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CTS 047: Troubleshooting WiFi With Wireshark

August 15, 2016 Rowell 00:23:40 13 Comments

It’s that time, a new episode about WiFi! Our main topic is Troubleshooting WiFi with Wireshark.

I saw this get shared on Twitter which is an article from The Guardian. Apparently, AirBnb WiFi is a security threat for travelers. This shouldn’t be a surprise to anyone but it is possible that the owner could be spying on your traffic, collecting information on you or even stealing your passwords. The best thing to do is not use the WiFi. I know, hard to do. From another perspective, a maliciuos hacker could break into your access point and install a backdoor and have his/her way with your WiFi. Now that’s a scarier thought.

I noticed Keith Parsons shared an interesting photo on social media. He displayed what he carries every day as part of his WLAN Professional toolkit. My toolkit is a lot lighter than that only because I hate carrying a lot of gear. Here’s a look into my toolkit:

For software I use:

What’s in your toolkit? Leave a comment below. I’m very curious what other professionals carry.

A WiFi Question from Lee Badman caught my attention, #WIFIQ 8/10/16 Have you ever had to deal with someone spoofing/copying your residential or business SSID? Circumstances, course of action?

On campus I know I’d find that rogue access point and shut it down after finding it.

But if it’s a neighboring tenant, what options do you have? The only thing I can think of is to simply ask them to change their SSID.

Troubleshooting WiFi with Wireshark

Download this sample pcap file to follow along.

My primary computer is a Macbook Pro. You can perform the same troubleshooting steps on a PC.

First step is to download the application at wireshark.org.

Before capturing wireless frames, there are a few things to take note. If you’re using a Macbook Pro/Air then you should be okay capturing frames using your built-in wireless adapter. I highly recommend using Airtool to assist in capturing frames on specific channels and channel widths. Airtool will conveniently save that capture for you on your desktop and open it right up in Wireshark.

If you’re using a PC, capturing wireless frames may not be that easy. Normally, the wireless adapter in Windows doesn’t allow you to capture frames in promiscuous mode. You’ll want to capture all the wireless frame details. Those frames I am referring to, not just the data frames, but also the frames used for management and control of the wireless medium.

On a Windows PC I have used the AirPcap adapter from Riverbed.

Once you’ve captured enough wireless frames, go ahead and stop it. Now we should be looking at Wireshark. The window is divided into three sections:

List of frames captured at the top pane
Middle pane shows the details of the frame selected at the top pane
Bottom pane shows the frame bytes of the selected frame.

We can see details such as the source mac address, destination mac address, and the details of the frame.

On the Info column, you can see what kind of frame is captured. For example, the first frame is a probe request from a device. What’s awesome about diving into wireless frames is being able to see so many details. Expand the Radiotap Header and we can see what data rate this frame was sent out on, which frequency, the signal, etc.

Expand IEEE 802.11 Probe Request and we can identify what kind of frame this is. It’s a Management frame with a subtype of 4 which is a Probe Request.

Now the meat of this specific frame is where you will expand IEEE 802.11 wireless LAN management frame. Here we will find the details of this probe request from the client device. It is probing for a specific SSID called test and has included all of the client’s capabilities.

We’re already seeing how powerful it is to analyze wireless frames when troubleshooting client devices.

So that’s looking at wireless frames. Let’s add more functionality to Wireshark. We can add columns to the frame list pane in order to see more details.

A few columns I like to have visible are:

Duration
Channel
Data rate
MCS Index

To add a column, right click on an existing column and select Column Preferences. Click on the Plus icon to add a new column. So for example, to add a Duration column, give the title of this column Duration, change the type to Custom and the in the field Name we will use what’s called a filter. For duration it is wlan.duration.

Display filters are your best friend. Display filters are used to find specific types of frames or packets. For example, if I wanted to see frames from a specific source MAC address, I would type in wlan.addr == mac_address in the display filter bar.

It is possible to filter from almost any type of frame.

Typically when capturing wireless frames, I capture everything without any filters. In Wireshark, it is possible to apply a capture filter. I don’t like this approach because you may miss a frame that may be required for troubleshooting. Instead, I capture everything and filter down from that capture. Sure it takes up a lot of hard disk space but that’s the life of a protocol analyzer. I know, I need a hobby.

But if you really want to conserve on space, Airtool has an option to not save layer 3-7 payloads. A neat little feature.

Download a PDF of display filters to use here.

So how is this useful? Let’s say an client is unable to join the wireless network and all you are able to do is perform wireless captures. So if it were me and this was my only option, I’d go to where the client is having issues. Assuming the client drivers are good and the SSID can be seen by the client and the only issue is it never connects to the SSID, we need to find out what channel to start capturing on.

We could use another useful tool such as WiFi Explorer, same author of Airtool, to find out what the strongest signal is on what channel. That’s where I would start capturing wireless frames, then while capturing frames, have the client try to connect. After the process fails, I would stop the capture.

Assuming we captured on the correct channel, we should be able to see the probe request coming from the MAC address of the client which you can obtain from the computer itself. After looking at the capture we should be able to see the 802.11 State Machine. If we don’t see successful authentication and association then that’s when we need to look closely at the capture. Maybe it’s because the client doesn’t support the requirements of the BSS such as a mandatory rate the client doesn’t support.

If you’re more of a visual person, Wireshark does have the capability to display the capture in a graph. What if we wanted to see how many retransmissions are occurring. In Wireshark, navigate to the Statistics menu and select I/O Graph. In the graph window, we will add a new data point by clicking on the plus icon. Rename it to Retries. The display filter to show retries is “wlan.fc.retry == 1”. Since this is bad we will color it as red. Next we modify the Y Axis to display Packets per second and also display All Packets so we can compare retries to all packets captured. That graph shows you the amount of retry frames compared to all frames captured.

There we have some basic Wireshark troubleshooting. That should be enough to get you going and it will take some practice. We went over installing Wireshark and how to capture wireless frames. Then we went over the different panes within Wireshark and how to add additional columns for easier viewing of frames. Next I went over how I use Wireshark to capture frames and troubleshoot an example issue. Also I provided two tool that will assist you in capturing frames, Airtool and WiFi Explorer.

In the news we talked about how insecure it is to use WiFi at an AirBnb. I know I wouldn’t.. The list of tools Keith Parsons has in his bag which is quite impressive. What’s in your bag? and a discussion of how to deal with someone spoofing your SSID.