[SOLVED] resolv.conf seems to always be wrong when building Docker container

Issue

I am attempting to write Dockerfile instructions to use yum and install a few packages. When I run my build command, I will always get an error...

class="lang-sh prettyprint-override">(28, 'Resolving timed out after 5000 milliseconds')

... due to the lack of network access. I have compared my host and container /etc/resolv.conf, and noticed they were different.

Example (Host)

# Generated by expressvpn
search expressvpn
nameserver 10.53.0.1

Example (Container)

search expressvpn
nameserver 10.1.2.3
nameserver 8.8.8.8

I attempted copying the host /etc/resolv.conf and overwriting the container /etc/resolv.conf as follows

$ echo "# Generated by expressvpn
> search expressvpn
> nameserver 10.53.0.1" > "/etc/resolv.conf"

Then immediately gained network access again and was able to use yum. However, if I try reading the Dockerfile with a build command, it does not seem to work anymore. How do I make docker use the host resolv.conf on build or resolve this issue correctly? It seemed to not have an issue with my VPN before. Is that the issue now?

Solution

I was able to at least get my docker to build the containers by taking the same naming conventions used in the Docker Daemon CLI options and applying them to a /etc/docker/daemon.json manually, then restarting the Docker Daemon.

1. Read the host /etc/resolv.config (Yours will likely be different)

$ cat /etc/resolv.config
# Generated by expressvpn
search expressvpn
nameserver 10.53.0.1

2. Make a new, or use the /etc/docker/daemon.json (I had to use Super User to write the file)

$ sudo touch /etc/docker/daemon.json

3. Use the Daemon file to manually set the Virtual Network to the host /etc/resolv.conf output as described in #1 (Again yours is likely to be different). You can find the different options here just use the CLI options as keys and arrays with strings as values.

{
    "dns": [
        "10.53.0.1"
    ],
    "dns-search": [
        "expressvpn"
    ]
}

4. Hard stop all docker processing

$ sudo ps axf | grep docker | grep -v grep | awk '{print "kill -9 " $1}' | sudo sh 

5. Restart Docker Daemon

$ sudo dockerd

This is not the most elegant solution, but I was able to at least get my Docker to build the Container and continue on with my work.

Answered By - VolksRat71

Answer Checked By - Pedro (WPSolving Volunteer)