Pi-Hole (v6) intermittent DNS failure

[CheshireChris]

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Expected Behaviour:

Pi-Hole should resolve sites without stalling or buffering. Happens on Windows10 & 11, Android

Actual Behaviour:

Several times a day name resolution stops and l cannot connect to any site - returns to normal after a few minutes.

Debug Token

Debug Token - 17032025@09:50

System is a 8GB Rpi5with a 500GGB nVMe drive, Latest OS and latest PiHole with Unbound
Router is a Fritz!Box 7530 on latest firmware.

If l bypass the PiHole for DNS and switch to public or my ISPs DNS servers everything works OK. This problem only started after 'upgrading' to v6 - this system is a fresh install of everything

Only error is a maximum number of DNS queries reached

[Bucking_Horn]

That error isn't present in your current debug log.

If that occurs again, let's see how many DNS requests Pi-hole had to handle in the minute before, and whether that would be related to certain domains or clients:

sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc');"

sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT domain, count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc') \
GROUP BY domain ORDER BY 2 DESC LIMIT 10;"

sudo pihole-FTL sqlite3 /etc/pihole/pihole-FTL.db "SELECT client, count(*) FROM queries \
WHERE timestamp > strftime('%s','2025-03-18 15:00:00.142', '-60 seconds', 'utc') \
AND timestamp <= strftime('%s','2025-03-18 15:00:00.142', 'utc') \
GROUP BY client ORDER BY 2 DESC LIMIT 10;"

Substitute 2025-03-18 15:00:00.142 with the timestamp as logged in Pi-hole's error message.

Maximum concurrent warnings are often caused by a DNS loop of sorts, or by upstream DNS server unresponsiveness, less often by a misbehaving client issuing DNS queries excessively.

@CheshireChris

Router is a Fritz!Box 7530 on latest firmware.

It seems there are at least three router type devices in your network, as your debug log shows three different RAs, originating from your Fritzbox, a Draytek and an unknown device.

Does anyone of those use your Pi-hole for DNS server?

Also, your debug log shows that your Fritzbox is advertising its own IPv6 addresses as local DNS servers:

*** [ DIAGNOSING ]: contents of /etc

-rw-r--r-- 1 root root 151 Mar 16 06:52 /etc/resolv.conf
   nameserver 192.168.104.39
   nameserver fd<redacted>89
   nameserver 2a<redacted>89

Fritzbox routers can be configured to not advertise any IPv6 DNS servers at all, see Unresolved ipv6 adress in my top list - #4 by Bucking_Horn.

[CheshireChris]

Thanks for the reply.

The other routers are used to provide Wifi in various rooms of the house. They are configured as WiFi access points; no DHCP, no routing. Exception is the Draytek that is also the DHCP server for my home network; my intention is to move DHCP to the Pi-Hole or the Fritz!Box at some point.

I've disabled IP6 on the Fritz!Box

I tried to upload a log file while the problem was happening but as it (my Pi-Hole server) couldn't resolve any names the upload failed so no Debug Token issued.

Chris