DNS failure from logs, resolves from shell OK

Sonarr version: 2.0.0.5344
Mono version: 4.6.2
OS: ubuntu 18.04
Debug logs:
Description of issue:

Hi,
I’ve spun up a new instance of sonarr on ubuntu and restored backup data to it. I have DNS errors in my logs:

20-5-23 17:26:59.8|Warn|XemService|Failed to update Xem series list.
[v2.0.0.5344] System.Net.WebException: DNS Name Resolution Failure: ‘thexem.de

I host a local DNS server in my network that successfully resolves this name from the shell using dig on the server sonarr is hosted on. No issue.

I have a openVPN service running on this server and iptables forcing all outbound connections to the VPN port, except DNS requests to my DNS server (and some other irrelevant things). Again, no issue.

I’m not 100% but I think that sonarr (mono?) is trying to resolve this using its own DNS server, not mine. I’m not sure how to check or test what the software is doing, beyond a command line ping or dig.

Self-reply:

So I was tearing my hair out for a while with DNS and firewall, ended up finding some other issue’s resolution in the firewall but still couldn’t fix this one.

Finally on a hunch, I renamed the MediaCover folder to .old, figured I might as well try it since most of the DNS errors were related to cover art. [the one I gave as an example wasn’t…i don’t think…]. Anyway, after letting Sonarr rebuild that folder and refreshing my library a couple of times, there are no longer any dns errors in the logs.

I can’t explain it. I’ve confirmed only .jpg files live in that folder. No configs. Maybe it was a coincidence. I’m just going to thank the media gods and be on my merry way.

It’s back.

How do I determine if this is a complete failure or this is one out of 100 meaning 99% success when performing this lookup? For now I’ve forked my DNS logs out from the syslog and will be waiting a day to see what shows up related to this domain.