If you are using IPv6, you might find a hung
$ sudo apt-get update
when it tries to connect to security.ubuntu.com servers.
At the moment there are problems connecting to some of the servers over IPv6. Till it gets fixed, the workaround is to give preference to IPv4 over IPv6.
Edit /etc/gai.conf and change the following line:
# # For sites which prefer IPv4 connections change the last line to # #precedence ::ffff:0:0/96 100
precedence ::ffff:0:0/96 100
Here’s the relevant bug to check the status.