Ubuntu uses the resolvconf program to configure local DNS resolution. The resolvconf package includes a simple database and a runtime for dynamically modifying name server information.

How to disable Ubuntu resolvconf

Typically, the program resolvconf runs through a network interface to push routines such as ifup, ifdown, NetworkManager, dhclient, and pppd, or local name servers such as dnsmasq to update the name server information.

If static IP addresses and DNS records are used on a host, the resolvconf package should be deactivated under Ubuntu, so that the DNS configuration from the dnsmasq daemon is not automatically made, the configuration that has been edited in /etc/resolv.conf and /etc/network/interfaces, will otherwise be overwritten by the resolvconf program.

Disable resolvconf

$ resolvconf --disable-updates

Disable resolvconf from boot level and quit the program.

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

Customize The Network Manager with default DNS.

$ vi /etc/NetworkManager/NetworkManager.conf
..
dns=default
..

Remove the symlink resolv.conf under /etc.

$ rm /etc/resolv.conf

and create a new resolv.conf file with the name servers. in this example, it’s Google Public DNS.

On a local network or ADS, the internal name servers should be used.

$ vi /etc/resolv.conf

nameserver 8.8.8.8
nameserver 8.8.4.4

Delete the resolv.conf file of the systemd configuration program.

$ rm /etc/systemd/resolved.conf

change the configuration.

$ service network-manager restart

The name servers can also be entered in the interface configuration.

$ vi /etc/network/interfaces

auto lo
iface lo inet loopback

auto ens160
iface ens160 inet static
  address 10.10.0.8
  gateway 10.10.0.1
  netmask 255.255.255.0
  network 10.10.0.0
  broadcast 10.10.0.255
  dns-nameservers 8.8.8.8 8.8.4.4
  dns-search my.local

The interface name (ens160) may differ and must correspond to that of the respective host.

The /etc/resolv.conf file should not be missing.

To enable the changed network configuration, it must be read into memory.

$ /etc/init.d/networking restart

Troubleshooting using DNS

Many network issues are due to incorrect DNS or incorrect configuration of resolver. In a home network there is often no internal DNS, whereby the router or the firewall can be used as a nameserver, such as the Linksys or Netgear router. Basically, it should be ensured that the firewall used has a DNS cache, with semi-professional firewalls such as the FortiGate that not every model provide such a cache. For open source-based firewalls most provide a cache through DNS forwarder or dnsmasq.

After open up a command prompt will changes to the nameservers in Windows, the DNS cache should be reset.

ipconfig /flushdns

For Linux, the DNS cache can be reset out from terminal, with one of the following commands, depending on which service is installed.

$ sudo /etc/init.d/nscd restart
$ service nscd restart
$ service nscd reload
$ sudo /etc/init.d/dnsmasq restart
$ service dnsmasq restart
$ rndc reload

In the Mac OS X terminal as root.

$ lookupd -flushcache

If there is no internal DNS in the local network, the name servers of the Internet provider can be used, or may the Cloudflare public DNS.

1.1.1.1 1.0.0.1

Example. of a name server querying its provider on Windows.

C:\> nslookup -type=ns green.ch
Server: dns1.agrinet.ch
Address: 81.221.250.11

Non-authoritative response:
green.ch nameserver = dns2.agrinet.ch
green.ch nameserver = dns1.agrinet.ch

dns1.agrinet.ch internet address = 81.221.250.11

Example. name server lookup query on Linux.

$ host -t ns green.ch
green.ch name server dns1.agrinet.ch.
green.ch name server dns2.agrinet.ch.

host dns1.agrinet.ch & host dns2.agrinet.ch
dns1.agrinet.ch has address 81.221.250.11
dns2.agrinet.ch has address 81.221.252.11
dns2.agrinet.ch has IPv6 address 2a01:2a8:2001:252::11

A ping -n1 resolves addresses to host names with parameters –a and –4 for IPv4 address.

ping -4 -n 1 -a www.google.com

Ping runs for www.google.com with [216.58.201.4]32 bytes of data:
Response from 216.58.201.4: Bytes=32 Time=32ms TTL=50

Ping statistics for 216.58.201.4:
    Packages: Sent = 1, Receive = 1, Lost = 0
    (0% loss),
Approx. times in millisec.:
    Minimum = 32ms, Maximum = 32ms, Mean = 32ms

Show of the current DNS name servers available use systemd resolver.

$ systemd-resolve --status