[Solved] apt-get update stuck: Connecting to security.ubuntu.com

The IPv6 setting for the network were ignored, but disabling IPv6 via /etc/sysctl.conf worked:

Add the following lines at the bottom of /etc/sysctl.conf:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

Run sudo sysctl -p to reload the /etc/sysctl.conf settings.

Reference: http://askubuntu.com/questions/620317/apt-get-update-stuck-connecting-to-security-ubuntu-com