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.