How nl80211 library & cfg80211 work?

Question

I want to learn about how nl80211 and cfg80211 works in detail. Function flow, how nl80211 interact with network tools like wpa_supplicant, iw.

Plz suggest me some useful links or books to refer.

Answer 1

To be able to control wireless drivers from userspace, some IPC communication processes between kernel and userspace are used.

• At first ioctl with vendor dependent APIs was used.
• In 1996, Jean Tourrilhes creates wireless extensions (WE or WEXT).

The Wireless Extension (WE) is a generic API allowing a driver to expose to the user space configuration and statistics specific to common Wireless LANs.

• In 2006, John Linville creates mac80211 and Johannes Berg creates cfg80211 and nl80211. Together it is intended to replace wireless extensions.

  +-------------+ |             | |  Userspace  | |             | +-------------+       ^ - - - | - - - -        | nl80211       v +-------------+ |             | |  cfg80211   | |             | +-------------+ +-------------+ |             | |  mac80211   | |   driver    | |             | +-------------+ 

An important point is that nl80211/cfg80211/mac80211 no longer use ioctl, they use netlink.

So, tools like iw, hostapd or the wpa_supplicant use some netlink libraries (like libnl or libnl-tiny) and the netlink interface public header which is of course nl80211.h.

There is not so much documentations, but I advise you to read the libnl documentation and then the iw source code (because iw use libnl).

Answer 2

A slightly more detailed picture of how nl80211 and cfg80211 work with other parts of the system (user space, kernel, and hardware).

• nl80211 is the interface between user space software (iw, wpa_supplicant, etc.) and the kernel (cfg80211 and mac80211 kernel modules, and specific drivers).
• The WiFi drivers and hardware could be Full-MAC or Soft-MAC (see Wireless_network_interface_controller).
• cfg80211_ops is a set of operations that Full-MAC drivers and mac80211 module register to cfg80211 module.
• ieee80211_ops is a set of operations that Soft-MAC drivers register to mac80211 module.