====== Ubuntu - Networking - Troubleshooting - Could not resolve ======

If you get errors like this when you try to access something on the network, for instance when running "apt-get update":

<code bash>
Err http://security.ubuntu.com trusty-security InRelease

Err http://security.ubuntu.com trusty-security Release.gpg
  Could not resolve 'security.ubuntu.com'

. . .
</code>

To fix, set your nameservers to outside nameservers.  This example uses Google's nameservers.  Open **/etc/network/interfaces** for editing:

<code bash>
sudo vi /etc/network/interfaces
</code>

Update the **dns-nameservers** line as shown:

<file bash /etc/network/interfaces>
. . .
iface eth0 inet6 static
        address 2604:A880:0800:0010:0000:0000:00B2:0001
        netmask 64
        gateway 2604:A880:0800:0010:0000:0000:0000:0001
        autoconf 0
        dns-nameservers 8.8.8.8 8.8.4.4
</file>
        
Refresh your network settings:

<code bash>
sudo ifdown eth0 && sudo ifup eth0
</code>


The expected output is:

Output

<code bash>
RTNETLINK answers: No such process
Waiting for DAD... Done
</code>

Next, create a new firewall rule to force IPv4 when it's available.  Create this new file:

<code bash>
sudo vi /etc/apt/apt.conf.d/99force-ipv4
</code>

Add this single line to the file:

<file bash /etc/apt/apt.conf.d/99force-ipv4>
Acquire::ForceIPv4 "true";
</file>

Save and close the file.  Now you should be able to use APT.