how to change DNS server permanently on Ubuntu 20.04?

Question

I know there are other posts here asking this, but those suggestions do not help, the configuration keeps changing and I simply cannot get my own changes to persist.

I have the Google 8.8.8.8 and 8.8.4.4 servers in my Network Manager configuration in the GUI, and

systemd-resolv --status returns

Link 2 (enp38s0f1)
      Current Scopes: DNS    
DefaultRoute setting: yes    
       LLMNR setting: yes    
MulticastDNS setting: no     
  DNSOverTLS setting: no     
      DNSSEC setting: no     
    DNSSEC supported: no     
  Current DNS Server: 8.8.8.8
         DNS Servers: 8.8.8.8
                      8.8.4.4
          DNS Domain: ~.   

Nevertheless, dig google.com shows

;; Query time: 0 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Mon Oct 05 11:08:26 EDT 2020
;; MSG SIZE  rcvd: 83

so my default route 192.168.1.1 as DNS. Furthermore, /etc/resolv.conf is a symlink to /run/systemd/resolve/resolv.conf, which in turn reads

# Generated by dhcpcd from enp38s0f1.dhcp, wlp0s20f3.dhcp
# /etc/resolv.conf.head can replace this line
domain fios-router.home
nameserver 192.168.1.1
# /etc/resolv.conf.tail can replace this line

The only enp38s0f1.dhcp file on this system (I've searched /) is /run/dhcpcd/resolv.conf/enp38s0f1.dhcp, which reads

# Generated by dhcpcd from enp38s0f1.dhcp
domain fios-router.home
search fios-router.home
nameserver 192.168.1.1

I have tried to write in other nameservers there, but they do not persist. In the past I've made the changes persistent by making the file immutable with chattr +i, but

lsattr /run/dhcpcd/resolv.conf/enp38s0f1.dhcp

returns

lsattr: Inappropriate ioctl for device While reading flags on /run/dhcpcd/resolv.conf/enp38s0f1.dhcp

so that attribute is not available here. And besides, I shouldn't have to make the file immutable anyway: there should presumably be some way I can control the DNS server..

---

Edit:

In response to comments below:

$ dpkg -l *dnsmasq*
---
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name             Version         Architecture Description
+++-================-===============-============-============================================
un  dnsmasq          <none>          <none>       (no description available)
ii  dnsmasq-base     2.80-1.1ubuntu1 amd64        Small caching DNS proxy and DHCP/TFTP server
un  dnsmasq-base-lua <none>          <none>       (no description available)

Solution 1

I believe I know how to resolve this. dhcpcd has a --nohook flag that instructs it to leave certain parts of your configuration alone. From my dhcpcd(8) man page:

-C, --nohook script
    Don't run this hook script. Matches full name, or prefixed with 2 numbers optionally ending with .sh.

    So to stop dhcpcd from touching your DNS settings you would do:-
    dhcpcd -C resolv.conf eth0 

So I altered my /lib/systemd/system/dhcpcd.service, changing the line

ExecStart=/usr/sbin/dhcpcd

to

ExecStart=/usr/sbin/dhcpcd -C /etc/resolv.conf

Afterwards, the nameservers 8.8.8.8 and 8.8.4.4 in /etc/resolv.conf survive the reboot.

Solution 2

If your current DNS server is still your router (i.e. 192.168.1.1), although you have declared the desired nameservers in /etc/netplan/[network-mager].yaml or via the GUI of NetworkManager, there are at least two solutions to try:

1. You may configure these settings using the already mentioned GUI:

   a) Choose a connection (from the Wired or Wireless tab) and click Edit. b) Click on the IPv4 Settings tab c) Choose 'Automatic (DHCP) addresses only' instead of just 'Automatic (DHCP)'. d) Enter the DNS servers in the “DNS servers” field, separated by spaces (e.g. 208.67.222.222 for OpenDNS). e) Click “Apply.”

Please, note that 'Automatic (DHCP) addresses only' means that the network you are connecting to uses a DHCP server to assign IP addresses but you want to assign DNS servers manually.

2. or, if your DNS settigs are messed up by multiple programs updating it, you can use resolvconf:

sudo apt install resolvconf 
sudo systemctl enable --now resolvconf.service

then, edit /etc/resolvconf/resolv.conf.d/head and insert your desired nameservers as:

nameserver 8.8.8.8 
nameserver 8.8.4.4

Finally, to update /etc/resolv.conf run:

sudo resolvconf -u

Comments

[user535733]

Seems like you are asking the wrong question. A better question would be "How can I troubleshoot why my configuration does not persist across a reboot?"

[Boris Hamanov]

Your setup actually looks ok. However, the /etc/resolv.conf symlink might be wrong. Edit your question and show me cat /run/resolvconf/resolv.conf, and dpkg -l *dnsmasq*.

[grobber]

cat: /run/resolvconf/resolv.conf: No such file or directory

[Boris Hamanov]

systemd-resolv --status correctly shows your desired DNS servers. What/where are you seeing a problem?

[Boris Hamanov]

Is DNS Automatic disabled in your NM GUI?

[grobber]

Re: the problem: as per the original post, dig google.com shows 192.168.1.1 as the server rather than the desired 8.8.8.8. And I don't see a DNS Automatic; I see DNS 8.8.8.8 8.8.4.4

[Boris Hamanov]

Right above DNS 8.8.8.8 8.8.4.4 is a slide switch that says "Automatic".

[Boris Hamanov]

Did you check the DNS Automatic setting that we were discussing?

[grobber]

there is no such switch; please see the screenshot at dropbox.com/s/hzlge170m0ws2u1/…

[Boris Hamanov]

You're looking at the wrong tab. Show me the ipv4 tab.

[grobber]

Ah, I see! Yes, it's set to Automatic.

[Boris Hamanov]

Turn that to DISABLED, undo your dhcpcd edits, reboot, and see what happens. Are the DNS servers listed there also?

[grobber]

I will, but as noted, I already have a working solution (which should be better documented anyway; these forceful DHCP resolv.conf rewrites come up much too often on these forums). Thanks for everything though!

[Boris Hamanov]

Yes, but your working solution may not be the most correct solution, if my way also works for you.

[grobber]

I don't know what "most correct" means, and I certainly prefer to know how to do this in a terminal; that's the more portable solution. So I prefer it to yours, I'm afraid. Again, thank you though.

[grobber]

Let us continue this discussion in chat.

[Thomas]

Rather than changing a file in /lib, a systemd drop-in file should be used to override the ExecStart line. Otherwise your changes might get overwritten by package upgrades.

[Bob Ortiz]

apt install doesn't work if you're without a working DNS. echo "nameserver 1.1.1.1" | sudo tee /etc/resolv.conf changes it temporarily to CloudFlare DNS. I used it before applying this answer as a permanent fix.