It would be delightful to use cellular as a backup to Ethernet

[terrillmoore]

At present, cellular takes priority over Ethernet: if cellular is up then Ethernet gets clobbered. (It might still be working, but everything gets routed to the cellular connection).

It would be nice to have a configuration where cellular is used only if Ethernet is not available. Maybe all we have to do is set a clever route metric? But anyway, if Ethernet is attached, we would want that to be preferred over cellular (and maybe not even bring cellular up). States would be:

Ethernet	Cellular	Action
no	no	bring up PPP
no	yes	use PPP
yes	no	do nothing (use Ethernet, and keep PPP out of the modem so we can diagnose)
yes	yes	bring down PPP.

For field service, this would be ideal: if the modem isn't working in practice, plug in an Ethernet hotspot, look around, and then unplug to force a retest.

[jchonig]

We could do this with defaultroute-metric in /etc/ppp/peers/CONF. Except that was introduced in 2.4.8 and mLinux 5.3.31 has 2.4.7.

Checking and 6.0.1 is also at that version.

We may be able to do something with post-processing.