary relations which require additional data (e.g.
a Value to match) to complete.
When you select a field from the field name list and select a binary relation
(such as the equality relation ==) you will be given the opportunity to enter a
value, and possibly some range information.
You may enter an appropriate value in the Value text box. The Value will
also indicate the type of value for the field name you have selected (like
- Predefined values
Some of the protocol fields have predefined values available, much like enum’s
in C. If the selected protocol field has such values defined, you can choose one
of them here.
A range of integers or a group of ranges, such as
When you have built a satisfactory expression click and a filter string
will be built for you.
You can leave the “Add Expression…” dialog box without any effect by
clicking the button.