Ethernet-briding a MBIM-interface

[Bjørn Mork]

Markus Gothe <nietzsche at lysator.liu.se> writes:

> Hi,
> has anyone worked out how to bridge an MBIM-interface with a regular ethernet interface?
> I think there are 2 ways to do it:
>
> 1) Using ebtables.
> 2) Something like this: http://www.spinics.net/lists/linux-usb/msg96303.html

True bridging of LTE/3G interfaces is simply impossible.  We can only
fake a bridge by routing the IP packets across point-to-point links and
moving the global IP address to the other end of this link.  This is
what the modem does when it presents its local end of the GTP tunnel as
an MBIM IP session to the host.

All LTE/3G modems presenting some sort of ethernet like interface to the
host does something like this.  Bridging these fake ethernet interfaces
might be possible, depending in the implementation.

But I believe it's impossible to bridge the interface created by the
Linux MBIM driver due to the way it forces the usage of the local MAC
address as a destination.  The ethernet frames delievered by this driver
will always go to the local host.  Creating a bridge will not change the
behaviour. You can probably do MAC address rewriting using ebtables to
overcome the problem, but that is unnecessarily complicated IMHO.

There is nothing preventing you from continuing the game played by the
modem over more point-to-point links, including ethernet links.  And you
can make the last one of these look pretty much like an ethernet LAN
with a DHCP server handing out a single address and a default gateway
answering ARP requests.  Making the modem firmware take care of the DHCP
requests will just complicate this (it can be done, but I do not see the
point).  Run your own DHCP server on the ethernet LAN interface instead,
using a config based on the MBIM IP configuraton data.  The rest is
plain IP forwarding between the ethernet and MBIM interface: A default
route out the MBIM interface and a /32 host route out the ethernet
interface, using proxy ARP on the ethernet interface to ensure that it
will answer requests for the (fake) gateway address.  Alternatively you
can configure the gateway address on the ethernet interface, but it will
then become unreachable for the connected host.  Whether that is a
problem or not is another question, but it does make the setup less
transparent.


Bjørn