Docker error IPv4 forwarding is disabled

Another common error is that the docker daemon cannot connect to the outside world to download anything during build time. This can be corrected in a number of ways, but I have done it thusly.

The error is commonly encountered as you are trying to build a docker image, the warning "[Warning] IPv4 forwarding is disabled. Networking will not work." tells you that you need to enable IPv4 forwarding.

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Step 10 : RUN set -x && pip install -U pip setuptools && pip install -r /tmp/requirements/dev.txt ...
 ---> [Warning] IPv4 forwarding is disabled. Networking will not work.
 ---> Running in b4b9fb134518
+ pip install -U pip setuptools
Retrying (Retry(total=4, connect=None, read=None, redirect=None)) after connection
broken by NewConnectionError(Failed to establish a new connection: [Errno -2]
Name or service not known,): /simple/pip/

First setup the docker daemon config file to have a DNS server or two available to make requests of:

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[mbacchi@centos7 ~]$ cat /etc/docker/daemon.json
{
    "dns": ["10.1.1.254","8.8.8.8","10.115.1.220"],
    "live-restore": true
}

You can see above that I have multiple local(intranet vpn) nameservers as well as the Google DNS server 8.8.8.8. This would require a docker daemon restart to be enabled(systemctl restart docker), but we will wait for the next step to be completed before performing that daemon restart.

Now we must enable forwarding between interfaces in the kernel. This will be done using the sysctl parameter net.ipv4.ip_forward. It can be set once from the command line but upon system restart will not be retained, so I set it in the docker sysctl file:

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[mbacchi@centos7 ~]$ cat  /usr/lib/sysctl.d/99-docker.conf
fs.may_detach_mounts=1
net.ipv4.ip_forward=1

This will then set the net.ipv4.ip_forward variable to true every time the docker daemon is started or restarted.

Now we can perform that restart and make sure both of these changes are enabled.

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[mbacchi@centos7 ~]$ sudo systemctl restart docker
[mbacchi@centos7 ~]$