No DNS lookup with Google's DNS servers

Question

Some mornings when my Ubuntu 18.04 wakes from hibernate, it takes around 30 minutes before I can do any DNS lookups.

I already disabled automatic DNS and added Google's DNS servers instead:

But nevertheless, I can ping 8.8.8.8 but not google.com:

Usually after much fruitless enabling/disabling Wifi, and connect and disconnecting Wifi networks, it suddenly springs into action and all is well for the rest of the day.

But here's the mystery: if Ubuntu is using 8.8.8.8 as its DNS lookup, how can it be the case that I can ping the IP but not the URL?

dig

Here's the output of dig:

➜  dig @8.8.8.8 www.google.com

; <<>> DiG 9.11.3-1ubuntu1.11-Ubuntu <<>> @8.8.8.8 www.google.com
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 39427
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;www.google.com.                        IN      A

;; ANSWER SECTION:
www.google.com.         297     IN      A       216.58.206.100

;; Query time: 21 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Thu Aug 20 19:52:43 BST 2020
;; MSG SIZE  rcvd: 59

tcpdump

sudo tcpdump -ni lo port 53

This command showed nothing until about 20 minutes after booting up, at which point it exploded into life with things like:

netstat

netstat -acpn | grep ":53"

This is the output of netstat when DNS lookup isn't working.

And with DNS working, later in the day:

Answer 1

Try

dig @8.8.8.8 www.ubuntu.com

just to rule out that DNS traffic somehow gets mangled.

edit: dig is a tool to query DNS-Servers. By using the the @serverIP syntax you can bypass the systems DNS settings and directly talk to a DNS server of your choice. IMHO this helps a lot in diagnosing if the problem is your machine (dig with @syntax working) or on the network side (dig with @syntax returning the same result)

Answer 2

It might be related to conflicts between the ancient system (ifupdown) and the new system (netplan,cloud-init,etc..). To only correct answer I could find to this modern nonsense was to :

apt remove ifupdown
apt install cloud-init
# comment out settings in /etc/network/interfaces
# complete settings in /etc/netplan/config.yaml

# Apply settings or reboot
netplan apply

The removal of ifupdown is needed to make the DNS resolver function properly.

[nb: my original post here: https://askubuntu.com/a/1255405/45849]

Answer 3

Have you tried disabling systemd-resolved and then checking how the resolution works? I had problems in the past with it.

sudo systemctl disable systemd-resolved
sudo systemctl stop systemd-resolved

Then in the [main] section of your /etc/NetworkManager/NetworkManager.conf

dns=default

Delete symlink to resolv.conf

rm /etc/resolv.conf

And then restart

sudo systemctl restart NetworkManager

Answer 4

I managed to get past this problem by forcing resolved to use a particular DNS server.

Previously, my resolv.conf was linked to /run/systemd/resolve/stub-resolv.conf:

$ l /etc/resolv.conf
lrwxrwxrwx 1 root root 39 Mar 30 10:32 /etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf

This contained the following:

nameserver 127.0.0.53
options edns0

(It also has a lot of comments about how the file is autogenerated and not to mess with it manually. I don't know how it got generated like that.)

So I created a separate resolv.conf file, with:

nameserver 8.8.8.8
options edns0

and replaced my symlink. Then restarted the service with:

sudo systemctl restart systemd-resolved

and suddenly it all sprang into life.

Answer 5

you do not need to dictate DNS address. Since the DNS toggle is switched to automatic you can leave the field empty.