Local swarm discovery not working on iOS

[matheus23]

Original user report:

Hello. I diagnosing an issue I am having with iroh 0.29 and LocalSwarmDiscovery. I have an app running 3 instances (on MacOS, iOS-Simulator, and an iPhone). Using endpoint.remote_info_iter() the Mac & iOS Simulator can see each other and the iPhone, however the iPhone does not receive mDNS registration for the other two (running on the Mac). I get this error spammed out in the iPhone log:

swarm_discovery::socket : error sending mDNS: No route to host (os error 65)

Any thoughts on why that may be? All devices are on the local LAN, iPhone via WIFI.

From googling around a bit, this seems to be due to apple restricting sending mDNS on iOS to apps that have an "entitlement" for this: https://developer.apple.com/documentation/bundleresources/entitlements/com.apple.developer.networking.multicast

Practically, this entitlement is going to be hard to get for every app developer. The alternative is to use higher-level APIs for service discovery, so the bonjour APIs directly. Specifically NWBrowser APIs seem to fulfill this role.

That said, them being high-level APIs probably means we're not going to have control over how discovery ends up happening there and can't make it work exactly like the algorithms in swarm-discovery.