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291 lines
11 KiB
Text
291 lines
11 KiB
Text
hostapd and Wi-Fi Protected Setup (WPS)
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=======================================
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This document describes how the WPS implementation in hostapd can be
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configured and how an external component on an AP (e.g., web UI) is
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used to enable enrollment of client devices.
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Introduction to WPS
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-------------------
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Wi-Fi Protected Setup (WPS) is a mechanism for easy configuration of a
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wireless network. It allows automated generation of random keys (WPA
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passphrase/PSK) and configuration of an access point and client
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devices. WPS includes number of methods for setting up connections
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with PIN method and push-button configuration (PBC) being the most
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commonly deployed options.
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While WPS can enable more home networks to use encryption in the
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wireless network, it should be noted that the use of the PIN and
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especially PBC mechanisms for authenticating the initial key setup is
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not very secure. As such, use of WPS may not be suitable for
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environments that require secure network access without chance for
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allowing outsiders to gain access during the setup phase.
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WPS uses following terms to describe the entities participating in the
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network setup:
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- access point: the WLAN access point
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- Registrar: a device that control a network and can authorize
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addition of new devices); this may be either in the AP ("internal
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Registrar") or in an external device, e.g., a laptop, ("external
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Registrar")
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- Enrollee: a device that is being authorized to use the network
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It should also be noted that the AP and a client device may change
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roles (i.e., AP acts as an Enrollee and client device as a Registrar)
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when WPS is used to configure the access point.
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More information about WPS is available from Wi-Fi Alliance:
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http://www.wi-fi.org/wifi-protected-setup
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hostapd implementation
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----------------------
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hostapd includes an optional WPS component that can be used as an
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internal WPS Registrar to manage addition of new WPS enabled clients
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to the network. In addition, WPS Enrollee functionality in hostapd can
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be used to allow external WPS Registrars to configure the access
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point, e.g., for initial network setup. In addition, hostapd can proxy a
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WPS registration between a wireless Enrollee and an external Registrar
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(e.g., Microsoft Vista or Atheros JumpStart) with UPnP.
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hostapd configuration
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---------------------
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WPS is an optional component that needs to be enabled in hostapd build
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configuration (.config). Here is an example configuration that
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includes WPS support and uses madwifi driver interface:
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CONFIG_DRIVER_MADWIFI=y
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CFLAGS += -I/usr/src/madwifi-0.9.3
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CONFIG_WPS=y
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CONFIG_WPS2=y
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CONFIG_WPS_UPNP=y
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Following section shows an example runtime configuration
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(hostapd.conf) that enables WPS:
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# Configure the driver and network interface
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driver=madwifi
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interface=ath0
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# WPA2-Personal configuration for the AP
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ssid=wps-test
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wpa=2
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wpa_key_mgmt=WPA-PSK
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wpa_pairwise=CCMP
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# Default WPA passphrase for legacy (non-WPS) clients
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wpa_passphrase=12345678
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# Enable random per-device PSK generation for WPS clients
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# Please note that the file has to exists for hostapd to start (i.e., create an
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# empty file as a starting point).
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wpa_psk_file=/etc/hostapd.psk
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# Enable control interface for PBC/PIN entry
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ctrl_interface=/var/run/hostapd
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# Enable internal EAP server for EAP-WSC (part of Wi-Fi Protected Setup)
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eap_server=1
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# WPS configuration (AP configured, do not allow external WPS Registrars)
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wps_state=2
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ap_setup_locked=1
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# If UUID is not configured, it will be generated based on local MAC address.
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uuid=87654321-9abc-def0-1234-56789abc0000
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wps_pin_requests=/var/run/hostapd.pin-req
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device_name=Wireless AP
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manufacturer=Company
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model_name=WAP
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model_number=123
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serial_number=12345
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device_type=6-0050F204-1
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os_version=01020300
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config_methods=label display push_button keypad
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# if external Registrars are allowed, UPnP support could be added:
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#upnp_iface=br0
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#friendly_name=WPS Access Point
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External operations
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-------------------
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WPS requires either a device PIN code (usually, 8-digit number) or a
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pushbutton event (for PBC) to allow a new WPS Enrollee to join the
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network. hostapd uses the control interface as an input channel for
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these events.
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The PIN value used in the commands must be processed by an UI to
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remove non-digit characters and potentially, to verify the checksum
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digit. "hostapd_cli wps_check_pin <PIN>" can be used to do such
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processing. It returns FAIL if the PIN is invalid, or FAIL-CHECKSUM if
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the checksum digit is incorrect, or the processed PIN (non-digit
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characters removed) if the PIN is valid.
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When a client device (WPS Enrollee) connects to hostapd (WPS
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Registrar) in order to start PIN mode negotiation for WPS, an
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identifier (Enrollee UUID) is sent. hostapd will need to be configured
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with a device password (PIN) for this Enrollee. This is an operation
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that requires user interaction (assuming there are no pre-configured
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PINs on the AP for a set of Enrollee).
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The PIN request with information about the device is appended to the
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wps_pin_requests file (/var/run/hostapd.pin-req in this example). In
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addition, hostapd control interface event is sent as a notification of
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a new device. The AP could use, e.g., a web UI for showing active
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Enrollees to the user and request a PIN for an Enrollee.
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The PIN request file has one line for every Enrollee that connected to
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the AP, but for which there was no PIN. Following information is
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provided for each Enrollee (separated with tabulators):
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- timestamp (seconds from 1970-01-01)
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- Enrollee UUID
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- MAC address
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- Device name
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- Manufacturer
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- Model Name
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- Model Number
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- Serial Number
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- Device category
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Example line in the /var/run/hostapd.pin-req file:
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1200188391 53b63a98-d29e-4457-a2ed-094d7e6a669c Intel(R) Centrino(R) Intel Corporation Intel(R) Centrino(R) - - 1-0050F204-1
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Control interface data:
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WPS-PIN-NEEDED [UUID-E|MAC Address|Device Name|Manufacturer|Model Name|Model Number|Serial Number|Device Category]
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For example:
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<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]
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When the user enters a PIN for a pending Enrollee, e.g., on the web
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UI), hostapd needs to be notified of the new PIN over the control
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interface. This can be done either by using the UNIX domain socket
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-based control interface directly (src/common/wpa_ctrl.c provides
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helper functions for using the interface) or by calling hostapd_cli.
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Example command to add a PIN (12345670) for an Enrollee:
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hostapd_cli wps_pin 53b63a98-d29e-4457-a2ed-094d7e6a669c 12345670
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If the UUID-E is not available (e.g., Enrollee waits for the Registrar
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to be selected before connecting), wildcard UUID may be used to allow
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the PIN to be used once with any UUID:
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hostapd_cli wps_pin any 12345670
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To reduce likelihood of PIN being used with other devices or of
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forgetting an active PIN available for potential attackers, expiration
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time in seconds can be set for the new PIN (value 0 indicates no
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expiration):
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hostapd_cli wps_pin any 12345670 300
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If the MAC address of the enrollee is known, it should be configured
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to allow the AP to advertise list of authorized enrollees:
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hostapd_cli wps_pin 53b63a98-d29e-4457-a2ed-094d7e6a669c \
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12345670 300 00:11:22:33:44:55
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After this, the Enrollee can connect to the AP again and complete WPS
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negotiation. At that point, a new, random WPA PSK is generated for the
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client device and the client can then use that key to connect to the
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AP to access the network.
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If the AP includes a pushbutton, WPS PBC mode can be used. It is
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enabled by pushing a button on both the AP and the client at about the
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same time (2 minute window). hostapd needs to be notified about the AP
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button pushed event over the control interface, e.g., by calling
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hostapd_cli:
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hostapd_cli wps_pbc
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At this point, the client has two minutes to complete WPS negotiation
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which will generate a new WPA PSK in the same way as the PIN method
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described above.
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When an external Registrar is used, the AP can act as an Enrollee and
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use its AP PIN. A static AP PIN (e.g., one one a label in the AP
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device) can be configured in hostapd.conf (ap_pin parameter). A more
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secure option is to use hostapd_cli wps_ap_pin command to enable the
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AP PIN only based on user action (and even better security by using a
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random AP PIN for each session, i.e., by using "wps_ap_pin random"
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command with a timeout value). Following commands are available for
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managing the dynamic AP PIN operations:
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hostapd_cli wps_ap_pin disable
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- disable AP PIN (i.e., do not allow external Registrars to use it to
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learn the current AP settings or to reconfigure the AP)
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hostapd_cli wps_ap_pin random [timeout]
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- generate a random AP PIN and enable it
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- if the optional timeout parameter is given, the AP PIN will be enabled
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for the specified number of seconds
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hostapd_cli wps_ap_pin get
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- fetch the current AP PIN
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hostapd_cli wps_ap_pin set <PIN> [timeout]
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- set the AP PIN and enable it
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- if the optional timeout parameter is given, the AP PIN will be enabled
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for the specified number of seconds
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hostapd_cli get_config
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- display the current configuration
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hostapd_cli wps_config <new SSID> <auth> <encr> <new key>
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examples:
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hostapd_cli wps_config testing WPA2PSK CCMP 12345678
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hostapd_cli wps_config "no security" OPEN NONE ""
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<auth> must be one of the following: OPEN WPAPSK WPA2PSK
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<encr> must be one of the following: NONE WEP TKIP CCMP
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Credential generation and configuration changes
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-----------------------------------------------
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By default, hostapd generates credentials for Enrollees and processing
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AP configuration updates internally. However, it is possible to
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control these operations from external programs, if desired.
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The internal credential generation can be disabled with
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skip_cred_build=1 option in the configuration. extra_cred option will
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then need to be used to provide pre-configured Credential attribute(s)
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for hostapd to use. The exact data from this binary file will be sent,
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i.e., it will have to include valid WPS attributes. extra_cred can
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also be used to add additional networks if the Registrar is used to
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configure credentials for multiple networks.
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Processing of received configuration updates can be disabled with
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wps_cred_processing=1 option. When this is used, an external program
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is responsible for creating hostapd configuration files and processing
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configuration updates based on messages received from hostapd over
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control interface. This will also include the initial configuration on
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first successful registration if the AP is initially set in
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unconfigured state.
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Following control interface messages are sent out for external programs:
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WPS-REG-SUCCESS <Enrollee MAC address <UUID-E>
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For example:
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<2>WPS-REG-SUCCESS 02:66:a0:ee:17:27 2b7093f1-d6fb-5108-adbb-bea66bb87333
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This can be used to trigger change from unconfigured to configured
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state (random configuration based on the first successful WPS
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registration). In addition, this can be used to update AP UI about the
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status of WPS registration progress.
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WPS-NEW-AP-SETTINGS <hexdump of AP Setup attributes>
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For example:
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<2>WPS-NEW-AP-SETTINGS 10260001011045000c6a6b6d2d7770732d74657374100300020020100f00020008102700403065346230343536633236366665306433396164313535346131663462663731323433376163666462376633393965353466316631623032306164343438623510200006024231cede15101e000844
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This can be used to update the externally stored AP configuration and
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then update hostapd configuration (followed by restarting of hostapd).
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