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

This worked for me, courtesy of Zach Adams (https://zach-adams.com/2015/01/apt-get-cant-connect-to-security-ubuntu-fix/):

Turns out this is an issue where connecting over IPv6 on some servers causes them to get stuck at this point. The fix is really simple.

Open /etc/gai.conf

Under the line

# For sites which prefer IPv4 connections change the last line to

Uncomment the following line by removing the #:

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

This will allow you to still use IPv6 but sets IPv4 as the precedence so that apt-get won’t get stuck.

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.

A better approach is to use apt-get as follows instead of editing gai.conf file.

sudo apt-get -o Acquire::ForceIPv4=true update

If you want to make this persistent just add an alias for 'apt-get' in your bash file.