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.

[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.

[kohanyirobert]

Sorry for the late reply. Life got in the way.

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.
Mine doesn't have anything like that unfortunately.

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.
I'm not familiar with DNS one shot resolving unfortunately. :(

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.
Absolutely, don't worry about it, just mentioned it since I was sitting in from my keyboard :)

There is also an option to sniff the traffic on android without root: https://github.com/emanuele-f/PCAPdroid
I have tried this. Solid app. I'm not sure what to look at. Currently things are complicated by that fact that .local resolving started to work again, whatever I do it just works (no matter I turn on/off private DNS, etc.).

When I turned sniffing on and went to Chrome I saw DNS requests from the app containing my .local domain name intermingled with what looked to me a root nameserver address/TLD/something. Not sure what to make of it. Could you perhaps try to resolve an mDNS address on your local network with this tool and attach your output as reference?

The apps puts my phone into a VPN to work. In the sniffed traffic I saw IP addresses in the 10.x.x.x range, not sure how to map that to my real network (as to what device the DNS query is sent to).

[hrzlgnm]

When I turned sniffing on and went to Chrome I saw DNS requests from the app containing my .local domain name intermingled with what looked to me a root nameserver address/TLD/something. Not sure what to make of it. Could you perhaps try to resolve an mDNS address on your local network with this tool and attach your output as reference?

I've captured some traffic from my pixel6 phone using pcapdroid, what I could observe while trying to ping Android.local using a local terminal app on my phone, and storing that to a pcap file:

Which usually would resolve to my Nvidia Shield when resolving via mDNS. Perhaps pcapdroid is not the right approach for capturing mDNS Traffic, as mDNS isn't something one would be using via a VPN connection.

The pcapdroid app creates a VPN on your device to be able to capture the traffic produced by the Phone without root. So using pcapdroid to capture mDNS-Traffic was perhaps a lead balloon.

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.

I'm not familiar with DNS one shot resolving unfortunately. :(

From my understanding, since Android 10 that happens automatically and transparent according to mdns-local-resolution, perhaps check the Android version of your Android devices. So if you ping a device using <hostname>.local it may be done automatically by the builtin android DNS resolver.

There is also an explicit example for using that with ping6 for example via adb:

Users can use the command adb shell ping6 HOSTNAME.local, where HOSTNAME is the hostname of a target device on the LAN, for example, adb shell ping6 ipad.local. VPN and mobile data connections are excluded from .local resolution.

Hence my comment about pcapdroid not being able to capture this.

[kohanyirobert]

VPN and mobile data connections are excluded from .local resolution.
Good catch.

Same here.

So using pcapdroid to capture mDNS-Traffic was perhaps a lead balloon.
No probs, seems like a useful tool. I saw in the logs a few apps making fishy queries here and there :) Good to know about that.

Neither ping6 nor ping was able to ping just know any of my local devices via .local. Also, my phone stopped exposing itself via its usual Android-2.local domain for reason (when I earlier commented everything just worked).

My Android version is 12, not sure which version number is useful to look at out of the different ones (build number, kernel version, etc.). It's a bummer.

I looked up what one-shot resolving in this context. I kind of get it. What doesn't seem to be right that when resolution of .local works it works consistently, no intermittent factor it, and when it doesn't work it just stops. Super weird, makes things hard to test.

An idea: could I setup my laptop as an AP and sniff traffic from my phone when it tries to resolve things? Wireshark would be the tool for that? My assumption is that when resolution works it does because the local Android DNS resolver does its job and when it doesn't work the tries to query a DNS server getting no answer (just like with using the PCAPdroid app).

[hrzlgnm]

Neither ping6 nor ping was able to ping just know any of my local devices via .local. Also, my phone stopped exposing itself via its usual Android-2.local domain for reason (when I earlier commented everything just worked).

The announcement for host names like "Android.local" is usually done by ADB as soon you enable developer mode on the device and also enable debugging via WiFi so the address to connect to can be automatically discovered. Perhaps if you stop debugging the "Android.local" also disappears.

For example when I start a local android emulator for debugging purposes, I also can see the following in my app when running on a Linux Desktop:

There you can observe "Android" as hostname in the left center box. I'm explicitly omitting .local there to spare some space in the UI.

An idea: could I setup my laptop as an AP and sniff traffic from my phone when it tries to resolve things? Wireshark would be the tool for that? My assumption is that when resolution works it does because the local Android DNS resolver does its job and when it doesn't work the tries to query a DNS server getting no answer (just like with using the PCAPdroid app).

Yes that's possible if you put all the devices to the same AP. But you may need to consider enabling device-to-device communication on that AP, otherwise mDNS may not work, as the devices will not see each other as the AP does not allow it. A Windows Mobile Hotspot for example disables that by default due to security considerations.

[kohanyirobert]

announcement for host names like "Android.local" is usually done by ADB as soon you enable developer mode on the device and also enable debugging via WiFi
Great catch again, I didn't notice this was the trigger, but this is it actually. One small mystery solver for me. Previously I was baffled as to why the phone disappears from the mDNS lookup. It stops advertising its name when I turn off debugging via WiFi and then does it again when I turn it on, so at least that's consistent for now :)

you may need to consider enabling device-to-device communication on that AP, otherwise mDNS may not work, as the devices will not see each other as the AP does not allow it. A Windows Mobile Hotspot for example disables that by default due to security considerations.
I checked the settings, didn't see anything remotely related to this. No client isolation whatsoever. I'll try this one of these days, it requires a bit of a fiddling.