CentOS 7.2 machines have a process "dnsmasq" running on port 53. DC/OS installation is not killing the process before installing the binaries and hence installation fails as port 53 (required by mesos-dns) is already in use

Should be fixed by https://github.com/dcos/dcos/pull/507

DC/OS with 1.8 can be configured two way internally
1. It never listens publicly on port 53 (works fine when you don't have to upgrade)
2. It listens publicly on port 53 on the masters only (The rest have their local spartan which binds to a host internal interface).