For Pi-OS/Raspbian, the default network manager is called dhcpcd and is responsible for what ends up in resolv.conf.
If the host is configured to acquire IP details automatically via DHCP, dhcpcd will configure the nameserver(s) in resolv.conf that are advertised via DHCP.
You can see/check those if run below (10.0.0.2 & 10.0.0.4 are both running Pi-hole):
pi@ph5b:~ $ pihole-FTL dhcp-discover
Scanning all your interfaces for DHCP servers
[..]
dns-server: 10.0.0.2
dns-server: 10.0.0.4
If instead have configured static IP details on the host like below:
The domain_name_servers= values will end up in resolv.conf.
In addition to that, if IPv6 is supported and enabled on your router for your LAN , clients normally also receive DNS server(s) via IPv6 router advertisement(RA).
So make sure your router is advertising Pi-hole's IPv6 ULA address for DNS!
Am not that familiar with diagnosing IPv6 RA DNS because I dont have IPv6 enabled on my LAN.
But you could check on clients what DNS server IP's are configured.
Below some hints:
I have as well configured static DNS addresses in /etc/dhcpcd.conf, these are visible too in resolv.conf.
Interestingly I got an error, I am not sure if it matters:
pi@PiHole:~ $ pihole-FTL dhcp-discover
Scanning all your interfaces for DHCP servers
Timeout: 10 seconds
WARN: Could not sendto() in send_dhcp_discover() (/__w/FTL/FTL/src/dhcp-discover.c:233): Network is unreachable
* Received 300 bytes from eth0:192.168.1.150
Offered IP address: 192.168.1.98
Server IP address: 192.168.1.150
Relay-agent IP address: N/A
BOOTP server: (empty)
BOOTP file: (empty)
DHCP options:
Message type: DHCPOFFER (2)
server-identifier: 192.168.1.150
lease-time: 86400 ( 1d )
renewal-time: 43200 ( 12h )
rebinding-time: 75600 ( 21h )
netmask: 255.255.255.0
broadcast: 192.168.1.255
dns-server: 192.168.1.150
domain-name: "lan"
router: 192.168.1.1
--- end of options ---
DHCP packets received on interface wlan0: 0
DHCP packets received on interface lo: 0
DHCP packets received on interface eth0: 1
As far as I understand things right now, my Vodafone Station is pushing the vodafone DNS through RA. I cannot change this.
On my Asus router I have enabled IPv6 passthrough and disabled automatic DNS server connection. Instead I have configured the local Ā“fe80Ā“ IPv6 pihole address. This last one is not being picked up.
I will disable IPv6 for some time and assess if there are strong enough reasons to enable it again like e.g. performance.
Non-technical advice, I recommwnd picking a debian pool out of the four, and blacklist the other three. I assume there are four for extra network use, debian forums probably knows. zI allow pool 0 for the ntp and packages.