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Wireshark Filters: 802.11 Retries

17/7/2019

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Wireshark filters help drill down to useful information among what can feel like a massive, overwhelming stream. Especially useful when doing 802.11 protocol analysis where the incoming frames can quickly accumulate to many thousands in a very short timeframe.

My favourite Wireshark filter of all time is the WLAN Retry filter. My favourite way to use it is with the I/O Graph. I really like to understand the detected retransmitted frames vs the total number of frames captured. It's really easy to visualise this.

First off - the filter for WLAN Retries is:
wlan.fc.retry == 1

Using this filter as a display filter of a 802.11 frame capture will show only frames that have the Retry bit set in the Frame Control Field in the MAC header.

If you have a frame selected you can tell if it is being re-transmitted by checking the flags exposed in the first IEEE 802.11 decode field below the 802.11 Radio Information - this is displayed in the Packet Details view within Wireshark. An example is shown below where the 'R' Flag is set on the currently selected Deauthentication frame.
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I've written about the Wireshark I/O Graph before. You can find this in the Statistics menu as I/O Graph. Within the I/O Graph window you are able to add custom filters which will be visualised. The default line graph shows 'All packets' which in this case represent every frame captured. Typically the y-axis of the graph represents Packets per second so the 'All packets' line indicates the number of 802.11 frames captured per second on the captured Wi-Fi channels.

I like to add my WLAN Retries display filter as a new red line so I can see it in high contrast to the default 'All packets' line. See an exampled below.
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In this capture a high proportion of the frames captured had the Retry bit set. They were frames being re-transmitted many times by a Wi-Fi station.

A high retry rate can often indicate that something in the WLAN is unhealthy. This might be a particular station, the medium (the channel and associated frequency), an Access Point or something else.

The retry filter is a useful tool in your Wi-Fi Professional tool-belt. Keep learning! Share your favourite Wireshark display filters below.
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Written by Matt Sutherland

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