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Wednesday, April 28, 2010

When Wireshark Gets Confused...

SIP/VoIP call setup is covered in Chapter 27 of
Wireshark Network Analysis.]

I was scrambling around in preparation for a VoIP training session recently
when I opened a new VoIP trace file that depicted a simple call set up routine
followed by the actual call.

Strangely, Wireshark had an issue identifying one side of the SIP connection -
as you can see in the graphic above. Wireshark dissected one-half of the
conversation as FF (Foundation Fieldbus) traffic.

What in the world is going on here?

Wireshark defines the protocol column value based on the highest layer of
decode that it can apply to the packet. In this case, Wireshark found something
in these packets to indicate the traffic was Foundation Fieldbus packets.

I compared the packets defined as "FF" to the packets correctly interpreted as

Aha! The port number fields tell the story. In the UDP header, packets that
contained the source port of 1089 are dissected as Foundation Fieldbus. Ok -
let's just skip past the fact that the sender in Frame 2 above is not responding
to the correct port number defined in Frame 1.

My focus was on getting Wireshark to dissect the packets marked Foundation
Fieldbus as SIP packets. I certainly don't want to alter the preferences of
Wireshark so that all packets containing source port 1089 are dissected as SIP
packets (as they likely are just ephemeral ports and not SIP at all).

The quick solution is to right click on the packets dissected as Foundation
Fieldbus and select Decode As. Selecting SIP as the desired dissection and
applying this to the trace file fixed the problem quickly. Wireshark now
dissected source port 1089 as SIP. Clicking the "Show Current" button in the
Decode As window displays all manually altered dissection configurations.

It's not often that I have to apply Wireshark's Decode As function - typically I hit
companies using non-standard port numbers for applications for various
understandable and just plain whacky reasons).

It's a great feature to know - just in case you hit a strange dissection.

Enjoy life...
in the Netherlands... one bit at a time!