[18.09 backport] Fix: network=host using wrong resolv.conf with systemd-resolved

[thaJeztah]

backport of moby#38579 for 18.09

When running a container in the host's network namespace, the container
gets a copy of the host's resolv.conf (copied to /etc/resolv.conf inside
the container).

The current code always used the default (/etc/resolv.conf) path on the
host, irregardless if systemd-resolved was used or not.

This patch uses the correct file if systemd-resolved was detected
to be running.

[thaJeztah]

ping @tiborvass @euanh PTAL

[prfcttone2]

I

[thaJeztah]

relates to moby#37485

[marcotuna]

This fix did break my configuration.
/etc/docker/daemon.json

{
    "dns": ["172.17.0.1"]
}

Usually inside the container it would write on /etc/resolv.conf
172.17.0.1

Now it overwrites and writes 127.0.0.11 after the update to docker 18.09.5

I am running on Debian 9 Stretch without systemd-resolved

Edit: I should note that this happened because on the affected containers they aren't using the default bridge network, if I check a container using the bridge network it is working fine.

[thaJeztah]

That's the expected behavior; 127.0.0.11 is the embedded DNS, which is used for resolving other containers on the network; other requests will be forwarded to the DNS that you specified

[zmrfzn]

is this also available on windows platform now?

[thaJeztah]

Windows doesn't use systemd-resolvd, so this doesn't really apply there

[zmrfzn]

Is there windows equivalent for network=host ?

docker ee engine :18.06
windows server 2016 standard

[thaJeztah]

No full equivalent afaik

[mcneiljt]

nevermind, my bad!