[systemd-devel] Waiting for network routes to appear

[Mantas Mikulėnas]

I think it's better to use a custom tool. RequiresMountsFor relies on
knowing the exact mount points from the beginning and just waiting for them
to be done – but that might not exactly match networking, where you might
have two /24 routes today and a /23 tomorrow, so you can't exactly
predefine that in units.

A custom tool could more easily express logic like waiting for any route
matching the specified address, or any non-/0 route, or any route through
the specified interfaces.

(If you're in this situation, you probably also have several routing
tables, and perhaps a ton of routes. I would not want all that memory usage
in pid 1 – already went through this for systemd-networkd, and then once
more for nss_myhostname...)

On Wed, Jun 15, 2022, 14:32 Kevin P. Fleming <kevin at km6g.us> wrote:

> I've got a number of systems that use BIRD to learn the routes
> available on their networks, and as a result some services on those
> systems attempt to start up before the routes have been learned. If
> those services attempt to make network connections (even just DNS
> queries), they will fail, and that's unpleasant.
>
> I can't use existing systemd facilities to make these services wait,
> because there's no mechanism available for BIRD to indicate that any
> specific route has been learned, or a way to configure a service to
> wait for a specific route.
>
> I'm considering just writing a smallish Python program which will
> accept (via configuration) a list of routes, and will listen to
> netlink to wait for all of those routes to appear. I'd then make my
> services dependent on this service reporting success. However, since
> networkd already listens to netlink, it would certainly be possible
> for it to provide this facility in some way.
>
> If you'll pardon the analogy, I'm thinking of something like
> RequiresMountsFor=, which makes service startup wait until mount units
> have succeeded. Of course following this analogy we'd end up creating
> the concept of a 'route unit', and I'm not sure that's really the
> right thing to do here.
>
> Is it worth trying to design some way for networkd to provide this
> facility? if not, I'll just continue down the road of doing this
> outside of systemd proper.
>
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