I am working on setting up my own cloud servers using Linux Containers (LXC). These cloud containers allow me to build one image, replicated it, configure it, and have a new server online in seconds.
I was having trouble getting access to the Internet from containers whenever I enabled UFW on the host machine. I tried a thousand different rules, but none worked. After much digging, I found the solution. UFW defaults to dropping forwarded packets. All my containers interact with the Internet via forwarding over a bridge (lxcbr0 in 12.04), so this is a big problem. Basically, with UFW enabled, the LXC containers could not talk to the internet. To solve the problem:
sudo nano /etc/default/ufw
----
# Change:
# DEFAULT_FORWARD_POLICY="DROP"
# to
DEFAULT_FORWARD_POLICY="ACCEPT"
Once I changed the rules, I just reload UFW, and all was well.
ufw reload