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Comment by wheybags | Hacker News Reader

As a total amateur when it comes to electronics, let alone radio, what do you think would be the consequences if one were to hack their openwrt router to ignore dfs and just use the reserved channels without checking for radar? Not on a huge corporate network, just something like a single suburban home. Would the interruptions from interference of radar be less intrusive than the drops from changing channel? And is that sort of violation the kind of thing your local regulator would be scanning for and send someone out to shut you down?

Obviously it would be illegal, and I promise I'm not gonna do it whatever your answer is :p

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No need to hack anything. There are countries where DFS does not exist. Move there, install OpenWrt, set the country code appropriately in the GUI for both radios, enjoy both the fast WiFi and the good weather.

And regarding the reasons for DFS - it is not due to the radars posing interference for access points, it is the other way round. They are supposed to be able to pick up reflections from clouds and airplanes that literally carry less than a millionth of the power that a typical AP would radiate.

Here in the Czech Republic you used to be able to see clear "wedges" on meteoradar images due to these types of interference, but it seems to be mostly solved now.

FCC fines for violating radio spectrum rules are huge, and they are authorized to confiscate equipment as well. I wouldn't recommend it.

Here is an example: https://www.fcc.gov/document/fcc-fines-hinson-dangerous-misu...

If I didn't see the post I would never think it's illegal. Also the FCC does not exist outside the US so is this relevant elsewhere?

https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat

> the FCC does not exist outside the US so is this relevant elsewhere?

Your own country likely has an analogue. Get thee to Google. Ofcom is the UK equivalent, for instance.

Given that 5.8GHz WiFi and 5.8GHz weather radar are both licence-exempt services within an ISM band, you literally cannot pay Ofcom to take an interest.

Given the level of corruption within the current UK government, I would not necessarily agree with your use of the word literally here. But at least in law and in principle yes.

> https://en.wikipedia.org/wiki/Ignorantia_juris_non_excusat

What do I care about some law student latin?

I know the analog of the FCC in my country. I have not ever heard about them knocking on anyone's door, ever so that's why I'm asking questions?

Ignorance of the law does not excuse one from breaking. There is no freebie.

The world is a bitch in that it is your oyster, right up until you start impacting something everyone else is counting on. In that case, it is not infrequent for one to be made an example of.

That law student Latin there is a reminder that in spite of you "not knowing" something, the world still turns, you are expected to do some level of research, and there are people whose job is to just search for violations, and dispense a Notice of Apparent Liability to the perpetrator.

Hell, Amateur radio operators run what they call foxhunts when they notice something odd going on, because spectrum misuse impacts everyone.

Thank you for explaining something that's not very complicated and was not relevant to my question I guess? Just because we have analogous agencies doesn't mean they are a carbon copy of each other and the radio spectrum stuff is not exacty mainstream knowledge.

Your country very likely has a government entity that regulates radio spectrum use and has means of enforcing it.

Messing with Doppler weather radar is a usually a big thing that impacts various important services. Even in relatively backwater countries you will get someone knocking at your door if you do this.

That makes sense, but I thought I'm messing with my router to have faster WiFi and not with some weather radar that I don't even know exists

The DFS checks exist to prevent your radio from interfering with the weather radar. You’re only allowed to own a radio that could cause such interference because it has those checks.

Here’s an example of what it can look like on the weather radar side: https://www.reddit.com/r/techsupportgore/comments/lnxdji/whe...

Is this from a consumer WiFi or from some bigger antenna?

This is why spectrum in many countries requires users and/or devices to be licensed.

probably not the best example. the guy impersonated a first responder resulting in no one responding to a fire alarm ; https://www.arrl.org/news/fcc-fines-north-carolina-man-for-u.... https://docs.fcc.gov/public/attachments/FCC-19-82A1_Rcd.pdf

There’s also this guy: https://www.fcc.gov/document/fcc-proposes-fine-interrupting-...

Pick a channel outside the DFS range. Not every 5GHz channel is affected, only a subset of channels is.

If you do cause interference with weather radar, there's actually a chance that the authorities take action. WiFi interference isn't hard to scan for at all, even or you disable broadcasting your SSID on the channels you're interfering on.

The interruption when the FCC knocks on your door would be pretty damn intrusive.

Radars are pretty good at locating things. So, if that rogue wifi device interferes much enough to be noticed by the radar operator on some nearby airfield, the reaction from radio authorities will likely be swift.

This is not how it works. The AP is not actually responding to or reflecting the radar ping so the range detection won't work. If it even sees it it'll show up as a line from the center of the radar screen in the direction of the AP. It won't pinpoint it. But most likely it'll just add to the noise floor.

It'll still be possible to locate it with that info but it's more work unless you manage to interfere with two radars and they can triangulate. That's pretty unlikely for an indoor access point with only a few milliwatts though.

I saw meteorologists publicly shame rogue APs by name. This implies they do have means (directional antenna) to probe the source of interference and get information such as AP name, MAC address and distance.

https://www.shmu.sk/sk/?page=2037

They wouldn't get this from the actual radar though. It must have been the result of further investigation.

I was mainly referring to the 'radars are good at pinpointing things' comment from the OP :) I wanted to clarify that an AP on this frequency won't show up as a dot on a radar screen. For the distance to be captured it must have an actual reflection.

You can even see some of the lines I mentioned in that pic!

Radar receivers have a secondary output (or at least a waveguide coupler) where you can connect arbitrary equipment. So you connect standard wifi card to it, enable monitor mode and listen to all the MAC addresses and SSIDs (if they are not hidden) - directly though the giant radar antenna. (I really need to try it with our radar, I currently only detect packets based on their wide spectrum, but I have never decoded them)

There is even a software-based wifi demodulator (https://www.nuand.com/bladerf-wiphy/) so you can try to decode it from the IQ data, but they usually don't have enough bandwidth.

The way this works is someone notices a disruption and notifies the relevant authorities, which have detector vans to track down the emission sources. And they're quite good at that, too.

Indeed, I just wanted to clarify that the radar itself won't identify you as a 2D dot just like that. It needs further investigation.

The comment mentions "something like a single suburban home". I suspect a line drawn on the map in the direction of the signal may give a good enough idea which few suburban homes may be the culprits, simplifying the task of locating it from a van.

I may overestimate the angular precision of that though.

The beamwidth of a standard weather radar is 1 degree. You can get to maybe 0.3° accuracy if you are pinpointing the maximum carefully. So this is 400m at 70km distance - which is not that bad. You then need to evaluate which parts of this sector are actually visible from your radar site (using a digital elevation model) and then search in them (I have never done this). Depending on the location, the entire 100km line may be visible...

But you need to specially do this scan and carefully evaluate the maximum (tricky if it's transmitting only once in a while). When you are scanning normally, you are pointing a bit above the ground (you are scanning the sky, of course) and the beam may have several degrees at the ground level.

even if you set a dfs channel manually (editing hostapd config or cli) if you are using qualcomm/atheros or broadcomm chipsets, they will automatically move you out once it detects radar. At least with QCA/Atheros chipsets they used to have both compiletime and runtime flags to enable/disable most features (including dfs). Also, FCC fines are huge when it comes to dfs violation - as dfs spectrum is for emergency communication (military, natural disasters etc). There are radar simulators with which you can test this.

> Would the interruptions from interference of radar be less intrusive than the drops from changing channel?

depends, hence the checking.

> And is that sort of violation the kind of thing your local regulator would be scanning for and send someone out to shut you down?

highly doubtfull

Well, you are both showing that you know it is illegal, yet asking "what would be the consequences ?". Of course if you get caught you would both have your hardware confiscated and experience a very severe fine (and possibly worse than just a fine if there were consequences in terms of interferences).

People who operate radars (in particular radars for weather forecasts) are now really upset of the recurring interferences that come from wifi devices with (illegally) disabled DFS. Keep in mind that weather forecasts is not only about consumer people watching TV to know whether tomorrow will be sunny or rainy, but is also safety-related (e.g. for flights and navigation. You don't want your flight to be caught in a storm !).

Those issues (significant interferences that are now almost impossible to solve) are periodically raising debates within RF administrations about whether to require locked hardware (locked bootloader and signed firmwares) in order to ensure that DFS cannot be disabled. That would be a pity for any embedded linux geek (openwrt is great both with regards to freedom, privacy, long-term support with security updates and therefore avoiding obsolescence, etc.), but a lot of people now think that it is the only way...

I understand the shortcomings of DFS, yet you should really understand the 5 GHz wifi band is a band dedicated to radars as primary users where wifi has been tolerated under the condition of DFS, so wifi users should really not complain too much about the consequences and limitations of DFS (if there was no DFS, there would be no 5 GHz wifi band at all ! ;)

No, there are plenty of modern weather radars now that have nothing to do with 5 GHz. This is a good example of how the FCC managing spectrum is the furthest thing possible from a good allocation; trillions of economic activity are driven by use of unlicensed bands, yet because there is nobody in particular ready to put up billions for a slice of spectrum, we end up with the FCC "design by committee" process for releasing them. That means whoever happened to be there is considered some sort of "lifetime incumbent" that must be worked around at all costs, even if their use of prime spectrum is absurdly wasteful. Challenged with the choice between "incur $ costs on few" or "incur $$$ costs on many", the FCC will consistently pick "incur $$$$ on all".

This comment above is a good example of someone having stereotypes about RF administrations without actually knowing their field of expertise... It's not the matter of "unlicensed application don't pay license fees so administrations don't want them" (even though whenever you want to move an application out of a band, someone has to pay for the new equipment. But that's another topic and various countries have various process for that), but a matter of the laws of physics : various frequency bands have various physical properties and this applies not only to radio propagation but also (maybe more importantly) to measurements related to the molecules you want to observe. There are reasons why some bands and not others are dedicated to radars (and even more : some bands are fully passive because they are required for calibration and scientific measurements). So yes there are weather radars in other bands than 5 Ghz... so what ? Different radars measure different things and they are complementary (just like having different low/high bands for mobile operators are complementary for them). Before saying that the use of spectrum is "absurdly wasteful", you should show your thorough knowledge of the topic...

Keep in mind that whenever a new application appears, detailed technical analysis and simulations have to be done with regards to the other applications in co-channel and adjacent channel/bands, both with regards to direct interference, aggregated interference taking into account the projected density of the new spectrum use, clutter/terrain, propagation models, geometries of antennas, filter characteristics, etc. And even then, administrations do their best to find the right balance and enable as many applications as possible (including unlicensed and SRD), but those are tradeoffs and sometimes administration take risks to foster innovation (and sometimes it proves it was too permissive for the new user, such as what is happening now with regards to wifi vs weather radars).

(N.B. I am not in the US, but I guess FCC works similarly to EU administrations - which I know very well).

Ensuring pilots and air traffic control can monitor for microbursts is very important. https://www.kahuna-fi.com/post/dfs-tpc-tdwr

Following the rules (DFS) is fair ask to be able to access these additional 5 GHz channels.

I certainly get why the FCC would tread lightly around "lifetime incumbents". In a civilized country people would look at the overall issues, recognize it as a collective problem, and support using shared money to reorganize things to society's long-term benefit. But in the US the incumbents would squawk about government overreach and creeping socialism and how President John Wayne gave their grandpappy that spectrum and it was their god-given right to use it without communist agitators trying to take it away, using taxpayer money to boot.

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