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Expected Behaviour:
I have 3 pi-hole servers but one of them has weird problem.
It should have around 1.6M of domains on my blocklist like the rest of the server.
This one has onle 1 Domains on Blocklist. Already restarted the server. Executed "pihole -r".
It also taking too long while updating.
Your gravity list is empty. Your debug log shows that Pi-Hole cannot resolve DNS queries from it's own DNS, but can using an external DNS. So, it can't get on the internet for the updates.
*** [ DIAGNOSING ]: Name resolution (IPv4) using a random blocked domain and a known ad-serving domain
[✗] Failed to resolve via localhost (127.0.0.1)
[✗] Failed to resolve via Pi-hole (10.125.166.47)
[✓] doubleclick.com is 172.217.26.78 via a remote, public DNS server (8.8.8.8)
Set the name server for the Pi to an external DNS as follows and gravity should rebuild:
sudo nano /etc/resolv.conf
Change 127.0.0.1 to 1.1.1.1, save and exit
pihole -g
The blocklists to which you subscribe (as shown in your debug log) should result in about 130,000 domains blocked, not 1.6 million.