Feuerfest

The app Termux is an Android terminal emulator and provides an Linux environment. I have it installed on my phone to have many of the various command-line tools ready to use.

However, there is one big problem if you to have working local DNS resolution in your home network: Termux uses the Google DNS server 8.8.8.8 and 8.8.4.4 per-default. As this is a problem for many people, it is regularly discussed on GitHub: https://github.com/termux/termux-app/issues/130

And while I'm not having much expertise in regards to Android development it seems that Termux is not allowed to get the Android system DNS settings and hence can't properly update their resolv.conf whenever the DNS server changes.

Also it seems that some tools use the Android system DNS and others don't. nslookup for example uses the resolv.conf provided by Termux. This just adds to the confusion.

It doesn't matter if you've disabled private DNS in your network settings or not. As such they have little other choice but to ship their own resolv.conf file under /data/data/com.termux/files/usr/etc/resolv.conf with the Google DNS servers, which are also the default setting on Android:

~ $ cat /data/data/com.termux/files/usr/etc/resolv.conf
options timeout:2
options attempts:2
options rotate
nameserver 8.8.8.8
nameserver 8.8.4.4

My workaround

If I install vi (to have a useable editor) and edit the resolv.conf so that it only contains my local DNS server it works.

~ $ cat /data/data/com.termux/files/usr/etc/resolv.conf
options timeout:2
options attempts:2
options rotate
nameserver ip1.ip1.ip1.ip1
#nameserver 8.8.8.8
#nameserver 8.8.4.4

But the big downside is: As soon as my phone leaves my Wifi many things regarding Termux will simply stop working. I then have to change the resolv.conf back. Sure, it done easily and an easy script also comes to mind.. 

A more permanent and better solution?

Termux however has Unbound already installed. I could just add dnsmasq to the mix and configure it to send DNS queries for my local lan domain to my Pi-holes. This way the the resolv.conf can be left untouched.

Something like this should do the trick..

server=/lan/192.168.0.x

Has anyone already done that?

Others have come to different solutions

There is https://www.zenz-solutions.de/personaldnsfilter-wp/ which servers as a DNS filter for Android. And apparently one can somehow hook into the DNS resolution process on Android. Okayyy... Wild.