[OpenWrt-Devel] Multiple Wi-Fi client/AP interfaces on one radio (was: Change OpenWrt Wifi default settings)

Roman Yeryomin leroi.lists at gmail.com
Tue Aug 4 14:14:28 EDT 2015

On 4 August 2015 at 17:58, Joshua Judson Rosen <jrosen at harvestai.com> wrote:
> On 2015-08-04 02:26, David Lang wrote:
>> A given radio can be either an AP or a client, but not both at once.
>> so if you use a radio to connect to another AP, you are making it a client, and
>> in client mode all it can do is connect to that other AP as shows up as the SSID
>> of that other AP.
>> you can do this with one radio, while using the other radio (assuming you have
>> two) to act as an AP for local clients.
> This is not necessarily true: with at least some hardware/drivers, it's
> possible to create multiple virtual interfaces on top of a single radio--
> and separate virtual interfaces can in fact operate in different modes
> (e.g.: one in STA mode, two in AP mode, one in mesh mode...).
> Assuming that your hardware/driver is capable of supporting multiple
> virtual interfaces on top of the single physical radio,
> you can create these interfaces by adding "wifi-iface" stanzas
> in /etc/config/wireless, e.g:
>         config wifi-device 'radio0'
>                 option type 'mac80211'
>                 option channel '11'
>                 option hwmode '11g'
>                 option path 'platform/ar933x_wmac'
>                 option htmode 'HT20'
>                 option country 'US'
>                 option txpower '20'
>         config wifi-iface 'ap1'
>                 option device 'radio0'
>                 option mode 'ap'
>                 option wds '1'
>                 option ssid 'my AP'
>                 option network 'lan'
>         config wifi-iface 'mesh1'
>                 option device 'radio0'
>                 option mode 'mesh'
>                 option mesh_id 'my mesh'
>                 option network 'lan'
> That creates two virtual interfaces using the same physical radio,
> and bridges them together onto the OpenWrt "lan network"
> (which is itself defined in /etc/config/network).
> I believe you could also have an interface with "mode 'sta'", but note
> that it would also need to use the same channel as the other interfaces--
> which means that the external AP to which you connect it would also
> need to use that same channel (11, in the example above).
> That's where having multiple *radios* helps: you can run them on
> different frequencies (either completely different bands [2.4 GHz vs. 5 GHz],
> or on different channels within the same band [e.g. 2.427 GHz = channel 4
> vs. 2.472 GHz = channel 13]) to increase efficiency by multiplexing
> less data over a single radio, or to increase compatibility with other
> APs outside of your control that you might want to connect to.
> Also note that, if you want to bridge a STA interface onto anything else,
> it'll need to be in "WDS" mode _and_ the the AP to which it's connecting
> will also need to be in "WDS" mode (note the "option wds '1'",
> in my example above), because the standard "3-address mode" of Wi-Fi
> isn't bridgeable (and note that WDS "4-address mode" is bridgeable,
> but not standardised across different platforms--so you're probably
> OK so long as all of your equipment is running Linux and using
> the same Wi-Fi driver, but don't expect to bridge a STA interface
> that's connected non-Linux Wi-Fi router).
> At least, I've seen this work with Atheros chips. There are some
> notes in the wiki about how Broadcom chips have their own, different
> solution implemented in hardware; and about how some drivers
> don't support this stuff at all....

Actually 3-address mode bridging is possible with the help of relayd
and it even works quite good.
But there is an issue (also mentioned in a parallel thread): when
client interface cannot connect then AP interface doesn't come up
also. Which may not be a problem unless you want to bridge ethernet
there also.
On the other hand you may not be able to switch the main AP into
4-address mode so relayd is a way out.

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