Single client not using Pihole even though 1st DNS is Pihole

[mr.pierre]

I posted this over at the reddit and wanted to see is anyone here could help troubleshoot.
I am posting what I did over at reddit:
I have a head scratcher I hope you can help me with. I searched quite a bit and either my google-fu is off today or there is not a well googled incident of this happening.
I have a single local client that has the IP for the Pihole server as the Primary DNS, but this one is getting ads and on the dashboard is not listed as a client. I have the Pihole+OVPN installed, so I decided to create an ovpn profile for this client and go that route, and even though it is connected with a VPN IP, it is still receiving ads and no activity on the dashboard. The OVPN client setup works for cell phone.
All other local clients are having ads blocked and activity on the dashboard.
Thanks in advance!
Edit: For troubleshooting, I assigned a static IP to the adapter with the only DNS server as the Pihole server - it times out.

Edit 2 - My next troubleshooting step will be to do a live USB boot and see what happens there.
Edit 3 - USB boot used the pihole and blocked ads. Same IP assigned from the router.
Edit 4 - Changed DHCP server from the router to the Pihole. Reset networking and received new IP. The only DNS server listed now is the Pihole IP. Still is receiving ads on this one client. Other clients have received new IPs and are listed on the dashboard....scratch scratch, scratch...

[deHakkelaar]

Whats outcome on Pi-hole for:

sudo netstat -nltup | grep "Proto\|:53 "

Whats outcome on a Windows client PC for:

ipconfig /all
*Am only interested in the DNS and DHCP bits.

Or if on a Linux client PC:

cat /etc/resolv.conf

And on a Windows or Linux client:

nslookup pi.hole

And:

nslookup pi.hole <IP_ADDRESS_OF_PIHOLE>

And this bit:

Why should Pi-hole be my only DNS server?

Reduced Ad Blocking Capability The main reason you should Pi-hole as your only DNS server is that you will see increased performance in the blocking of ads. If you have two DNS servers (Pi-hole and something else), your network clients may not always query Pi-hole for name resolution. If a query happens to be answered from a non-Pi-hole DNS server, your block lists will not apply (since that DNS server doesn't know about them). All Queries Need To Go Through Pi-hole First Since other DNS se…

[mr.pierre]

Boom goes the dynamite! The resolv.config had opendns ips.
Thank you!

[deHakkelaar]

You can edit this resolv.conf file manualy but chances are at the next reboot, these changes will be gone again.
Usually some network manager populates this file.
Depending on OS distribution, you need to search how to configure local DNS server(s) for your distro.
Whats in:

cat /etc/issue

Or:

cat /etc/redhat-release

[mr.pierre]

I am running Ubuntu 16.04.3 on this client. Did some googling on how the resolv.conf file gets written to and though the research was brief, there are several answers, which none of them match what is on my computer(files that are to be edited do not exist).
Edit: created the file /etc/resolvconf/resolv.conf.d/base and added nameserver
and rebooted. /etc/resolv.conf now has IP of pihole.
Thanks for your help!

[deHakkelaar]

@mr.pierre

Edit: created the file /etc/resolvconf/resolv.conf.d/base and added nameserver

Had to do something similar just now that required me to look for configured IP in some unknown config file(s) somewhere.
As root, I grepped for that IP address recursively in the "/etc" folder containing most of the configuration files but excluded "gravity" as that would be too many results from the Pi-hole list files.
In bash, that translates to:

sudo grep 10.0.0.2 -R /etc | grep -v gravity

But to be honest, why not configure network to acquire the DNS server automatically via DHCP from your router or Pi-hole DHCP server ?

EDIT: Not sure if below link is still current:
https://help.ubuntu.com/community/DnsAndDhcp