This page states, that the default user is "core" for CoreOS respectively "centos" for CentOS. However that's not true at least if you are using Terraform to install DC/OS on Azure. Terraform sets the admin user to "dcos_admin". I don't know what the default user is using some other method to install DC/OS, but I feel like this needs to be clarified in the docs. It took me hours to realize I was using a wrong username to ssh into my cluster.