My docker container has no internet

Question

I had it working allright but now it stopped. I tried the following commands with no avail:

docker run -dns 8.8.8.8 base ping google.com

docker run base ping google.com

sysctl -w net.ipv4.ip_forward=1 - both on the host and on the container

All I get is unknown host google.com. Docker version 0.7.0

Any ideas?

P.S. ufw disabled as well

Answer 1

First thing to check is run cat /etc/resolv.conf in the docker container. If it has an invalid DNS server, such as nameserver 127.0.x.x, then the container will not be able to resolve the domain names into ip addresses, so ping google.com will fail.

Second thing to check is run cat /etc/resolv.conf on the host machine. Docker basically copies the host's /etc/resolv.conf to the container everytime a container is started. So if the host's /etc/resolv.conf is wrong, then so will the docker container.

If you have found that the host's /etc/resolv.conf is wrong, then you have 2 options:

1. Hardcode the DNS server in daemon.json. This is easy, but not ideal if you expect the DNS server to change.

2. Fix the hosts's /etc/resolv.conf. This is a little trickier, but it is generated dynamically, and you are not hardcoding the DNS server.

---

1. Hardcode DNS server in docker daemon.json

• Edit /etc/docker/daemon.json

  {
      "dns": ["10.1.2.3", "8.8.8.8"]
  }
  

• Restart the docker daemon for those changes to take effect:
  sudo systemctl restart docker

• Now when you run/start a container, docker will populate /etc/resolv.conf with the values from daemon.json.

---

2. Fix the hosts's /etc/resolv.conf

A. Ubuntu 16.04 and earlier

• For Ubuntu 16.04 and earlier, /etc/resolv.conf was dynamically generated by NetworkManager.

• Comment out the line dns=dnsmasq (with a #) in /etc/NetworkManager/NetworkManager.conf

• Restart the NetworkManager to regenerate /etc/resolv.conf :
  sudo systemctl restart network-manager

• Verify on the host: cat /etc/resolv.conf

B. Ubuntu 18.04 and later

• Ubuntu 18.04 changed to use systemd-resolved to generate /etc/resolv.conf. Now by default it uses a local DNS cache 127.0.0.53. That will not work inside a container, so Docker will default to Google's 8.8.8.8 DNS server, which may break for people behind a firewall.

• /etc/resolv.conf is actually a symlink (ls -l /etc/resolv.conf) which points to /run/systemd/resolve/stub-resolv.conf (127.0.0.53) by default in Ubuntu 18.04.

• Just change the symlink to point to /run/systemd/resolve/resolv.conf, which lists the real DNS servers:
  sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf

• Verify on the host: cat /etc/resolv.conf

Now you should have a valid /etc/resolv.conf on the host for docker to copy into the containers.