Bail if unable to bind

[IngmarStein]

Context:

I'm running DnsServer on macOS as a background service using launchd. This is comparable to systemd on Linux machines, but it doesn't have the concept of targets. Specifically, one can't start a service after reboot at a time when the network is guaranteed to be available. There used to be a NetworkState setting, but Apple removed it ("This key is no longer implemented as it never acted how most users expected.") because network interfaces can be dynamically added or removed at runtime.

The problem:

When DnsServer fails to bind to a port because the respective interface is not yet available, it falls back to defaults (e.g. binds to 0.0.0.0:53443 instead of 192.168.1.6:443) and continues startup. Of course, this then doesn't match how clients are configured and I have to restore the settings manually.

Solution idea:

Add a setting (maybe a command line parameter) that makes bind errors fatal. If the process exits, launchd will try to restart the service after a delay and this will eventually succeed once the network interface is up.

[ShreyasZare]

Thanks for the post. Does using 0.0.0.0:443 work for your scenario? It would fix the issue as it would work when the network interface comes up later.

Is it possible to add any kind of sleep statement before starting the DNS service? In that case, a sleep for a few seconds would also work.

[IngmarStein]

0.0.0.0:443 does not work in my scenario, because the host is serving multiple services on the same port, so each must bind to a specific IP.

The launchd service could start a wrapper around the DNS service, but finding the right amount of time seems brittle and waiting until a given IP is reachable would be specific to my scenario and hence wouldn't work with the package which is the same for everyone.
systemd and launchd take a similar approach here: if a service isn't healthy, it should exit so that the system can decide when to retry, e.g. with throttle intervals. Docker also adopted that philosophy with container restarts.

[ShreyasZare]

Thanks for the details. It seems that a command line option that you suggested would be a proper solution. Will get that added in the next update.

[IngmarStein]

Oh nice, thank you!

[ShreyasZare]

You're welcome!

[ShreyasZare]

Just for your reference, the command line option --webservice-no-fallback will be implemented.

[IngmarStein]

Will that just affect the webservice or also the DNS services (which are subject to the same issue)?