This guide makes an interesting choice with regards to etcd security, which I'm not sure I'd go with.

etcd stores a load of sensitive cluster information, so unauthorised access to it is a bad thing.

There's an assumption in the guide that you have a "secure network" and therefore don't have to worry about etcd authentication/encryption. The thing is if you have a compromised container (say) and that container, which has an in-cluster IP address can see your etcd server, then it can easily dump the etcd database and get access to the information held in it...

Personally I'd recommend setting up a small CA for etcd and using it's authentication features, there's a good guide to this on the CoreOS site https://coreos.com/etcd/docs/latest/op-guide/security.html