Docker build “Could not resolve 'archive.ubuntu.com'” apt-get fails to install anything

Question

I\'ve been trying to run Docker build on various files which previously worked before, which are now no longer working.

As soon as the Docker file included any line

Answer 1

I just wanted to add a late response for anyone coming across this issue from search engines.

Do NOT do this: I used to have an option in /etc/default/docker to set iptables=false. This was because ufw didn't work (everything was opened even though only 3 ports were allowed) so I blindly followed the answer to this question: Uncomplicated Firewall (UFW) is not blocking anything when using Docker and this, which was linked in the comments

I have a very low understanding of iptables rules / nat / routing in general, hence why I might have done something irrational.

Turns out that I've probably misconfigured it and killed DNS resolution inside my containers. When I ran an interactive container terminal: docker run -i -t ubuntu:14.04 /bin/bash

I had these results:

root@6b0d832700db:/# ping google.com
ping: unknown host google.com

root@6b0d832700db:/# cat /etc/resolv.conf
search online.net
nameserver 8.8.8.8
nameserver 8.8.4.4

root@6b0d832700db:/# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=56 time=1.76 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=56 time=1.72 ms

Reverting all of my ufw configuration (before.rules), disabling ufw and removing iptables=false from /etc/default/docker restored the DNS resolution functionality of the containers.

I'm now looking forward to re-enable ufw functionality by following these instructions instead.