OpenWRT, ModemManager, Sierra EM7455, IP passthrough

Peter Naulls peter at
Tue Dec 7 14:27:09 UTC 2021

On 12/4/21 1:31 PM, James Thomas wrote:
> Evening all,
> I’ve been racking my brains trying to figure out how I might, in OpenWRT, get 
> modemmanager and netifd to ‘passthrough’ the IP assigned in the bearer when 
> making an mbim connection to a client connecting to eth1.
> My aim, as I’m sure is apparent, is to make OpenWRT transparent to the device 
> connected to eth1 whilst granting that device access to the data connection made 
> through modemmanager, OpenWRT and the Sierra EM7455 card.

Good luck. We do passthrough in out solution, but it was a a lot of work, and I 
don't think it would be entirely appropriate to reveal all the details (some
pieces are proprietary anyway).

What I can say is that ModemManager as it stands, isn't up to it - the OpenWrt
setup needs a new protocol for this type of connection. To that end, we don't
presently use ModemManager in this case; I did start work on such a protocol,
but without fixing up all the other pieces, it's of limited use. Maybe it's
something I can contribute one day.

Also, this would be a bridging setup, vs the default NAT setup in OpenWrt,
but I think that's pretty obvious.


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