hostapd and Wi-Fi Protected Setup (WPS)
=======================================

This document describes how the WPS implementation in hostapd can be
configured and how an external component on an AP (e.g., web UI) is
used to enable enrollment of client devices.


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


hostapd implementation
----------------------

hostapd includes an optional WPS component that can be used as an
internal WPS Registrar to manage addition of new WPS enabled clients
to the network. In addition, WPS Enrollee functionality in hostapd can
be used to allow external WPS Registrars to configure the access
point, e.g., for initial network setup. The current version of hostapd
does not support use of external WPS Registrars for adding new client
devices.


hostapd configuration
---------------------

WPS is an optional component that needs to be enabled in hostapd build
configuration (.config). Here is an example configuration that
includes WPS support and uses madwifi driver interface:

CONFIG_DRIVER_MADWIFI=y
CFLAGS += -I/usr/src/madwifi-0.9.3
CONFIG_EAP=y
CONFIG_WPS=y


Following section shows an example runtime configuration
(hostapd.conf) that enables WPS:

# Configure the driver and network interface
driver=madwifi
interface=ath0

# WPA2-Personal configuration for the AP
ssid=wps-test
wpa=2
wpa_key_mgmt=WPA-PSK
wpa_pairwise=CCMP
# Default WPA passphrase for legacy (non-WPS) clients
wpa_passphrase=12345678
# Enable random per-device PSK generation for WPS clients
# Please note that the file has to exists for hostapd to start (i.e., create an
# empty file as a starting point).
wpa_psk_file=/etc/hostapd.psk

# Enable control interface for PBC/PIN entry
ctrl_interface=/var/run/hostapd

# Enable internal EAP server for EAP-WSC (part of Wi-Fi Protected Setup)
eap_server=1

# WPS configuration (AP configured, do not allow external WPS Registrars)
wps_state=2
ap_setup_locked=1
# If UUID is not configured, it will be generated based on local MAC address.
uuid=87654321-9abc-def0-1234-56789abc0000
wps_pin_requests=/var/run/hostapd.pin-req
device_name=Wireless AP
manufacturer=Company
model_name=WAP
model_number=123
serial_number=12345
device_type=6-0050F204-1
os_version=01020300
config_methods=label display push_button keypad


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. hostapd uses the control interface as an input channel for
these events.

When a client device (WPS Enrollee) connects to hostapd (WPS
Registrar) in order to start PIN mode negotiation for WPS, an
identifier (Enrollee UUID) is sent. hostapd will need to be configured
with a device password (PIN) for this Enrollee. This is an operation
that requires user interaction (assuming there are no pre-configured
PINs on the AP for a set of Enrollee).

The PIN request with information about the device is appended to the
wps_pin_requests file (/var/run/hostapd.pin-req in this example). In
addition, hostapd control interface event is sent as a notification of
a new device. The AP could use, e.g., a web UI for showing active
Enrollees to the user and request a PIN for an Enrollee.

The PIN request file has one line for every Enrollee that connected to
the AP, but for which there was no PIN. Following information is
provided for each Enrollee (separated with tabulators):
- timestamp (seconds from 1970-01-01)
- Enrollee UUID
- MAC address
- Device name
- Manufacturer
- Model Name
- Model Number
- Serial Number
- Device category

Example line in the /var/run/hostapd.pin-req file:
1200188391	53b63a98-d29e-4457-a2ed-094d7e6a669c	Intel(R) Centrino(R)	Intel Corporation	Intel(R) Centrino(R)	-	-	1-0050F204-1


When the user enters a PIN for a pending Enrollee, e.g., on the web
UI), hostapd needs to be notified of the new PIN over the control
interface. This can be done either by using the UNIX domain socket
-based control interface directly (src/common/wpa_ctrl.c provides
helper functions for using the interface) or by calling hostapd_cli.

Example command to add a PIN (12345670) for an Enrollee:

hostapd_cli wps_pin 53b63a98-d29e-4457-a2ed-094d7e6a669c 12345670

If the UUID-E is not available (e.g., Enrollee waits for the Registrar
to be selected before connecting), wildcard UUID may be used to allow the PIN to be used once with any UUID:

hostapd_cli wps_pin any 12345670


After this, the Enrollee can connect to the AP again and complete WPS
negotiation. At that point, a new, random WPA PSK is generated for the
client device and the client can then use that key to connect to the
AP to access the network.


If the AP includes a pushbutton, WPS PBC mode can be used. It is
enabled by pushing a button on both the AP and the client at about the
same time (2 minute window). hostapd needs to be notified about the AP
button pushed event over the control interface, e.g., by calling
hostapd_cli:

hostapd_cli wps_pbc

At this point, the client has two minutes to complete WPS negotiation
which will generate a new WPA PSK in the same way as the PIN method
described above.
