Home Assistant unable to resolve hosts - works everywhere else

Installation Home Assistant OS
1

I’m using the latest version of hass.io on Ubuntu 16.04 LTS.

Home Assistant is running, however it is apparently unable to resolve any host, either internal (another docker image) or external via DNS.

For example, it is failing to connect to met.no for the weather sensor with the following error:
Cannot connect to host aa015h6buqvih86i1.api.met.no:443 ssl:None [Try again]

I couldn’t find anything related to that error, however I noticed it also wouldn’t connect to the Mosquitto broker via the “core-mosquitto” host. After installing the SSH plugin and connecting via SSH to the HA image, I was able to resolve an connect to both hosts mentioned.

When changing the mqtt broker setting to the internal docker IP address of the core-mosquitto image (172.30.33.0), instead of “core-mosquitto” hostname, it started working, which made me think it HA is somehow unable to resolve the hosts.

Any ideas?

Thanks.