Did not know that, I will have to search the forum unless you know a link that can read about it.
Thank's I may be figuring out another way to solve the problem this weekend, I have a couple of devices that don't respect my DHCP servers wishes with "option6".
I actually know about Openwrt and it's option 6 box and I believe your second example is called tagging.
For over a year now I have been disabling Openwrt's DHCP server and using 2 Pihole"s, each running DHCP and DNS.
I just got a new Samsung S8 phone before Christmas and noticed in my logs that even though I had setup option6 on the DHCP server's to point to my PiHoles it would bypass them and go strait to Google's DNS servers every other device would use what I told it to.
This is a new feature that newer Android devices have started (for me) since Android 8 Oreo and the only cure I have found is to block Google's IPv4 and IPv6 DNS servers, This forces my phone to use my server and the iptable rules help which I might change over the weekend for a more approved method.
More and more devices in the future are going to hard code in their own preferred DNS servers.
I applaud Google for making the internet a bit easier for the masses but I wish they included a way to disable it for fringe cases like me, I have no idea what corporation's do who have strict rules about logging and what sites people are aloud to visit.
At the moment I'm back on my stock Linksys WRT1900AC firmware after I realised my ISP DNS servers weren't working on my own compiled OpenWRT build. I first tried my ISP router and everything worked so I then checked the stock firmware for my router and that seems to be working. As soon as I'm the only one on the internet I'm going to flash a stable 18.06.2 build and see if I can get the ISP DNS servers working, because that me a reason why my firewall ports weren't behaving. If no luck I will try
However, I did notice that with the the box 'Use DNS servers advertised by peer' checked and setting DHCP-option 6 to '6,208.67.220.220,208.67.222.222' actually established connections.
Also in regards to the hard encoded Google DNS servers, I can confirm that under the advanced options there is default options in the DNS server boxes.
Okay I have downloaded the pre-built build of OpenWrt and my ISP DNS servers are working. So far I have set DNS server under the WAN connection and then I tried to add the following to the firewall before I couldn't connect to any website:-
The Android issue does not seem to be the case for me I am on Pie on a Note 9 and I use a PiHole on my network as my only DNS. Even with it hard coded as you say I can see my device logs within my PiHole simply use the dhcp-options 6 choice and all my clients are receiving my PiHole DNS including all those who use our guest network.
I am of course able to bypass my PiHole by adding my own DNS servers under static.