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. In addition, hostapd can proxy a
WPS registration between a wireless Enrollee and an external Registrar
(e.g., Microsoft Vista or Atheros JumpStart) with UPnP.
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_WPS=y
CONFIG_WPS2=y
CONFIG_WPS_UPNP=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
# if external Registrars are allowed, UPnP support could be added:
#upnp_iface=br0
#friendly_name=WPS Access Point
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.
The PIN value used in the commands must be processed by an UI to
remove non-digit characters and potentially, to verify the checksum
digit. "hostapd_cli wps_check_pin <PIN>" can be used to do such
processing. It returns FAIL if the PIN is invalid, or FAIL-CHECKSUM if
the checksum digit is incorrect, or the processed PIN (non-digit
characters removed) if the PIN is valid.
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
Control interface data:
WPS-PIN-NEEDED [UUID-E|MAC Address|Device Name|Manufacturer|Model Name|Model Number|Serial Number|Device Category]
For example:
<2>WPS-PIN-NEEDED [53b63a98-d29e-4457-a2ed-094d7e6a669c|02:12:34:56:78:9a|Device|Manuf|Model|Model Number|Serial Number|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
To reduce likelihood of PIN being used with other devices or of
forgetting an active PIN available for potential attackers, expiration
time in seconds can be set for the new PIN (value 0 indicates no
expiration):
hostapd_cli wps_pin any 12345670 300
If the MAC address of the enrollee is known, it should be configured
to allow the AP to advertise list of authorized enrollees:
hostapd_cli wps_pin 53b63a98-d29e-4457-a2ed-094d7e6a669c \
12345670 300 00:11:22:33:44:55
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.
When an external Registrar is used, the AP can act as an Enrollee and
use its AP PIN. A static AP PIN (e.g., one one a label in the AP
device) can be configured in hostapd.conf (ap_pin parameter). A more
secure option is to use hostapd_cli wps_ap_pin command to enable the
AP PIN only based on user action (and even better security by using a
random AP PIN for each session, i.e., by using "wps_ap_pin random"
command with a timeout value). Following commands are available for
managing the dynamic AP PIN operations:
hostapd_cli wps_ap_pin disable
- disable AP PIN (i.e., do not allow external Registrars to use it to
learn the current AP settings or to reconfigure the AP)
hostapd_cli wps_ap_pin random [timeout]
- generate a random AP PIN and enable it
- if the optional timeout parameter is given, the AP PIN will be enabled
for the specified number of seconds
hostapd_cli wps_ap_pin get
- fetch the current AP PIN
hostapd_cli wps_ap_pin set <PIN> [timeout]
- set the AP PIN and enable it
- if the optional timeout parameter is given, the AP PIN will be enabled
for the specified number of seconds
hostapd_cli get_config
- display the current configuration
hostapd_cli wps_config <new SSID> <auth> <encr> <new key>
examples:
hostapd_cli wps_config testing WPA2PSK CCMP 12345678
hostapd_cli wps_config "no security" OPEN NONE ""
<auth> must be one of the following: OPEN WPAPSK WPA2PSK
<encr> must be one of the following: NONE WEP TKIP CCMP
Credential generation and configuration changes
-----------------------------------------------
By default, hostapd generates credentials for Enrollees and processing
AP configuration updates internally. However, it is possible to
control these operations from external programs, if desired.
The internal credential generation can be disabled with
skip_cred_build=1 option in the configuration. extra_cred option will
then need to be used to provide pre-configured Credential attribute(s)
for hostapd to use. The exact data from this binary file will be sent,
i.e., it will have to include valid WPS attributes. extra_cred can
also be used to add additional networks if the Registrar is used to
configure credentials for multiple networks.
Processing of received configuration updates can be disabled with
wps_cred_processing=1 option. When this is used, an external program
is responsible for creating hostapd configuration files and processing
configuration updates based on messages received from hostapd over
control interface. This will also include the initial configuration on
first successful registration if the AP is initially set in
unconfigured state.
Following control interface messages are sent out for external programs:
WPS-REG-SUCCESS <Enrollee MAC address <UUID-E>
For example:
<2>WPS-REG-SUCCESS 02:66:a0:ee:17:27 2b7093f1-d6fb-5108-adbb-bea66bb87333
This can be used to trigger change from unconfigured to configured
state (random configuration based on the first successful WPS
registration). In addition, this can be used to update AP UI about the
status of WPS registration progress.
WPS-NEW-AP-SETTINGS <hexdump of AP Setup attributes>
For example:
<2>WPS-NEW-AP-SETTINGS 10260001011045000c6a6b6d2d7770732d74657374100300020020100f00020008102700403065346230343536633236366665306433396164313535346131663462663731323433376163666462376633393965353466316631623032306164343438623510200006024231cede15101e000844
This can be used to update the externally stored AP configuration and
then update hostapd configuration (followed by restarting of hostapd).