hostapd/doc/driver_wrapper.doxygen

/**
\page driver_wrapper Driver wrapper implementation (driver.h, drivers.c)

All hardware and driver dependent functionality is in separate C files
that implement defined wrapper functions. Other parts
of the wpa_supplicant are designed to be hardware, driver, and operating
system independent.

Driver wrappers need to implement whatever calls are used in the
target operating system/driver for controlling wireless LAN
devices. As an example, in case of Linux, these are mostly some glue
code and ioctl() calls and netlink message parsing for Linux Wireless
Extensions (WE). Since features required for WPA were added only recently to
Linux Wireless Extensions (in version 18), some driver specific code is used
in number of driver interface implementations. These driver dependent parts
can be replaced with generic code in \ref driver_wext.c once the target driver
includes full support for WE-18. After that, all Linux drivers, at
least in theory, could use the same driver wrapper code.

A driver wrapper needs to implement some or all of the functions
defined in \ref driver.h. These functions are registered by filling struct
\ref wpa_driver_ops with function pointers. Hardware independent parts of
wpa_supplicant will call these functions to control the driver/wlan
card. In addition, support for driver events is required. The event
callback function, \ref wpa_supplicant_event(), and its parameters are
documented in \ref driver.h. In addition, a pointer to the 'struct
\ref wpa_driver_ops' needs to be registered in \ref drivers.c file.

When porting to other operating systems, the driver wrapper should be
modified to use the native interface of the target OS. It is possible
that some extra requirements for the interface between the driver
wrapper and generic wpa_supplicant code are discovered during porting
to a new operating system. These will be addressed on case by case
basis by modifying the interface and updating the other driver
wrappers for this. The goal is to avoid changing this interface
without very good reasons in order to limit the number of changes
needed to other wrappers and hardware independent parts of
wpa_supplicant. When changes are required, recommended way is to
make them in backwards compatible way that allows existing driver
interface implementations to be compiled without any modification.

Generic Linux Wireless Extensions functions are implemented in
\ref driver_wext.c. All Linux driver wrappers can use these when the kernel
driver supports the generic ioctl()s and wireless events. Driver
specific functions are implemented in separate C files, e.g.,
\ref driver_hostap.c. These files need to define struct \ref wpa_driver_ops
entry that will be used in \ref wpa_supplicant.c when calling driver
functions. struct \ref wpa_driver_ops entries are registered in \ref drivers.c.

In general, it is likely to be useful to first take a look at couple
of driver interface examples before starting on implementing a new
one. \ref driver_hostap.c and \ref driver_wext.c include a complete
implementation for Linux drivers that use wpa_supplicant-based control
of WPA IE and roaming. \ref driver_ndis.c (with help from \ref driver_ndis_.c)
is an example of a complete interface for Windows NDIS interface for
drivers that generate WPA IE themselves and decide when to roam. These
example implementations include full support for all security modes.


\section driver_req Driver requirements for WPA

WPA introduces new requirements for the device driver. At least some
of these need to be implemented in order to provide enough support for
wpa_supplicant.

\subsection driver_tkip_ccmp TKIP/CCMP

WPA requires that the pairwise cipher suite (encryption algorithm for
unicast data packets) is TKIP or CCMP. These are new encryption
protocols and thus, the driver will need to be modified to support
them. Depending on the used wlan hardware, some parts of these may be
implemented by the hardware/firmware.

Specification for both TKIP and CCMP is available from IEEE (IEEE
802.11i amendment). Fully functional, hardware independent
implementation of both encryption protocols is also available in Host
AP driver (driver/modules/hostap_{tkip,ccmp}.c). In addition, Linux 2.6
kernel tree has generic implementations for WEP, TKIP, and CCMP that can
be used in Linux drivers.

The driver will also need to provide configuration mechanism to allow
user space programs to configure TKIP and CCMP. Linux Wireless Extensions
v18 added support for configuring these algorithms and
individual/non-default keys. If the target kernel does not include WE-18,
private ioctls can be used to provide similar functionality.

\subsection driver_roaming Roaming control and scanning support

wpa_supplicant can optionally control AP selection based on the
information received from Beacon and/or Probe Response frames
(ap_scan=1 mode in configuration). This means that the driver should
support external control for scan process. In case of Linux, use of
new Wireless Extensions scan support (i.e., 'iwlist wlan0 scan') is
recommended. The current driver wrapper (\ref driver_wext.c) uses this for
scan results.

Scan results must also include the WPA information element. Support for
this was added in WE-18. With older versions, a custom event can be used
to provide the full WPA IE (including element id and length) as a hex
string that is included in the scan results.

wpa_supplicant needs to also be able to request the driver to
associate with a specific BSS. Current Host AP driver and matching
\ref driver_hostap.c wrapper uses following sequence for this
request. Similar/identical mechanism should be usable also with other
drivers.

- set WPA IE for AssocReq with private ioctl
- set SSID with SIOCSIWESSID
- set channel/frequency with SIOCSIWFREQ
- set BSSID with SIOCSIWAP
  (this last ioctl will trigger the driver to request association)

\subsection driver_wpa_ie WPA IE generation

wpa_supplicant selects which cipher suites and key management suites
are used. Based on this information, it generates a WPA IE. This is
provided to the driver interface in the associate call. This does not
match with Windows NDIS drivers which generate the WPA IE
themselves.

wpa_supplicant allows Windows NDIS-like behavior by providing the
selected cipher and key management suites in the associate call. If
the driver generates its own WPA IE and that differs from the one
generated by wpa_supplicant, the driver has to inform wpa_supplicant
about the used WPA IE (i.e., the one it used in (Re)Associate
Request). This notification is done using EVENT_ASSOCINFO event (see
\ref driver.h). wpa_supplicant is normally configured to use
ap_scan=2 mode with drivers that control WPA IE generation and roaming.

\subsection driver_events Driver events

wpa_supplicant needs to receive event callbacks when certain events
occur (association, disassociation, Michael MIC failure, scan results
available, PMKSA caching candidate). These events and the callback
details are defined in \ref driver.h (\ref wpa_supplicant_event() function
and enum \ref wpa_event_type).

On Linux, association and disassociation can use existing Wireless
Extensions event that is reporting new AP with SIOCGIWAP
event. Similarly, completion of a scan can be reported with SIOCGIWSCAN
event.

Michael MIC failure event was added in WE-18. Older versions of Wireless
Extensions will need to use a custom event. Host AP driver used a custom
event with following contents: MLME-MICHAELMICFAILURE.indication(keyid=#
broadcast/unicast addr=addr2). This is the recommended format until
the driver can be moved to use WE-18 mechanism.

\subsection driver_wext_summary Summary of Linux Wireless Extensions use

AP selection depends on ap_scan configuration:

ap_scan=1:

- wpa_supplicant requests scan with SIOCSIWSCAN
- driver reports scan complete with wireless event SIOCGIWSCAN
- wpa_supplicant reads scan results with SIOCGIWSCAN (multiple call if
  a larger buffer is needed)
- wpa_supplicant decides which AP to use based on scan results
- wpa_supplicant configures driver to associate with the selected BSS
  (SIOCSIWMODE, SIOCSIWGENIE, SIOCSIWAUTH, SIOCSIWFREQ,
   SIOCSIWESSID, SIOCSIWAP)

ap_scan=2:

- wpa_supplicant configures driver to associate with an SSID
  (SIOCSIWMODE, SIOCSIWGENIE, SIOCSIWAUTH, SIOCSIWESSID)


After this, both modes use similar steps:

- optionally (or required for drivers that generate WPA/RSN IE for
  (Re)AssocReq), driver reports association parameters (AssocReq IEs)
  with wireless event IWEVASSOCREQIE (and optionally IWEVASSOCRESPIE)
- driver reports association with wireless event SIOCGIWAP
- wpa_supplicant takes care of EAPOL frame handling (validating
  information from associnfo and if needed, from scan results if WPA/RSN
  IE from the Beacon frame is not reported through associnfo)
*/