Fix DNS /etc/resolv.conf systemd-resolved


DNS resolution of /etc/resolv.conf and systemd-resolved doesn’t work after upgrade Ubuntu or Linux Mint.

How to Fix DNS resolution using systemd-resolved and /etc/resolv.conf after upgrade

Fix DNS /etc/resolv.conf systemd-resolved

Symptom: Could not resolve hostname. Name or service not known

After upgrading Ubuntu or Linux Mint querying DNS resolution fails. It seems the DNS servers which get by DHCP doesn’t work anymore.

Name resolution

resolvconf is a set of script and hooks managing DNS resolution. The most noticeable experience for the user is that any changes made manually to /etc/resolv.conf will be lost as they will be overwritten the next time resolvconf is triggered. resolvconf uses DHCP client hooks, a Netplan NetworkManager plugin and /etc/network/interfaces to generate a list of nameservers and domain to put in /etc/resolv.conf.

DNS client configuration

Traditionally, the file /etc/resolv.conf was a static configuration file that rarely needed to be changed, also it automatically changed via DHCP client hooks. systemd-resolved handles nameserver configuration, and it should be interacted with through the systemd-resolve command. Netplan configures systemd-resolved to generate a list of nameservers and domains to write in /etc/resolv.conf, which is a symlink:

/etc/resolv.conf -> ../run/systemd/resolve/stub-resolv.conf

Note. Ubuntu Server network interface configuration utility is ifup and it is configured by the file /etc/network/interfaces.

Ubuntu Gnome Desktop or Linux Mint Mate or Cinnamon the network interface configuration is by Netplan configure utility.

DNS Resolution /etc/resolv.conf systemd-resolved

The Netplan NetworkManager daemon attempts to make networking configuration and operation as painless and automatic as possible by managing the primary network connection and other network interfaces.

Netplan NetworkManager is also configured by Edit Connections in the graphical network connection utility. However, for network interfaces configured by DHCP it normally isn’t necessary to change any settings manually.

If you are considering to configure the DNS resolution manually, you must first change the NetworkManager.conf file.

$ sudo vi /etc/NetworkManager/NetworkManager.conf

default: NetworkManager will update /etc/resolv.conf to reflect the nameservers provided by currently active connections.

# Set to default DNS processing mode.

Add dns default key to the main section in the NetworkManager.conf file.

If the key is unspecified, default is used, unless /etc/resolv.conf is a symlink to /run/systemd/resolve/stub-resolv.conf, /run/systemd/resolve/resolv.conf, /lib/systemd/resolv.conf or /usr/lib/systemd/resolv.conf. In that case, systemd-resolved is chosen automatically.

Now restart network management daemon to apply changes.

$ sudo systemctl restart NetworkManager

After an os upgrade, the symlink may need to be recreated. Make symlink to /run/systemd/resolve/stub-resolv.conf

$ sudo ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf

Restart systemd-resolved DNS Network Name Resolution manager.

$ sudo systemctl restart systemd-resolved.service

Verify DNS Resolution using systemd-resolved /etc/resolv.conf

Verify processing and also oprations of Network Name Resolution.

$ journalctl -b -u systemd-resolved

Check DNS status of systemd-resolved Network Name Resolution.

$ resolvectl status

Using resolvectl command to resolve domain names, IPv4 and IPv6 addresses.

$ resolvectl query

Set DNS Resolver completely manually

If you want to configure the DNS resolution manually, you must first change the NetworkManager.conf file.

$ sudo vi /etc/NetworkManager/NetworkManager.conf

Change NetworkManager configuration dns key to none.


Now is a good moment to verify the NetworkManager settings.

$ NetworkManager --print-config

Apply changes of network management daemon.

$ sudo systemctl reload NetworkManager

Unlink resolv.conf symlinked to /run/systemd/resolve/stub-resolv.conf.

$ sudo unlink /etc/resolv.conf

Stop and disable systemd-resolved Network Name Resolution manager.

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

Manually configuring the /etc/resolv.conf file

Create new or edit existing /etc/resolv.conf file.

$ sudo vi /etc/resolv.conf

Add the following lines in to the /etc/resolv.conf file.

options edns0 trust-ad
search localdomain

Note. The number of name servers is limited to 3 nameserver.

Finaly verify DNS name resolution.

$ resolvectl query                     -- link: ens160

-- Information acquired via protocol DNS in 13.0ms.
-- Data is authenticated: no

$ resolvectl status

$ dig

; <<>> DiG 9.16.48-Ubuntu <<>>
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 45042
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

; EDNS: version: 0, flags:; udp: 65494
;                        IN      A

;; ANSWER SECTION:         18466   IN      A

;; Query time: 6 msec
;; WHEN: Tue May 28 12:53:01 CEST 2024
;; MSG SIZE  rcvd: 59

what does systemd-resolved do?

systemd-resolved is a system service that provides network name resolution to local applications. It also implements a caching and validating DNS/DNSSEC stub resolver, as well as an LLMNR and MulticastDNS resolver and responder. Local applications may submit network name resolution requests via three interfaces:

systemd-resolved maintains the /run/systemd/resolve/stub-resolv.conf file for compatibility with traditional Linux programs. This file may be symlinked from /etc/resolv.conf. This file lists the DNS stub (see above) as the only DNS server. It also contains a list of search domains that are in use by systemd-resolved. The list of search domains is always kept up-to-date. Note that /run/systemd/resolve/stub-resolv.conf should not be used directly by applications, but only through a symlink from /etc/resolv.conf.

A static file /usr/lib/systemd/resolv.conf is provided that lists the DNS stub as only DNS server. This file may be symlinked from /etc/resolv.conf in order to connect all local clients that bypass local DNS APIs to systemd-resolved. This file also does not contain any search domains.

more help get also the man pages
$ man systemd-resolved.service
$ man NetworkManager.conf
$ man NetworkManager
$ man resolv.conf
$ man resolvectl

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