Cant set custom DNS for WiFi on Rpi3 B

[z3huti]

Rpi 3 B connected with cable and WiFi at the same time
If I try to set custom DNS with dietpi-config for WiFi I get this

[  OK  ] DietPi-WiFiDB | mkdir -p /etc/wpa_supplicant
[  OK  ] DietPi-WiFiDB | eval > /var/lib/dietpi/dietpi-wifi.db
[ INFO ] DietPi-WiFiDB | Applied WiFi DB slot 0 with SSID "A73GT822KDT27A"
[ INFO ] DietPi-WiFiDB | Applied WiFi DB slot 1 with SSID ""
[ INFO ] DietPi-WiFiDB | Applied WiFi DB slot 2 with SSID ""
[ INFO ] DietPi-WiFiDB | Applied WiFi DB slot 3 with SSID ""
[ INFO ] DietPi-WiFiDB | Applied WiFi DB slot 4 with SSID ""
[  OK  ] DietPi-WiFiDB | chown 0:0 /var/lib/dietpi/dietpi-wifi.db /etc/wpa_supplicant/wpa_supplicant.conf
[  OK  ] DietPi-WiFiDB | chmod 600 /var/lib/dietpi/dietpi-wifi.db /etc/wpa_supplicant/wpa_supplicant.conf
[ SUB1 ] DietPi-Set_hardware > wifimodules (enable)
[ INFO ] DietPi-Set_hardware | Please wait, enabling WiFi modules...
[  OK  ] DietPi-Set_hardware | modprobe cfg80211
[  OK  ] DietPi-Set_hardware | modprobe brcmfmac
[  OK  ] DietPi-Set_hardware | modprobe brcmutil
[ INFO ] DietPi-Set_hardware | Checking for required APT packages: iw wpasupplicant wireless-regdb
[  OK  ] wifimodules (enable) | Completed
[ INFO ] DietPi-Config | Restarting network connections, please wait...
[  OK  ] DietPi-Config | systemctl daemon-reload
[ INFO ] DietPi-Config | systemctl restart ifup@eth0, please wait...
Jul 07 15:42:41 DietPi systemd[1]: Stopping ifup@eth0.service - ifup for eth0...
[  OK  ] DietPi-Config | systemctl restart ifup@eth0
[ INFO ] DietPi-Config | systemctl restart ifup@wlan0, please wait...
Jul 07 15:42:42 DietPi systemd[1]: Starting ifup@wlan0.service - ifup for wlan0...
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: Successfully initialized wpa_supplicant
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: nl80211: kernel reports: Match already configured
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: ctrl_iface exists and seems to be in use - cannot override it
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: Delete '/run/wpa_supplicant/wlan0' manually if it is not used anymore
Jul 07 15:42:42 DietPi wpa_supplicant[1333]: Failed to initialize control interface 'DIR=/run/wpa_supplicant GROUP=netdev'.
                                             You may have another wpa_supplicant process already running or the file was
                                             left by an unclean termination of wpa_supplicant in which case you will need
                                             to manually remove this file before starting wpa_supplicant again.
Job for ifup@wlan0.service failed because the control process exited with error code.
See "systemctl status ifup@wlan0.service" and "journalctl -xeu ifup@wlan0.service" for details.
[FAILED] DietPi-Config | systemctl restart ifup@wlan0
[ INFO ] DietPi-Config | Reloading networking data, please wait...
[  OK  ] DietPi-Config | Network restarted

[MichaIng]

There is two things:

• You have Ethernet and WiFi enabled concurrently. Note that applying a default route for both cannot work, so whether Ethernet or WiFi is used for network access is basically random, and answers can be lost/dropped if done through the other interface. If one interface is needed for LAN access only, better create a dedicated /etc/network/interfaces.d config for it. If you want WiFi as fallback for Ethernet vice versa, this can be done properly via ifplugd or similar: DietPi-WiFi-Monitor | Option: Check for Ethernet, else fallback to WiFi #3254
• However, the actual error seems to be a concurrent wpa_supplicant process. Is wpa_supplicant.service probably active? It would conflict with the one invoked by ifupdown, hence disable it:

  sudo systemctl disable --now wpa_supplicant

  Else, check htop or so for wpa_supplicant processes, and possibly conflicting network daemons which invoke it.