A strange issue with /etc/hosts & DNS name servers in 18.04 droplets

A few days ago I created two new droplets with Ubuntu 18.04.

Both of these droplets have identical setups. I provisioned each with the same Ansible playbooks.

One of them is dedicated to Wordpress. Yesterday when performing a health check I saw that the REST API was unavailable and the loopback connection wasn’t working. In addition, it reported that a plugin had deactivated wp_version_check()

I know that this wasn’t a plugin issue, as it affected all of the WP sites on this droplet. I confirmed by using troubleshoot mode and by manually disabling all plugins.

The errors was associated with a curl error:

curl: (6) Could not resolve hostname: domain.com

Ping, nslookup, and wget all gave similar errors for any locally hosted domains. Unable to resolve the hostname. All worked as expected for any non-local domain.

This was not the case on the other, identical, droplet. Curl, wget, etc. all worked as expected for local and non-local domains.

After spending the evening trying all the fixes I could find, they were either inapplicable or just didn’t work.

A moment ago I found this answer on Ask Ubuntu https://askubuntu.com/a/512311

/etc/resolv.conf contained the following on both droplets:

nameserver 127.0.0.53
options edns0

On the affected droplet, I changed the DNS name servers according to the Ask Ubuntu post:

nameserver 8.8.8.8
nameserver 8.8.4.4
nameserver 2001:4860:4860::8888
nameserver 2001:4860:4860::8844
options edns0

…and just like that, problem solved.

I’m not too fussed about this as long as it’s working. But it is something that needs a long term fix.

Does anyone have any idea why the DNS name servers would work on one droplet and fail completely for local hostnames on another with identical specs?