My answer to https://android.stackexchange.com/q/213045/218526 explains the role of Android's mDNS Daemon which is an
init service, started on demand. It provides https://en.wikipedia.org/wiki/Multicast_DNS (host/domain name resolution) and https://en.wikipedia.org/wiki/Zero-configuration_networking#DNS-SD (DNS-Based Service Discovery); both part of Zero Configuration Networking.
How to resolve Android's mDNS hostname to IP address?
Technically it's possible for DNS-SD to use the old unicast DNS to resolve the domain name found in the SRV record (of a broadcasted service) to an IP address. But mostly mDNS and DNS-SD are bound to each other. And this is the case with Android.
Android doesn't expose any setting or API to do only the name resolution (as mDNS Responder or Querier) and not the service discovery. Neither the https://android.stackexchange.com/a/218209/218526 uses mDNS. Android's
mdnsd serves as mDNS broadcaster / responder only to DNS-SD service. You cannot use either of the both independently (as to my knowledge).
But there are a few options:
Run an app on your Android device with a discoverable service (even a dummy), which uses Android's official API (which in turn interacts with
mdnsd underneath) to https://developer.android.com/training/connect-devices-wirelessly/nsd.html#register and https://developer.android.com/training/connect-devices-wirelessly/nsd.html#discover services. So any other host on the local network with mDNS/DNS-SD capability can discover the service. For instance use https://www.systutorials.com/docs/linux/man/1-avahi-browse command on a Linux system. Resolving the service provides the IP address of the Android device.
mdnsd is running, Android device is already broadcasting itself with a https://android.googlesource.com/platform/external/mdnsresponder/+/refs/tags/android-11.0.0_r1/mDNSPosix/mDNSPosix.c#489 :
Android.local. If you don't want to broadcast a service, there's another option.
ADB relies on mDNS to discover devices on local network by registering a service of type
_adb._tcp (see references https://android.googlesource.com/platform/system/core/+/refs/tags/android-11.0.0_r1/adb/daemon/mdns.cpp#45 , https://developer.android.com/things/hardware/raspberrypi.html#step_2_connect_the_hardware and https://source.android.com/devices/automotive/virtualization/tools ). So enable "USB Debugging" in Developer Options and you'll see
mdnsd running on your device.
Or on a rooted device start
mdnsd alone, without starting ADB, by executing
setprop ctl.start mdnsd or simply
In both above cases, when
mdnsd is running, executing https://www.systutorials.com/docs/linux/man/1-avahi-resolve-host-name
Android.local on a Linux PC should return IP address of your Android device.
What if two Android devices have the same hostname?
In case of name conflict, as the name
Android.local is hard-coded, the Android device which joins later automatically renames itself to
Android-3.local and so on (see reference https://android.googlesource.com/platform/external/mdnsresponder/+/refs/tags/android-11.0.0_r1/mDNSCore/DNSCommon.c#940 ). So you can resolve the names accordingly.
Any option to change the mDNS hostname?
Coming to the next problem, what if your OEM/ROM developer removed or modified the
mdnsd service, or if you want to run with a name of your choice, not the hard-coded one?
Build and run your own more configurable mDNS responder like https://github.com/lathiat/avahi on Android device.
Even simpler, patch and rebuild Android's
mdnsd to receive a commandline argument for name, or read from a config file if exists.
No, it doesn't need a rooted device. The patched
mdnsd binary should run fine from
adb shell, or even on a terminal emulator app like Termux. As of this writing, there's only one syscall in https://android.googlesource.com/platform/external/mdnsresponder which the
seccomp blocks for apps. But it can be removed easily. Neither the DAC or MAC block the execution.