Smoothwall Firewall project

Saturday 4 November 2017

DNS resolution in Docker containers with Ubuntu Artful on AWS

This post is solution to a problem I discovered - so I hope others will find it useful.
Spinning up AWS Ubuntu Zesty - 17.04 - images with Docker installed was straight forward with Ansible and Terraform , but then arrived Ubuntu Artful - 17.10 , and the containers spun up could not resolve DNS, regardless of which version of Docker I installed.
After a lot of testing , it appeared to me that the host computer was passing through the wrong DNS server entry into resolv.conf within the container - so it would never work.
The Solution:
With systemd and docker, the preferred way to change a daemon setting is to create a new file in /etc/docker called daemon.json.
In that file add the following to get it use the AWS VPC default DNS resolver - - like so
   "dns": [""]
Restart the docker daemon , and the containers can now resolve DNS. There may be other ways to resolve this issue, but this works perfectly , and uses methods preferred by the docker community.
I hope this helps others who may run into this problem.
Other settings that can be made in that file can be found here. Dockerd settings documentation

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