Just use CSF to lockdown your MN. Very easy.
Can you use CSF on Ubuntu? On their homepage they mention supporting RedHat, CentOS, CloudLinux and Fedora.
Also, can anyone give me a rundown of how to get darkcoin masternode working on Ubuntu? I have Ubuntu 32 bit on a VM but when following Chaeplin's guide I got stuck at the local part

I tried the same steps as for the EC2, but at step 9 (install darkcoin) but at : time make -f makefile.unix, something went wrong. Didn't check the response as I tried a few other things. Will try again on fresh install, not sure if I'm doing it right though.