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Re: About filter
From: Christopher Maynard <Christopher.Maynard () gtech com>
Date: Tue, 16 Apr 2013 22:59:36 +0000 (UTC)

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.

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