Enable Networking fails DNS configuration with systemd-resolved

[Nicolas Michel]

I'm testing Ubuntu 17.10. The DNS is not well configured when choosing "enable networking".

Steps to reproduce
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- Choose recovery mode from grub
- choose "enable networking"
- Choose "drop to root shell prompt"
- You can test name resolution with ping www.google.com or apt update. It fails.

Explanation
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- /etc/resolv.conf is a symlink to /var/run/systemd/resolve/resolv.conf
- The file is configured to use the local DNS forwarder made available by the systemd-resolved service
- In recovery mode, systemd-resolved service is not started and trying to start it by hand doesn't seem to work

Workaround
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Replace the symlink /etc/resolv.conf by a text file and configure a known working DNS. For example:
nameserver 8.8.8.8