Some additional comment on this, as been there...
Linux networkmanager will assign a higher metric on non-ethernet interfaces
(ideally) to de-preference wireless over wired, but they still both get an
address. In the same subnet, the metric is what determines preference.
You can tweak metrics, but usually depend on the network interface and
system preferences.
This has been a problem for decades, but generally managed by networking
stack setting metric preference on routes. Wired == best, wireless, vpn,
others, less. VPN sometimes add more specific routes to force traffic for
longest prefix match routing preference (openvpn advertises 0.0.0.0/1 and
128.0.0.1/1 to override 0.0.0.0/0), but it comes down to standard
networking preference of prefix-match or metric. This is standard
networking.
Linux uses bridges in dd-wrt boxes to bridge wireless to wired networks,
this is how 99.8% residential routers work, they *can* be separate, but
simply never works in a house. Things like apple's mullticast media (like
everything) implementation mean different subnets break everything apple
(and everyone that sniffs their heels) as they don't work when using
multiple subnets. This is mostly true for android too with anything use
mdns multicast. Same reason apple devices are cantankerous in every
enterprise, nothing of theirs is designed to operate across subnets in
anything but a house properly. This usually requires mdns multicast
reflection and other magic vendors build in just to appease apple.
Why I say apple's don't belong in enterprise - they were never built for
it, but rather as a speak&spell for your grandma. This from someone with a
mac128k and an se/30 still sitting around somewhere.
Short terms, don't rely on multi-homing between subnets if you can
avoid it. It usually never works out well, workstations or servers.
-mb
On Mon, Dec 9, 2019 at 1:48 PM <
kitepilot@kitepilot.com> wrote:
> OK, I'll narrow this down:
> We will have a router serving the same subnet in wireless and wire.
> We'll have a laptop with 2 interfaces, wifi0 and eth0.
> We'll not do any routing configuration beyond a default.
> Finally, this explanation is watered down to dilution because I don't have
> a
> lot of time right now.
>
> In a nutshell, when the protocol is sending a packet, it will look for an
> interface that matches the subnet of the packet, and if it finds it, it
> will
> send a "who has" request over that interface.
>
> Otherwise, the packet is handed to the "gateway" (default routing) which
> is
> not what we are discussing here.
>
> There is no guarantee (that I know of) that the kernel will search the
> network interfaces in any particular order for a matching subnet, and the
> search will stop as the first one is found.
>
> That creates a race condition with the ARP table where a packed may be
> sent
> while the ARP table gets refreshed and moved to the other interface, and
> those packets will die a slow death. That will create random connection
> drops and transmission slowdowns.
>
> I've seen it...
> tctpdump(it), and you'll see it too.
> ET
>
>
>
> Matt Graham writes:
>
> > On 2019-12-07 14:20, kitepilot@kitepilot.com wrote:
> >> Mark Phillips writes:
> >>> dd-wrt router (ASUS RT_N16) would do this. I then
> >>> noticed that the firmware was over 2 years old, so I thought, I should
> >>> upgrade the firmware. Long story short, I may have bricked my router.
> >>> My question is, can I run the wifi on SUBNET (192.168.25.x) and my
> wired
> >>> connection on another SUBNET
> >> You *HAVE* to configure different subnets in each interface or you'll
> >> have a chaos.
> >
> > Not necessarily. I have a bog-standard Netgear consumer grade
> > wireless/wired gateway. It serves up addresses in 192.168.2.0/24 to
> wired
> > and wireless clients. The option for having a separate subnet for
> > (whatever) is called "guest network" in this, consult your man page for
> > dd-wrt for what that's called there.
> >
> > My device is probably doing something funky involving bridging in its
> guts
> > so that it allows 192.168.2.1 to be accessible over wired and wireless
> > interfaces. I think I turned on both wired and wireless networking on
> my
> > laptop at some point, and it didn't break everything. I'll have to wait
> a
> > few hours to try that out again though. This is *not* recommended, but
> it
> > should not be the horrible failure you got in the 2000s if you had 2
> wired
> > Ethernet devices on the same machine in the same subnet. ICBW though.
> >
> > --
> > Crow202 Blog: http://crow202.org/wordpress
> > There is no Darkness in Eternity
> > But only Light too dim for us to see.
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