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====== Ubuntu - Networking - DNS - /etc/resolv.conf ======

<WRAP danger>
**DANGER**:  Do not manually make changes to **/etc/resolv.conf**.

  * Any changes will be overridden, since Ubuntu 12.04.
  * **/etc/resolv.conf** is nowadays maintained by systemd.
  * If DNS resolver changes are needed then do this through systemd.
</WRAP>

----

===== List /etc/resolv.conf =====

<code bash>
ls -al /etc/resolv.conf
</code>

returns:

<code bash>
lrwxrwxrwx 1 root root 32 Jan 10 18:50 /etc/resolv.conf -> /run/systemd/resolve/resolv.conf
</code>

<WRAP info>
**NOTE:**  This shows that **/etc/resolv.conf** is a symlink to **/run/systemd/resolve/resolv.conf**.

It may instead be a symlink to **/run/systemd/resolve/stub-resolv.conf**.  Also ok.

If not then to fix try:

<code bash>
sudo ln -sf /run/systemd/resolve/resolv.conf /etc/resolv.conf
</code>

</WRAP>

----

===== Display /etc/resolv.conf =====

<code bash>
cat /etc/resolv.conf
</code>

returns:

<code bash>
# This file is managed by man:systemd-resolved(8). Do not edit.
#
# This is a dynamic resolv.conf file for connecting local clients directly to
# all known uplink DNS servers. This file lists all configured search domains.
#
# Third party programs must not access this file directly, but only through the
# symlink at /etc/resolv.conf. To manage man:resolv.conf(5) in a different way,
# replace this symlink by a static file or a different symlink.
#
# See man:systemd-resolved.service(8) for details about the supported modes of
# operation for /etc/resolv.conf.

nameserver 192.168.1.1
search localdomain
</code>

<WRAP info>
**NOTE:**  If the **nameserver** is **127.0.0.53** then this is fine too - It is set as default by Ubuntu.

Usually it would only show as 192.168.1.1 or similar if DNS changes had been made in networking settings; such as in /etc/netplan.

</WRAP>


----

===== To Make Resolver Changes =====

Edit the **/etc/resolvconf/resolv.conf.d/head** file.  

<WRAP info>
**NOTE**:  Ignore the scary warning.

**/etc/resolv.conf** is auto-generated; so the warning is there so it will get put in /etc/resolv.conf when /etc/resolv.conf is generated. 
</WRAP>

To the end of the file, add:

<file bash /etc/resolvconf/resolv.conf.d/head>
nameserver <ip_of_nameserver>
</file>

<WRAP info>
**NOTE:**  Resolvconf has a **/etc/resolvconf/resolv.conf.d/** directory that can contain “base”, “head”, “original” and “tail” files.  All in resolv.conf format.
  * **base**: Used when no other data can be found
  * **head**: Used for the header of resolv.conf, can be used to ensure a DNS server is always the first one in the list
  * **original**: Just a backup of your resolv.conf at the time of resolvconf installation
  * **tail**: Any entry in tail is appended at the end of the resulting resolv.conf. In some cases, upgrading from a previous Ubuntu release, will make tail a symlink to original (when we think you manually modified resolv.conf in the past)
</WRAP>

Regenerate **/etc/resolv.conf** so the changes are applied right now:

<code bash>
sudo resolvconf -u
</code>

----

===== Common DNS configuration changes =====

To make DNS configuration changes for some common use cases:

  * For a static configuration using ifup, use **dns-nameservers** and **dns-search** entries in /etc/network/interfaces.
  * For a static configuration using NetworkManager, select the Automatic (DHCP) addresses only Method and enter addresses and domain names in the Additional DNS servers and Additional search domains fields in the Connection Editor.
  * If using netplan, set the DNS within **/etc/netplan/01-network-manager-all.yaml**.
  * To override resolvconf's default configuration, use base, head and tail in /etc/resolvconf/resolv.conf.d/. Only enter addresses and domain names in these files as a temporary measure, since they really belong elsewhere, as described above.

<WRAP warning>
**WARNING:**  Disabling resolvconf is not recommended, but you can replace the symlink /etc/resolv.conf with a regular file.
</WRAP>

----


===== Test =====

setup nslookup or dig command:

<code bash>
dig www.google.com
nslookup www.google.com
</code>