[systemd-devel] Bridging a wifi access point?

[Mantas Mikulėnas]

On Mon, Apr 14, 2025, 18:47 Hadmut Danisch <hadmut at danisch.de> wrote:

>
> On 14.04.25 18:36, Mantas Mikulėnas wrote:
> > I don't remember seeing client-mode wlan interfaces working as bridge
> > members on regular Linux kernel. Did that change recently?
>
>
> I actually didn't try it recently with systemd, but I remember
> positively, that I had used it before systemd (years ago), and if I
> remember correctly, when trying the tests, I was able to configure the
> wifi interface as a client with netplan/networkd, and manually put it
> into a bridge.
>
>
> After all: How do Linux based routers like OpenWRT do this? It can't be
> regular routing, if ethernet and wifi do share the same address range.
>


I know some products like Mikrotik or Ubiquiti proprietary Linux-based
firmware implement L2 NAT (i.e. MAC address masquerading) to emulate
bridging of client-mode wlans, and I think I've heard of OpenWrt having a
kernel patch for it, too.

(Same goes for VirtualBox's custom bridge driver, as well as the built-in
bridge feature in Windows: they can bridge client wlan interfaces because
they have code to rewrite MAC addresses in flight.)

Technically, semi-regular routing with lots of Proxy-ARP can also be used
to imitate bridging; not sure if any products do that but I've seen
software for it on github.

Mostly though Linux-based routers are used to host an AP, and there's no
issue with bridging an AP-mode interface.

(Client-mode interfaces also become bridgeable when they're in the
"4-address" mode, which Linux supports natively, but APs are likely to
reject 4addr frames from clients unless manually set up to accept them.
It's often also called "WDS bridge" mode.)
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