I am not a parent, but I am very much into parental control. I don't want to see any porn on my computer. Period. It's a distraction and I want to become a good father and husband one day, so I am training myself by abstaining. (And to not sin)
I used OpenDNS on my Linux machine but it was not enough. Google search did not enforce Safe Search, allowing racy and naughty pictures on my screen. It was a naughty no-no.
That's where Dan's guardian came to rescue. If you follow my instruction, you will be able to enforce VerySafeSearch on you ubuntu machine. Also OpenDNS does not have an extensive blacklist, just domain blacklist, but Dan's Guardian has everything. It can effectively select and ban: IPs, Sites, URLs, Phrases, MIME Types, and File Extensions. Only feature it lacks is the scheduled blocking, but I am fine without it.
FYI, I use K9 Web protection and OpenDNS on my Windows machine.