This is just a short blip for people running Docker on CentOS who have encountered problems accessing containers from outside the localhost.
The long and short of it is this command:
Why? Read this answer on StackExchange first. When Docker configures your
iptables rules for network access, it likes to create a
docker0 interface alongside any other network interfaces (like
eth0) that CentOS creates by default.
Therefore, you must enable IP forwarding to allow those packets arriving on your externally-listening interface to be forwarded (or routed) to the docker interface that all of your containers are attached to.
Hopefully this helps somebody else out who may encounter similar problems.