mDNS and Android!

[kohanyirobert]

Hey! I found your project while fiddling with an ESP32S3 dev board and my Android phone.

I run an mDNS service on my dev board and all my other devices (Windows laptop, MacBook Air, iPad, etc.) can see the .local address of the device, except my phone (at least some of the times, it's quite maddening really).

So, if use adb and connect to my phone and run ping something.local it doesn't get resolved ... most of the times, sometimes it does get resolved, and I have no idea as to why.

However if run your app on my phone it detects my service available on my local network.

Any idea what kind of problem could cause this? I know it's not exactly related to your project, but I thought you might be interested in this or have some sort of insight on how to debug this.

Thanks a bunch!

[hrzlgnm]

Hey! That's a very valid question. I'll try to answer it as best as I can.

First of all, as far as I know, Android does not have a mDNSResponder running. Which would be responsible for resolving .local addresses via mDNS.

Therefore it doesn't work if you try to ping something.local from an android device.

There is a High Level API provided to resolve mDNS Services from an Android application perspective: https://developer.android.com/develop/connectivity/wifi/use-nsd

An explanation for why it works sometimes: Your WiFi-router may resolve the .local addresses additionally when it provided an address to your device and your Android-Phone uses the DNS-Server of your WiFi-router.

To reason why my app is able to resolve your device: The app utilizes it's own mDNS Resolver on a lower system level via C bindings to the rust library https://github.com/keepsimple1/mdns-sd.

If you need further information, please let me know.

[kohanyirobert]

Hey, thanks for the answer first and foremost.

First of all, as far as I know, Android does not have a mDNSResponder running. Which would be responsible for resolving .local addresses via mDNS.
I'm not an Android expert, but I found this: https://source.android.com/docs/core/ota/modular-system/dns-resolver#mdns-local-resolution. This explicitly states something like adb shell ping6 HOSTNAME.local should work and my own experience is that sometimes this indeed works, but only sometimes (hence the reason me asking around :D).

I've connected to my phone via adb and tested this right now. I have tried to ping my Android phone, my Android TV, my Windows laptop, my ESP32 device and a Xiaomi Smart Hub thingy via their .local names (found by your mDNS browser, great stuff! 👏 ).

Android is my TV, Android-2is my phone. wake-on-esp32s3is my ESP device, ZEPHYRUS is my laptop and that rtlinux thingy is the smart hub.

(Window should also advertise the system via HOSTNAME.local and that's broken as well it seems.)

Some of the works. Since my phone is not rooted I couldn't figure out what service gave back the IPs unfortunately (I haven't tried Wireshark to sniff around yet, because it's seems like a complicated task).

An explanation for why it works sometimes: Your WiFi-router may resolve the .local addresses additionally when it provided an address to your device and your Android-Phone uses the DNS-Server of your WiFi-router.
I'm not sure how to verify this, but as far as know my router doesn't act as a DNS server (it's a TP-link Deco M4R V2, latest firmware). I don't have an option to turn on a DNS server (it might still have one tho'). It allows me to configure alternative DNS servers instead of my ISP's but that's all.

Any tips on how to debug this?

This popped into my mind, but your "might" benefit from an "mDNS tester" or something which uses not your app's builtin DNS resolving functionality but the system's default or something. Just an idea. I don't have any business use-case for it other than to satisfy my own selfish needs :D

[hrzlgnm]

Usually WiFi routers support sniffing traffic on their interfaces, perhaps give it try to see whether those DNS Request are resolved by your WiFi-router.

[hrzlgnm]

There is also an option to sniff the traffic on android without root: https://github.com/emanuele-f/PCAPdroid

[hrzlgnm]

This popped into my mind, but your "might" benefit from an "mDNS tester" or something which uses not your app's builtin DNS resolving functionality but the system's default or something. Just an idea. I don't have any business use-case for it other than to satisfy my own selfish needs :D

I would rate that feature out of scope from a mDNS Browsing perspective. I personally don't have a use case for DNS debugging. There are probably apps out there that cover your use case.

[hrzlgnm]

Some of the works. Since my phone is not rooted I couldn't figure out what service gave back the IPs unfortunately (I haven't tried Wireshark to sniff around yet, because it's seems like a complicated task).

There is support for one shot resolving using mDNS on android since Android 10.0 https://source.android.com/docs/core/ota/modular-system/dns-resolver, this might also explain why it sometimes works.