If you have server on Linode what ever Ubuntu 14.04 or 16.04 and update the system using apt-get update eventually it would reach a point where it got stuck. Open file

$ nano /etc/gai.conf

and uncomment the following line:

#

For sites which prefer IPv4 connections change the last line to

# precedence ::ffff:0:0/96 100

This allows you to still use IPv6 but IPv4 as the precedence that is generally more stable.