Docker containers running in Ubuntu 18 can’t resolve DNS as they are trying to use the default Google DNS server 8.8.8.8 as their DNS server.

Since Ubuntu 18, the file /etc/resolv.conf is a symlink to /run/systemd/resolve/stub-resolv.conf (systemd-resolved is set as default on Ubuntu 18) and its content always contains the local ip 127.0.0.53:

$ cat /etc/resolv.conf
nameserver 127.0.0.53
search somedomain.name

By default, Docker reads /etc/resolv.conf and list all non-localhost IP. If the list is empty, then it uses google DNS servers 8.8.8.8 and 8.8.4.4:

$ docker run -it --rm ubuntu:18.10 cat /etc/resolv.conf
search somedomain.name

nameserver 8.8.8.8
nameserver 8.8.4.4

That means your containers do not use your host DNS servers, thus, your internal URLs are not accessible inside your Docker containers.

Forcing the host network

One way is to force the container to use the same network as the host by using the parameter --network host:

$ docker run --network host --rm ubuntu:18.10 cat /etc/resolv.conf
nameserver 127.0.0.53
search somedomain.name

Setting the DNS server

Another way is to set your DNS server when running your containers

# First find what your DNS server is
$ nmcli dev show | grep 'IP4.DNS'
IP4.DNS[1]:                             192.168.4.1
$ docker run --dns 192.168.4.1 --rm ubuntu:18.10 cat /etc/resolv.conf
search somedomain.name
nameserver 192.168.4.1
nameserver 8.8.8.8

If you want to fix permanently without the need to set the DNS each time, you need to:

  • create/edit the file /etc/docker/daemon.json and add your DNS server:
{
  "dns": ["192.168.4.1", "8.8.8.8"]
}
  • Restart your docker service:
sudo systemctl restart docker

Some sources: