Automatically connect plugged in modems

[Hugo Osvaldo Barrera]

Hi,

Thanks for the feedback...

On Mon, 18 Apr 2022, at 14:51, Aleksander Morgado wrote:
> Hey,
>
>>
>> After having set up modemmanager properly, I still need to run `mmcli simple-connect...` each time I plug in the modem. Plus, the ID sometimes changes when re-plugging it, so I have to query the ID (sometimes the modem takes a while to show up too), and THEN manually connect it.
>>
>> Not a huge deal TBH, but still low-key annoying; it gets repetitive rather quickly.
>>
>
> Yes, that is all expected. If you want an "automated" connection
> management, you need something on top of MM to do it. MM just provides
> APIs to control the modem, it doesn't do any logic by itself. No one
> is expected to run --simple-connect every time to connect a modem,
> there should be some other process in charge of doing that for you.
>

That makes sense: do one thing and to it well :)

>> So I wrote a very little helper that listens on the D-Bus interface for new modems, and automatically connects them to the network (sim-pin and profile-id are read from a config file). Maybe this can be of use to others?:
>>
>> https://git.sr.ht/~whynothugo/mmctl
>>
>> Configuration of the network itself (e.g.: DHCP) is handled by the network manager service (systemd-networkd in my case) once wlan0 is up.
>>
>
> Thanks for sharing! I'm sure this could be helpful to others.
>
> One thing I quickly spotted is that you're assuming DHCP can be run in
> the modem, and this is not always the case. Instead, you should look
> at the generated connected bearer object and see what are the
> Ip4Config and Ip6Config property values. The bearer may report
> "static" if the user should configure the IP settings provided by the
> modem in the bearer object, or "dhcp" if the user should run DHCP (or
> SLAAC for IPv6), or "ppp" if the user should run pppd over a specific
> TTY port.
>

I don't exactly assume DHCP; the helper limits itself to connecting the modem
to the cellular network and doesn't handle the IP layer at all. Establishing
the upper-layer network connection is not handled deliberately, since that's
handled by the network management service itself (systemd-networkd in my case).

Presently, I _do_ have wwan0 configured to use DHCP (since my ISP uses this).
However, systemd-networkd is adding native support for grabbing the details
from ModemManager[1], so I'll just rely on that in future.

[1]: https://github.com/systemd/systemd/pull/22315

>> Aside from sharing this very tiny tool, I wanted to also kind of suggest incorporating this functionality into modem-manager itself; adding support for configuration files where one can define sim-pin, profile-id, etc, and have modem-manager automatically connect when it's detected.
>>
>
> This is not a task of ModemManager really. MM just provides a mean to
> configure the modem, it is not in charge of the actual logic to
> perform the connection, or what to do with the connection. Upper
> layers like NetworkManager (in most distros), netifd (in openwrt) or
> Shill (in ChromeOS) are the ones in charge of triggering the
> connection setup and then configuring the net interface as requested.
> All those upper layers already have ways to store connection settings
> like APN, PIN, user/pass... Adding the complexity of managing all that
> by MM automatically is not something I'd personally like to do.
> Setting up a connection, along with default routes and every other
> detail should be done by the network management logic in the system,
> be it a ethernet, a wireless or a modem device.
>

Right!

I currently use `iwd` to tell the wifi adapter "please connect to this AP".
systemd-networkd then handles the IP configuration.

My goal with mmctl is to have an analogous for modems; something that ONLY
connects the modems, but leaves the IP configuration to the network management
service.

> That said, we could have a simple program like yours as an example in
> the ModemManager sources, or even something that could be used in
> setups where no other upper layer is available (e.g. assuming there
> will be one single default route via the modem and such). I have
> several examples like those that I could adapt, and ideally they would
> be using libmm-glib (e.g. in python via GObject Introspection). I have
> absolutely no idea if you can have automatic bindings like the ones we
> have in python or javascript via GIR when using rust, is that a thing?
>

I'm currently using the D-Bus API directly (using zbus, a Rust library for
connecting to D-Bus). Is is possible to generate bindings via GIR, but I'm not
sure if there's any benefit so as to justify the added complexity.

Maybe just adding a link to it in examples is enough. It might be enough for
someone, or at least a sufficient reference.

> Cheers!
>
> -- 
> Aleksander
> https://aleksander.es

-- 
Hugo Osvaldo Barrera