Wireshark · Wireshark-users: Re: [Wireshark-users] About filter

[Christopher Maynard <Christopher.Maynard@xxxxxxxxx>]

Guy Harris <guy@...> writes:

> > It's all right till here, but when I export the capture as plain text,
the ICMP packets appear again. Seems
> like the filter just works in the main screen of Wireshark. What can I do
to capture just UDP traffic? 
> 
> Use the filter "udp", and use it as a *capture* filter (when starting the
capture with Capture -> Options)
> rather than as a *display* filter (in the main screen).
> 
> "udp && !icmp" is used as a *display* filter to handle the fact that some
ICMP packets contain a partial or
> complete copy of the packet for which they're reporting a problem, so they
might contain UDP packets.  For
> *capture* filters, only the outermost packet layer is tested, so "udp"
matches only UDP packets, not ICMP
> packets that happen to contain a copy of a UDP packet.

To isolate only the UDP packets from the capture files you might already
have, you will first need to apply a display filter of "udp && !icmp" as
you've done, then export only those displayed packets to a new capture file
using "File -> Export Specified Packets -> ...".  After that, you can load
the newly created capture file containing only the UDP packets and finally
'File -> Export Packet Dissections -> as "Plain Text" file...'.

But to avoid that two-step process going forward, just follow Guy's advice.