Ubuntu – systemd-resolve does not forward DNS requests to stated DNS server

I experienced similar problems, for example with adding an extra USB wifi dongle. First I disabled dnsmasq in networkmanager as described above and I stopped dnsmasq (service dnsmasq stop)

I noticed that when resolving broke during my VPN connecting, the routing table looks slightly different (output of route command). The name of the Gateway is DD-WRT in the case it does not work and simply 'gateway' when it does work. The output of this did not change:

nmcli device show wlp1s0 | grep IP4.DNS

It kept showing my router IP. A workaround to get it to work for a while is to restart systemd-resolvd:

sudo service systemd-resolved restart

Since dnsmasq is out of the equation, it is either systemd-resolvd that is the cause of the issue, or anything changing the routing table.

So this is the only difference I see:

ubuntu@ubuntu-Lenovo-Yoga-2-11:~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         gateway         0.0.0.0         UG    601    0        0 

which works. And this when it does NOT work:

ubuntu@ubuntu-Lenovo-Yoga-2-11:~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         DD-WRT          0.0.0.0         UG    601    0        0 wlp1s0

And the same name difference on the VPN line :

vpn-dns.name gateway         255.255.255.255 UGH   0      0        0 wlp1s0

Who knows what may influence the routing table? It would be great if we can identify this so a bug report can be filed. I am getting seriously sick and tired of pursuing all these bugs, but I would like to get them fixed so future users and us will be happy :).

[update] It seems stopping systemd-resolved may fix this and not negatively impact other stuff. You can try that and let it know if it does break stuff. I saw when running systemd-resolvd in debug when it broke:

Removing scope on link wlp1s0, protocol llmnr, family AF_INET
Removing scope on link wlp1s0, protocol llmnr, family AF_INET6
Removing scope on link *, protocol dns, family *

To disable:

sudo systemctl disable systemd-resolved.service

I updated the Ubuntu report with suggestions. [/update] Add: Note: the bug report : https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1624317 has a patch for 17.04 for some issues. Please check the bug report and if possible test the patch. Thank you!

[update]

Please check the above mentioned bug report, the issue seems to be resolved for 17.10 and with a simple command DNS leakage can be disabled too.

[/update]

I've found a solution.

First, disable systemd-resolve:

sudo systemctl disable systemd-resolved.service
sudo systemctl stop systemd-resolved

Remove the symlink for resolv.conf and create a file:

rm -rf /etc/resolv.conf
touch /etc/resolv.conf
chmod a+rw /etc/resolv.conf

Now change the configuration file for Network Manager to restore the expected result: vi /etc/NetworkManager/NetworkManager.conf:

[main]
plugins=ifupdown,keyfile
# add this line
dns=default

and leave the rest of the file as is. Restart Network Manager:

sudo /etc/init.d/network-manager restart

and configure your preferred DNS servers. Done!