Docker container can reach DNS but not resolve hos

Question

I'm having an interesting problem running docker containers: out of the blue, I'm not able to resolve DNS from within the container.

Here's a rundown:

1. Nothing is resolving; apt-get, pip, one-off ping containers, etc. Running docker run -it --dns=8.8.8.8 ubuntu ping www.google.com results in ping: unknown host www.google.com both with and without the --dns flag.

2. I can reach 8.8.8.8 both from inside and outside the containers. docker run -it ubuntu ping 8.8.8.8 works.

3. I've configured the containers to use both 8.8.8.8, 8.8.8.4, and my local network DNS servers (in various permutations) both by editing /etc/sysconfig/docker to add DOCKER_OPTS="--dns 8.8.8.8 --dns 8.8.8.4" and by setting the --dns 8.8.8.8 flag on the containers at runtime. systemctl restart docker was run between each change.

4. ip_forward is enabled. (see here)

5. I've done a full reset as specified here.

I'm running Fedora 21, docker client version 1.5.0.

Any ideas? I'm at a complete loss as to what's preventing docker from accessing the Internet successfully.

Answer 1

You could install tools like dig inside the docker image to alanyse the issue.

use static hosts in your /etc/hosts file te make apt-get run inside the docker image. add these lines to your hosts file:

213.32.5.7 debian.mirrors.ovh.net
141.76.2.4 ftp.de.debian.org 
217.196.149.233 mirror-conova-security.debian.org
212.211.132.250 lobos.debian.org
212.211.132.250 security.debian.org 
5.153.231.4  http.debian.net 
151.101.12.204 cdn-fastly.deb.debian.org
151.101.12.204 security-cdn.debian.org

Answer 2

After all this, a full reboot solved the problem - although, that still doesn't answer what it was.

If anyone knows what the actual cause was, I'm still curious, but for now the problem is gone.

Answer 3

They have fixed the issue in 1.8: https://github.com/docker/docker/issues/13381 Cheers.