4bc523382c
Update wpa_supplicant/hostapd 2.7 --> 2.8 Upstream documents the following advisories: - https://w1.fi/security/2019-1/sae-side-channel-attacks.txt - https://w1.fi/security/2019-2/eap-pwd-side-channel-attack.txt - https://w1.fi/security/2019-3/sae-confirm-missing-state-validation.txt - https://w1.fi/security/2019-4/eap-pwd-missing-commit-validation.txt - https://w1.fi/security/2019-5/eap-pwd-message-reassembly-issue-\ with-unexpected-fragment.txt Relnotes: yes MFC after: 1 week (or less) Security: CVE-2019-9494, VU#871675, CVE-2019-9495, CVE-2019-9496, CVE-2019-9497, CVE-2019-9498, CVE-2019-9499
161 lines
8.1 KiB
Plaintext
161 lines
8.1 KiB
Plaintext
hostapd, wpa_supplicant and the Multi-AP Specification
|
|
======================================================
|
|
|
|
This document describes how hostapd and wpa_supplicant can be configured to
|
|
support the Multi-AP Specification.
|
|
|
|
Introduction to Multi-AP
|
|
------------------------
|
|
|
|
The Wi-Fi Alliance Multi-AP Specification is the technical specification for
|
|
Wi-Fi CERTIFIED EasyMesh(TM) [1], the Wi-Fi Alliance® certification program for
|
|
Multi-AP. It defines control protocols between Wi-Fi® access points (APs) to
|
|
join them into a network with centralized control and operation. It is targeted
|
|
only at routers (repeaters, gateways, ...), not at clients. Clients are not
|
|
involved at all in the protocols.
|
|
|
|
Most of the Multi-AP specification falls outside of the scope of
|
|
hostapd/wpa_supplicant. hostapd/wpa_supplicant is only involved for the items
|
|
summarized below. The rest of the protocol must be implemented by a separate
|
|
daemon, e.g., prplMesh [2]. That daemon also needs to communicate with hostapd,
|
|
e.g., to get a list of associated clients, but this can be done using the normal
|
|
hostapd interfaces.
|
|
|
|
hostapd/wpa_supplicant needs to be configured specifically to support:
|
|
- the WPS onboarding process;
|
|
- configuring backhaul links.
|
|
|
|
The text below refers to "Multi-AP Specification v1.0" [3].
|
|
|
|
|
|
Fronthaul and backhaul links
|
|
----------------------------
|
|
|
|
In a Multi-AP network, the central controller can configure the BSSs on the
|
|
devices that are joined into the network. These are called fronthaul BSSs.
|
|
From the point of view of hostapd, there is nothing special about these
|
|
fronthaul BSSs.
|
|
|
|
In addition to fronthaul BSSs, the controller can also configure backhaul
|
|
links. A backhaul link is a link between two access point devices, giving
|
|
internet access to access point devices that don't have a wired link. The
|
|
Multi-AP specification doesn't dictate this, but typically the backhaul link
|
|
will be bridged into a LAN together with (one of) the fronthaul BSS(s) and the
|
|
wired Ethernet ports.
|
|
|
|
A backhaul link must be treated specially by hostapd and wpa_supplicant. One
|
|
side of the backhaul link is configured through the Multi-AP protocol as the
|
|
"backhaul STA", i.e., the client side of the link. A backhaul STA is like any
|
|
station and is handled appropriately by wpa_supplicant, but two additional
|
|
features are required. It must send an additional information element in each
|
|
(Re)Association Request frame ([3], section 5.2, paragraph 4). In addition, it
|
|
must use 4-address mode for all frames sent over this link ([3], section 14).
|
|
Therefore, wpa_supplicant must be configured explicitly as the backhaul STA
|
|
role, by setting 'multi_ap_backhaul_sta=1' in the network configuration block
|
|
or when configuring the network profile through the control interface. When
|
|
'multi_ap_backhaul_sta=1', wpa_supplicant includes the Multi-AP IE in
|
|
(Re)Association Request frame and verifies that it is included in the
|
|
(Re)Association Response frame. If it is not, association fails. If it is,
|
|
wpa_supplicant sets 4-address mode for this interface through a driver
|
|
callback.
|
|
|
|
The AP side of the backhaul link is called a "backhaul BSS". Such a BSS must
|
|
be handled specially by hostapd, because it must add an additional information
|
|
element in each (Re)Association Response frame, but only to stations that have
|
|
identified themselves as backhaul stations ([3], section 5.2, paragraph 5-6).
|
|
This is important because it is possible to use the same BSS and SSID for
|
|
fronthaul and backhaul at the same time. The additional information element must
|
|
only be used for frames sent to a backhaul STA, not to a normal STA. Also,
|
|
frames sent to a backhaul STA must use 4-address mode, while frames sent to a
|
|
normal STA (fronthaul, when it's a fronthaul and backhaul BSS) must use
|
|
3-address mode.
|
|
|
|
A BSS is configured in Multi-AP mode in hostapd by setting the 'multi_ap'
|
|
configuration option to 1 (backhaul BSS), 2 (fronthaul BSS), or 3
|
|
(simultaneous backhaul and fronthaul BSS). If this option is set, hostapd
|
|
parses the Multi-AP information element in the Association Request frame. If the
|
|
station is a backhaul STA and the BSS is configured as a backhaul BSS,
|
|
hostapd sets up 4-address mode. Since there may be multiple stations connected
|
|
simultaneously, and each of them has a different RA (receiver address), a VLAN
|
|
is created for each backhaul STA and it is automatically added to a bridge.
|
|
This is the same behavior as for WDS, and the relevant option ('bridge' or
|
|
'wds_bridge') applies here as well.
|
|
|
|
If 'multi_ap' is 1 (backhaul BSS only), any station that tries to associate
|
|
without the Multi-AP information element will be denied.
|
|
|
|
If 'multi_ap' is 2 (fronthaul BSS only), any station that tries to associate
|
|
with the Multi-AP information element will be denied. That is also the only
|
|
difference with 'multi_ap' set to 0: in the latter case, the Multi-AP
|
|
information element is simply ignored.
|
|
|
|
In summary, this is the end-to-end behavior for a backhaul BSS (i.e.,
|
|
multi_ap_backhaul_sta=1 in wpa_supplicant on STA, and multi_ap=1 or 3 in
|
|
hostapd on AP). Note that point 1 means that hostapd must not be configured
|
|
with WPS support on the backhaul BSS (multi_ap=1). hostapd does not check for
|
|
that.
|
|
|
|
1. Backhaul BSS beacons do not advertise WPS support (other than that, nothing
|
|
Multi-AP specific).
|
|
2. STA sends Authentication frame (nothing Multi-AP specific).
|
|
3. AP sends Authentication frame (nothing Multi-AP specific).
|
|
4. STA sends Association Request frame with Multi-AP IE.
|
|
5. AP sends Association Response frame with Multi-AP IE.
|
|
6. STA and AP both use 4-address mode for Data frames.
|
|
|
|
|
|
WPS support
|
|
-----------
|
|
|
|
WPS requires more special handling. WPS must only be advertised on fronthaul
|
|
BSSs, not on backhaul BSSs, so WPS should not be enabled on a backhaul-only
|
|
BSS in hostapd.conf. The WPS configuration purely works on the fronthaul BSS.
|
|
When a WPS M1 message has an additional subelement that indicates a request for
|
|
a Multi-AP backhaul link, hostapd must not respond with the normal fronthaul
|
|
BSS credentials; instead, it should respond with the (potentially different)
|
|
backhaul BSS credentials.
|
|
|
|
To support this, hostapd has the 'multi_ap_backhaul_ssid',
|
|
'multi_ap_backhaul_wpa_psk' and 'multi_ap_backhaul_wpa_passphrase' options.
|
|
When these are set on an BSS with WPS, they are used instead of the normal
|
|
credentials when hostapd receives a WPS M1 message with the Multi-AP IE. Only
|
|
WPA2-Personal is supported in the Multi-AP specification, so there is no need
|
|
to specify authentication or encryption options. For the backhaul credentials,
|
|
per-device PSK is not supported.
|
|
|
|
If the BSS is a simultaneous backhaul and fronthaul BSS, there is no need to
|
|
specify the backhaul credentials, since the backhaul and fronthaul credentials
|
|
are identical.
|
|
|
|
To enable the Multi-AP backhaul STA feature when it performs WPS, a new
|
|
parameter has been introduced to the WPS_PBC control interface call. When this
|
|
"multi_ap=1" option is set, it adds the Multi-AP backhaul subelement to the
|
|
Association Request frame and the M1 message. It then configures the new network
|
|
profile with 'multi_ap_backhaul_sta=1'. Note that this means that if the AP does
|
|
not follow the Multi-AP specification, wpa_supplicant will fail to associate.
|
|
|
|
In summary, this is the end-to-end behavior for WPS of a backhaul link (i.e.,
|
|
multi_ap=1 option is given in the wps_pbc call on the STA side, and multi_ap=2
|
|
and multi_ap_backhaul_ssid and either multi_ap_backhaul_wpa_psk or
|
|
multi_ap_backhaul_wpa_passphrase are set to the credentials of a backhaul BSS
|
|
in hostapd on Registrar AP).
|
|
|
|
1. Fronthaul BSS Beacon frames advertise WPS support (nothing Multi-AP
|
|
specific).
|
|
2. Enrollee sends Authentication frame (nothing Multi-AP specific).
|
|
3. AP sends Authentication frame (nothing Multi-AP specific).
|
|
4. Enrollee sends Association Request frame with Multi-AP IE.
|
|
5. AP sends Association Response frame with Multi-AP IE.
|
|
6. Enrollee sends M1 with additional Multi-AP subelement.
|
|
7. AP sends M8 with backhaul instead of fronthaul credentials.
|
|
8. Enrollee sends Deauthentication frame.
|
|
|
|
|
|
References
|
|
----------
|
|
|
|
[1] https://www.wi-fi.org/discover-wi-fi/wi-fi-easymesh
|
|
[2] https://github.com/prplfoundation/prplMesh
|
|
[3] https://www.wi-fi.org/file/multi-ap-specification-v10
|
|
(requires registration)
|