Wifi on linux
unix networking
Published: 2019-01-21

It’s been sometime since I posted anything proper on the internet, but recently I was reading about how WiFi is deployed in enterprise and the wireless subsystem in the Linux kernel. So I thought it would be nice to have a self note about it in form of a short blogpost.

Wireless communication

A WNIC can operate in two ways:

  • Infrastructure mode ( AP Topology )
    • uses Infrastructure BSS
    • BSSID is usually the MAC address of the AP
  • AdHoc mode
    • uses Independent BSS ( IBSS )
    • BSSID for AdHoc is the IBSS which is a randomly generated bit string for each peer
    • Do not use a mac address
    • IBSS
      • A BSS that forms a self-contained network, and in which no access to a distribution system (DS) is available.

In the infrastructure mode there are two types of Service sets:

  • BSS (Basic SS)
    • A BSS consists of a group of computers and one AP, which links to a wired LAN
  • ESS (Extended SS)
    • Multiple BSS
    • An ESS consists of more than one AP. An ESS lets mobile users roam anywhere within the area covered by multiple APs.
    • When in an ESS, user is unaware of which BSS they belong to they will reuse the SSID/ESSID.

A note about SSID, BSSID and ESSID

  • Service Set Identifier (SSID) : SSID is the Name of a Network (name of the WLAN)
  • Basic Service Set Identifier (BSSID) : Identify Access Points and Their Clients in a multiple AP setup
  • ESSID : is the same as SSID here.

How our machines connect to our WNICs

+-----------+   +--------+   +------+  +-----------+
| userspace +-->+ kernel +-->+ WNIC +-->external AP|
+-----------+   +--------+   +------+  +-----------+

Most of the info here on will be Linux specific. It’s important to understand there are 2 paths in which userspace communicates with the kernel when we’re talking about WiFi:

  • Data Path/(TX/RX) Path: The data being received is passed from the wireless driver to the netdev core
  • Control Path: scan/authentication/association

WNIC

WNICs can be of two types based on management of MLME (MAC Sublayer Management Entity) :

  • SoftMAC
    • Example Hardware: Most laptops
    • Example Hardware Driver: iwlwifi
  • HardMAC/FullMAC
    • Example Hardware: Rpi3
    • Example Hardware Driver: brcm80211

If the WNICs wants to talk to the kernel then it has to use the cfg80211 interface.

  • HardMAC WNICs using HardMAC drivers have to implement the cfg80211 interfaces fully themselves.
  • SoftMAC WNICs using SoftMAC drivers can use linux kernel’s mac80211 framework to talk to cfg80211.

NOTE: MLME (MAC Sublayer Management Entity) MLME is the management entity where the Physical layer (PHY) MAC state machines reside.

Kernel

  • cfg80211: Kernel side of configuration management for wireless devices.
    • works with FullMAC-drivers and also with mac80211-based drivers.
  • mac80211 A driver API for SoftMAC wireless cards.
    • mac80211 registers itself with cfg80211 by using the cfg80211_ops
    • specific HW driver(eg. iwlwifi) registers itself with mac80211 by using the ieee80211_ops

Userspace

  • nl80211 : User-space side of configuration management for wireless devices. It is a Netlink-based user-space protocol.

    • Netlink is designed and used for transferring miscellaneous networking information between the kernel space and userspace processes.
      • The libnl suite is a collection of libraries providing APIs to netlink protocol based Linux kernel interfaces. It is designed for building command line tools and is blocking in nature for a generic netlink family discovery.
      • ELL (successor to libnl, maybe?)

  • wpa_supplicant is supposed to handle MLME commands

    • If you want to use WPA/WPA2 you need to use wpa_supplicant
    • it is probably being replaced by iwd
      • iwd has a interactive command mode called iwctl

  • iw: It is a new nl80211 based CLI configuration utility for wireless devices.

    • successor to iwconfig
    • not related to iwd in terms of development
    • When using this tool, it is important to distinguish between the WiFi hardware (or PHYsical layer) and the WiFi interface (or MAC layer).
      • PHY layer: iw phy
      • Interface Layer: iw dev

Additional Jargons:

  • WifiAP == Hotspot
  • Wifi Client == Wifi station

About Multiple interfaces

Multiple interfaces can be used to run something like a station on wlan0 and an AP on wlan1 (depending on the HW). Otherwise we can also have the connection coming on a eth0 and the AP on wlan0. You can use linux virtual networking devices like bridge to pass the internet over.

Some random commands

$ sudo iw dev wlp2s0 scan | less
$ sudo iwlist wlp2s0 scanning