wpa_supplicant and Wi-Fi Protected Setup (WPS)
==============================================
This document describes how the WPS implementation in wpa_supplicant
can be configured and how an external component on the client (e.g.,
management GUI) is used to enable WPS enrollment and registrar
registration.
Introduction to WPS
-------------------
Wi-Fi Protected Setup (WPS) is a mechanism for easy configuration of a
wireless network. It allows automated generation of random keys (WPA
passphrase/PSK) and configuration of an access point and client
devices. WPS includes number of methods for setting up connections
with PIN method and push-button configuration (PBC) being the most
commonly deployed options.
While WPS can enable more home networks to use encryption in the
wireless network, it should be noted that the use of the PIN and
especially PBC mechanisms for authenticating the initial key setup is
not very secure. As such, use of WPS may not be suitable for
environments that require secure network access without chance for
allowing outsiders to gain access during the setup phase.
WPS uses following terms to describe the entities participating in the
network setup:
- access point: the WLAN access point
- Registrar: a device that control a network and can authorize
addition of new devices); this may be either in the AP ("internal
Registrar") or in an external device, e.g., a laptop, ("external
Registrar")
- Enrollee: a device that is being authorized to use the network
It should also be noted that the AP and a client device may change
roles (i.e., AP acts as an Enrollee and client device as a Registrar)
when WPS is used to configure the access point.
More information about WPS is available from Wi-Fi Alliance:
http://www.wi-fi.org/wifi-protected-setup
wpa_supplicant implementation
-----------------------------
wpa_supplicant includes an optional WPS component that can be used as
an Enrollee to enroll new network credential or as a Registrar to
configure an AP. The current version of wpa_supplicant does not
support operation as an external WLAN Management Registrar for adding
new client devices or configuring the AP over UPnP.
wpa_supplicant configuration
----------------------------
WPS is an optional component that needs to be enabled in
wpa_supplicant build configuration (.config). Here is an example
configuration that includes WPS support and Linux wireless extensions
-based driver interface:
CONFIG_DRIVER_WEXT=y
CONFIG_EAP=y
CONFIG_WPS=y
WPS needs the Universally Unique IDentifier (UUID; see RFC 4122) for
the device. This is configured in the runtime configuration for
wpa_supplicant (if not set, UUID will be generated based on local MAC
address):
# example UUID for WPS
uuid=12345678-9abc-def0-1234-56789abcdef0
The network configuration blocks needed for WPS are added
automatically based on control interface commands, so they do not need
to be added explicitly in the configuration file.
WPS registration will generate new network blocks for the acquired
credentials. If these are to be stored for future use (after
restarting wpa_supplicant), wpa_supplicant will need to be configured
to allow configuration file updates:
update_config=1
External operations
-------------------
WPS requires either a device PIN code (usually, 8-digit number) or a
pushbutton event (for PBC) to allow a new WPS Enrollee to join the
network. wpa_supplicant uses the control interface as an input channel
for these events.
If the client device has a display, a random PIN has to be generated
for each WPS registration session. wpa_supplicant can do this with a
control interface request, e.g., by calling wpa_cli:
wpa_cli wps_pin any
This will return the generated 8-digit PIN which will then need to be
entered at the Registrar to complete WPS registration. At that point,
the client will be enrolled with credentials needed to connect to the
AP to access the network.
If the client device does not have a display that could show the
random PIN, a hardcoded PIN that is printed on a label can be
used. wpa_supplicant is notified this with a control interface
request, e.g., by calling wpa_cli:
wpa_cli wps_pin any 12345670
This starts the WPS negotiation in the same way as above with the
generated PIN.
If the client design wants to support optional WPS PBC mode, this can
be enabled by either a physical button in the client device or a
virtual button in the user interface. The PBC operation requires that
a button is also pressed at the AP/Registrar at about the same time (2
minute window). wpa_supplicant is notified of the local button event
over the control interface, e.g., by calling wpa_cli:
wpa_cli wps_pbc
At this point, the AP/Registrar has two minutes to complete WPS
negotiation which will generate a new WPA PSK in the same way as the
PIN method described above.
If the client wants to operate in the Registrar role to configure an
AP, wpa_supplicant is notified over the control interface, e.g., with
wpa_cli:
wpa_cli wps_reg <AP BSSID> <AP PIN>
(example: wpa_cli wps_reg 02:34:56:78:9a:bc 12345670)
This is currently only used to fetch the current AP settings instead
of actually changing them. The main difference with the wps_pin
command is that wps_reg uses the AP PIN (e.g., from a label on the AP)
instead of a PIN generated at the client.
Scanning
--------
Scan results ('wpa_cli scan_results' or 'wpa_cli bss <idx>') include a
flags field that is used to indicate whether the BSS support WPS. If
the AP support WPS, but has not recently activated a Registrar, [WPS]
flag will be included. If PIN method has been recently selected,
[WPS-PIN] is shown instead. Similarly, [WPS-PBC] is shown if PBC mode
is in progress. GUI programs can use these as triggers for suggesting
a guided WPS configuration to the user. In addition, control interface
monitor events WPS-AP-AVAILABLE{,-PBC,-PIN} can be used to find out if
there are WPS enabled APs in scan results without having to go through
all the details in the GUI. These notification could be used, e.g., to
suggest possible WPS connection to the user.
wpa_gui
-------
wpa_gui-qt4 directory contains a sample GUI that shows an example of
how WPS support can be integrated into the GUI. Its main window has a
WPS tab that guides user through WPS registration with automatic AP
selection. In addition, it shows how WPS can be started manually by
selecting an AP from scan results.
Credential processing
---------------------
By default, wpa_supplicant processes received credentials and updates
its configuration internally. However, it is possible to
control these operations from external programs, if desired.
This internal processing can be disabled with wps_cred_processing=1
option. When this is used, an external program is responsible for
processing the credential attributes and updating wpa_supplicant
configuration based on them.
Following control interface messages are sent out for external programs:
WPS-CRED-RECEIVED <hexdump of Credential attribute(s)>
For example:
<2>WPS-CRED-RECEIVED 100e006f10260001011045000c6a6b6d2d7770732d74657374100300020020100f000200081027004030653462303435366332363666653064333961643135353461316634626637313234333761636664623766333939653534663166316230323061643434386235102000060266a0ee1727