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Attackers can sniff Wi-Fi networks in monitor mode to intercept the data without connecting to the router. Is it possible to check if somebody is sniffing my network?

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Unfortunately WPA personal sucks and allows passive interception by anybody knowing the network password. – CodesInChaos Feb 15 at 11:07
up vote 35 down vote accepted

No, they're just getting the radio waves out of the air. As long as they're not sending anything, you can't tell that they're receiving. (It's like how FM radio stations can't tell who or how many people are listening.)

I have heard that some wireless access points can direct the radio signals to the appropriate clients, which is pretty neat, but you shouldn't rely on just physical location for information security.

If you're concerned about people sniffing your wireless traffic, enable encryption/authentication and use a strong password. Even better, encrypt important traffic at another layer (TLS/HTTPS) so even the access point can't see anything sensitive.

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Actually, while this answer is good - thinking more about the problem - the answer should not be "no", it should be "maybe". It must be possible to set up a honeypot router - this won't tell if they are passively listening, but might allow you to leak credentials (either by using WEP or WPS) and see if they attempt to connect. – davidgo Feb 14 at 21:20
@davidgo True. Though, strictly speaking, you still can't tell if somebody is only listening. – Ben N Feb 14 at 21:23
It's also the "please watch and support X TV show! we need the ratings!" is complete nonsense - unless you're one of the 0.015% of the US population keeping a Nielson journal. – Lightness Races in Orbit Feb 15 at 0:28
@slebetman: I'm talking about TV, that is broadcast video media. – Lightness Races in Orbit Feb 15 at 10:23
A note about beam forming: compare it to talking with your head pointing towards someone. Although the person in front of you will hear you best, someone standing behind you will also pick up enough to understand you. In other words, some of the transmitted energy is directed towards the intended client/AP (more than with a pure omnidirectional antenna), but there will always be plenty of "leakage" in other directions. – Mels Feb 15 at 14:24

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