Is it possible that someone made an attack (DoS or something else) to my Wi-Fi router (without knowing of the password) and make my router's signal unavailable?
1) How it can be done?
2) What are remedies?
There's a lot of ways you can attack a WiFi without knowing any passwords:
You have to realize that it's a privilege, not a right, to have your WiFi use a channel. WiFi happens in the so-called ISM bands (Industrial, Scientific, Medical usage), where operators of transmitters don't have to have an explicit license. That means it's OK for everyone to use that spectrum, as long as they don't intentionally harm other devices and are not easily damaged by interference.
So, it's absolutely legal for someone to operate a high-definition digital camera stream that occupies the whole WiFi channel. That will effectively shut down your WiFi.
If you need something that no-one can mess with, wireless is, by definition, not the way to go.
There are devices that you can buy that will 'jam' a Wi-Fi signal and make it unusable.
Also Wi-Fi has different 'channels' that they can be set to run on. If your Wi-Fi is running on a channel that is saturated due to lots of other Wi-Fi devices around you using the same channel then the performance of your connection will degrade the further you are from the device.
I have used cloning of ESSID and BSSID to do this (set BSSID with
ifconfig wlanX hw ether xx:xx:xx:xx:xx:xx. Even if you don't output more power to the clients than their own AP, it will cause problems. Eventually they will notice that there's an AP with the same name of course.
Aireplay-ng deauth is more useful. If you are not interested in using too much time on this, use mdk or wifite which automate it. Mdk has whitelist/blacklist functionality for client link layer addresses.
The deauth attack is clearly visible with tcpdump's '-y foo' - unfortunately I don't remember the flag off-hand. You may need to set RFMON first (with e.g.
airmon-ng check kill ; airmon-ng start wlanX $channel).
On OpenBSD it is '-Y IEEE802_11', on Linux it is similar.
You set the duration field in frames to the maximal value and possibly use low data rate (duration is expressed as data units, not time), this forces the AP and other clients to honor your transmission time, do that often enough and the network will be badly impacted.
There is no simple defence against this kind of attack/misbehaviour.
The first generations of apple mobile products used to do that by mistake (aka a bug)
As Sane already mentioned, you can interrupt the certain frequency on which the router and the devices are listening. Every channel has its own frequency, but depending on the quality of the router and the WLAN standard there is something called crosstalking.
This makes it really hard to focus one specific device. Cheap jammers will work more like a machine gun in the hands of an ape instead of a precise sniper rifle used by Simo Häyhä. So be careful what you're doing due to legal consequences...
There are really only 3 channels of interest on 2.4GHz band - 1, 6, 11. If you have highpower APs ( can be done with USB wifi cards at 1 watt ) and saturate them with traffic ( pingflood broadcast address, looped endless transfers of random data w e.g. netcat ) other APs in the vicinity will be disrupted, or perform very poorly. This is not illegal. The 5GHz band which is used in 802.11n etc. doesn't have the range or wall-punching power to prevent this.
Yes. Using aireplay-ng someone could send deauth packets to all of the clients connected to an AP, these deauth packets trick devices into thinking they come from the router. This causes clients to be disconnected from the router, although devices tend to connect back automatically, if the attacker keeps on sending these packets they could “jam” your WiFi AKA keep on causing devices to disconnect.