Weirdness in Wi-Fi: Canon printers

Wi-Fi Taxonomy is a mechanism to identify the species of a Wi-Fi client device when it joins a network. Implementing this involved a great deal of staring at packet captures, and I'd like to share some of the strange and wonderful things flying by on Wi-Fi networks.

Some models of Canon Printer always send Probe Request frames of maximum size (1500 bytes). The frame is filled with zeros, which is interpreted as an empty SSID Information Element which means it is a Broadcast Probe, repeated over and over and over.

No.     Time           Source                Destination           Protocol Length Info
      1 0.000000       Canon_3d:97:f4        Canon_3d:97:f4        802.11   1553   Probe Request, SN=1363, FN=0, Flags=........C, SSID=Broadcast

Frame 1: 1553 bytes on wire (12424 bits), 1553 bytes captured (12424 bits)
Radiotap Header v0, Length 25
802.11 radio information
IEEE 802.11 Probe Request, Flags: ........C
    Type/Subtype: Probe Request (0x0004)
    Frame Control Field: 0x4000
    .000 0000 0000 0000 = Duration: 0 microseconds
IEEE 802.11 wireless LAN
    Tagged parameters (1500 bytes)
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0000  00 00 19 00 6f 08 00 00 21 e7 5c 16 00 00 00 00   ....o...!.\.....
0010  12 0c 80 09 80 04 e5 a5 00 40 00 00 00 60 12 8b   .........@...`..
0020  3d 97 f4 60 12 8b 3d 97 f4 60 12 8b 3d 97 f4 30   =..`..=..`..=..0
0030  55 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   U...............
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0050  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
0060  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
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0080  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00   ................
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0600  00 00 00 00 00 00 00 00 00 00 00 00 00 7e 2a ac   .............~*.
0610  af                                                .

Wi-Fi Taxonomy Talk at DEFCON25

I gave a talk at DEFCON 25 describing a technique to identify the type of Wi-Fi client connecting to an Access Point. It can be quite specific: it can distinguish an iPhone 5 from an iPhone 5s, it can tell a Samsung Galaxy S7 from an S8, etc. Classically in security literature this type of mechanism would have been called "fingerprinting," but in modern usage that term has evolved to mean identification of a specific individual user. Because this mechanism identifies the species of the device, not the specific individual, we refer to it as Wi-Fi Taxonomy.

The mechanism works by examining Wi-Fi management frames, called MLME frames. It extracts the options present in the client's packets into a signature string, which is quite distinctive to the combination of the Wi-Fi chipset, device driver, and client OS.

The slides are available in PDF format from the DEFCON media server, and the speaker notes on the slides contain the complete talk. At the time of this writing the video has not yet been posted, but will appear on the DEFCON Conference YouTube channel at some point. The database of signatures to identify devices is available as open source code with an Apache license as a GitHub repository.

There is also a paper which describes the mechanism, and which goes a level of detail deeper into how it works. It is available from arXiv.

Software Engineering Maxim #20: Everyone finds their own experience most compelling

(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)

"We should do A. I did that on my last project, and it was great."

"No, we should do B. I did that on my last project, and it was great."

Comparing experiences rarely builds consensus, everyone believes their own experiences to be the most convincing. Comparing experiences really only works when there is a power imbalance, when the person advocating A or B also happens to be a Decider.

In most cases, simply being aware of this phenomena is sufficient to avoid damaging disagreements. The team needs to find other ways to pick their path forward, such as shared experiences or quantitative measurements, not just firmly held belief.

Software Engineering Maxim #19: Goodwill can be spent. Spend wisely.

(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)

Doing good work accumulates goodwill. It is helpful to have a pile of goodwill, it tends to make interactions smoother and generally makes things easier.

Nonetheless, it is possible to spend goodwill on something important: to redirect a path, to right a wrong, etc. Sometimes spending goodwill is the right thing to do. Don’t spend it frivolously.

Software Engineering Maxim #18: Manage without a net

(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)

Managers and Tech Leads cannot depend on escalation. We sometimes believe that the layers of management above us exist in order to handle things which the lower layers are having trouble with. In reality, those upper layers have their own goals and priorities, and they generally do not include handling things bubbling up from below.

Do not rely on Deus Ex Magisterio from above, organizations do not work that way.

Software Engineering Maxim #17: Software Managers must code

(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)

When one first transitions from being an individual contributor software engineer to being a manager, there is a decision to be made: whether to stop doing work as an individual contributor and focus entirely on the new role in guiding the team, or to keep doing IC work as well as management.

There are definitely incentives to focus entirely on management: one can have a much bigger impact via effective use of a team than by one’s own effort alone. When a new manager makes that choice, they get a couple of really good years. They have more time to plan, more time to strategize, and the team carries it all out.

The danger in this path comes later: one forgets how hard things really are. One forgets how long things take. The plans and strategies become less effective because they no longer reflect reality.

Software managers need to continue do engineering work, at least a little, to stay grounded in reality.