Re: NetworkManager, ModemManager, and pppd

[Dan Williams <dcbw redhat com>]

On Thu, 2019-03-14 at 16:52 +1300, Harry Mander via networkmanager-list 
wrote:
> Hi,
>
> I have a PPP connection brought up by pppd running on a ttyUSB
> connection
> to an LTE modem, which is managed by ModemManager. When pppd is
> started
> after simple-connecting the modem using ModemManager it is able to
> get an
> IP address fine (which can be seen from ifconfig and ip route etc.)
> and I
> can connect to the network using the modem. However, nmcli shows the
> ppp0
> connection as disconnected.

When used in combination with ModemManager, NetworkManager handles PPP
through a pppd plugin. MM itself does not do any IP configuration, that
is left to a connection manager like NetworkManager since IP config
(and routing, and DNS, etc) is pretty complicated and MM shouldn't re-
invent that wheel.

So, typically you run MM and NM at the same time, and NM will see the
modem and allow you to 'nmcli con up <modem connection>' and NM will
handle talking to MM and then the IP configuration.

What you're probably missing is the NetowrkManager "WWAN" plugin. I see
below you have the ADSL, OVS, and WiFi plugins. But you should also
have a WWAN plugin in the same directory.

That's likely a build-time problem. NM must be built with the --with-
modem-manager-1 flag. It should also be built with the --with-pppd flag
so it knows where to put the pppd plugin that it uses.

But that brings up the question, why is your modem using PPP at all?
Typically LTE-capable devices use a pseudo-ethernet device or a netdev
that just does IP ("raw-ip"). PPP is typically not used as it limits
network speeds due to its overhead. I'd expect your device to use
something like QMI, MBIM, or a CDC-NCM port for data instead of PPP.

Dan

> How can I configure NetworkManager to see the PPP connection
> properly? Is
> it possibly related to the pppd plugin since we have libnm-ppp-
> plugin.so
> present in /usr/lib/NetworkManager/1.14.4 but the log does not show
> any
> pppd plugin being loaded:
>
> > NetworkManager[285]: <info>  [1552534718.0779] Loaded device
> > plugin:
> > NMAtmManager (/usr/lib/NetworkManager/1.14.4/libnm-device-plugin-
> > adsl.so)
> > NetworkManager[285]: <info>  [1552534718.1004] Loaded device
> > plugin:
> > NMOvsFactory (/usr/lib/NetworkManager/1.14.4/libnm-device-plugin-
> > ovs.so)
> > NetworkManager[285]: <info>  [1552534718.1181] Loaded device
> > plugin:
> > NMWifiFactory (/usr/lib/NetworkManager/1.14.4/libnm-device-plugin-
> > wifi.so)
>
> This is running on an iMX7 based board using Yocto. I have added
>
> > --enable-ppp=yes
> > --with-pppd-plugin-dir=${libdir}/pppd/2.4.5
>
> to the networkmanager recipe file.
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