New Ubuntu 16.04 LTS Xenial prefers IPv6 connections. In some networks, this breaks software updates.
You can easily fix this by making Ubuntu prefer IPv4 connections over IPv6.
The Bug
Updating the list of available packages fails in some network conditions. Not getting security updates can cause security issues.
$ sudo apt-get update Get:2 http://mirror.example.com/ubuntu xenial-updates InRelease [94.5 kB] ... 0% [Connecting to security.ubuntu.com (2001:67c:1562::16)]^C
The Fix
Tell computer to prefer old style IPv4 connections over the new IPv6.
$ sudoedit /etc/gai.conf
Uncomment the line
precedence ::ffff:0:0/96 100
It works
Now sudo apt-get update runs normally.
$ sudo apt-get update Hit:1 http://mirror.example.com/ubuntu xenial InRelease ... Reading package lists... Done
Sources
Ubuntu bug #1412943: security.ubuntu.com (2001:67c:1562::XXX) not reachable
Adams 2015: Apt-get can’t connect to ‘security.ubuntu.com’
Ubuntuforums: 0% [Connecting to us.archive.ubuntu.com (2001:67c:1562::14)] – Install not happening
Adminstrivia
Tested on Ubuntu 16.04 LTS x86_64.