Failed lookup IPv4 DNS

Troubleshooting
,
1

CMK version: 2.1.0p5 cre
OS version: Ubuntu 20.04.3 LTS

Error message: Failed to lookup IPv4 address of [hostname] via DNS: [Errno -5] No address associated with hostname

Since this week all the services Check_MK Discovery and Check_MK HW/SW Inventory of all hosts give Failed to lookup IPv4 address of [hostname] via DNS: [Errno -5] No address associated with hostname. As far as I can find out, nothing changed.

Earlier this week, checked Checkmk server, it was set by netplan using dhcp config. Since we had new modem months ago which forced IPv6 onto our network, we had some issues that modem was interfering with internal DNS promotion.
So to be sure in case Ubuntu got influenced somehow, gave it using netplan a static config with internal DNS ip set. After applying that and disabling DNS caching in Checkmk, all error messages were resolved (pun intented).

Sadly, lasted about a day, all errors returned. )-:

Not sure were to look for now, and since I’m the complete IT department here, hope you might help me out a bit. (-:

Any suggestions?

Edit: removed possible bug part to turn it into seperate thread

2

Using resolvectl command I found out when I disabed DHCP, I forgot to disable IPv6, which was an opening for the modem to ‘secretly’ promote its own external IPv6 DNS servers onto the Checkmk server.

Also with DHCP previously used, the DNS search domain was promoted.
So I had to set that on Ubuntu with static config too, so the hostnames could be resolved using the FQDN on internal DNS server.

Above caused the IPv4 issues … which seems be resolved … again.
Hopefully permanently now … (-;

Netplan config, in case others see use for it when encountering similar issue:
image
See Netplan docs for more info.

Solved. (-:

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