How do applications such as Airodump-ng determine the channel of probe packets they receive? To my understanding, when the receiver is scanning channel 11 it can pick up packets all the way down to channel 6 without much trouble, but these probe packets (to my knowledge) don't actually say what channel they're transmitting on, yet somehow the program knows. Is this determined by the program itself, or is it happening at a more basic level in the receiver?
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The Beacon frame itself contains this information.
The 802.11 spec describes this as an optionally-present field on a beacon frame denoting a channel switch; however beacon frames may carry this for a basic beacon packet not denoting a channel switch.
Probe responses also carry channel usage info if the request specifies to send this.
In addition, certain monitoring or capture software could potentially read field 7 of association request packets, which describes the supported set of channels by a client
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