resolv.conf - No DNS servers known

Question

I am using focal Ubuntu 20.04.1 LTS (hosted virtual machine)

Occasionally after a reboot DNS resolution fails (e.g. ping google.com > Resource temporarily unavailable)

To 'fix' this I reboot the machine.

What is the proper fix and why is this occurring sporadically ?

The contents of resolv.conf indicates the file is being managed by systemd-resolved

# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients directly to
# all known uplink DNS servers. This file lists all configured search domains.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.
# No DNS Servers known

example error:

wal@www:~$ ping google.com
ping: google.com: Resource temporarily unavailable

Solution 1

If /etc/netplan is empty then netplan does not configure your network.

Your network is probably configured in old ifup/ifdown style. That means you find the network configuration in /etc/network/interfaces.

In addition there is systemd-resolve with its DNS configuration file /etc/systemd/resolved.conf where you can put your DNS configuration in.

[Resolve]
DNS=8.8.4.4 8.8.8.8 2001:4860:4860::8844 2001:4860:4860::8888
#FallbackDNS=
Domains=example.com
#LLMNR=no
#MulticastDNS=no
#DNSSEC=no
#Cache=yes
#DNSStubListener=yes

You can check your DNS settings with

systemd-resolve --status

You can restart systemd-resolve with

systemctl restart systemd-resolved

Alternatively you can put your settings in /etc/resolvconf/resolv.conf.d/tail which will be added to /run/resolvconf/resolv.conf during boot.

Solution 2

Ubuntu 20.04 uses /etc/netplan to set DNS. Do you have nameservers section in your /etc/netplan/?.yaml file?

yaml-file should have a section similar like this:

            nameservers:
                addresses:
                - 8.8.8.8
                - 2001:4860:4860::8888

resolve.conf is generated from netplan during boot and should should look like this.

nameserver 127.0.0.53
options edns0 trust-ad
search example.com

There is also a /etc/systemd/resolved.conf where static configuration may be strored. You can read more details man resolved.conf

Furthermore you can investigate journalctl -b and search for systemd-resolved messages.

Comments

[wal]

@Nmath question updated. also How do you know that this is a problem with DNS resolution? - ping google.com fails (as above) but ping 8.8.8.8 works

[ubfan1]

Check the /etc/resolv.conf link, it should point to the stub-resolv.conf, but yours points to the resolv.conf, and for some reason, that file does not contain any DNS servers (Some recommend using that to cut out systemd-resolv completely, but then a real DNS server, like the router should be included).

[starkus]

you can read through that documentation to configure your symlink for DNS resolving: wiki.archlinux.org/index.php/Systemd-resolved

[rtaft]

What does sudo systemd-resolve --status say when this happens?

[wal]

/etc/netplan folder is empty (the folder exists however)... ...is generated from netplan during boot and should should look like this <-- it does look like that now (and is working) but as above on some reboots it is blank -

[wal]

this answer has helped (i cannot upvote due to reputation being used for bounty) - i'm able to work around the problem by assigning the DNS field in /etc/systemd/resolved.conf - when i do this i no longer see the entry with search (all entries are nameserver: ... - still unsure why prior resolved.conf was seemingly randomly empty

[Thomas Aichinger]

You probably did not install netplan. I have no idea why. Without netplan you need another way to configure resolv.conf. This is in your case systemd-resolved now. search is probably empty because you did not set the Domains parameter in /etc/systemd/resolved.conf