Docker container can reach DNS but not resolve hosts

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.

Solution 1

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

Solution 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.

Comments

[Reto Gmür]

I'm having the problem repeatedly, reboot helps but can't be the solution. The problem doesn't just occur with the ubuntu image, but all I've tried.

[DNelson]

Could you link to the fix for posterity?

[Matthew Sprankle]

Yeah, link was a pain to re-find lol. I really need to start tracking the Q&A of my problems for reference.