Ubuntu missing package sources

While trying to install Directus in an ubuntu server I got an error from a composer dependency that php5-mcrypt is not installed.

php5-mcrypt : Depends: libmcrypt4 but it is not installable

Running apt-get install php5-mcrypt didn't work.

Running apt-get update first didn't work either.

After some time figuring out what was the problem, I end up noticing that the source.list file was missing.

The next question would be where do I find the official ubuntu repositories? Luckily I found this generator where you can select a ubuntu release and all the repositories you need and it generates a source.list file.

Copying and pasting the new generate content to source.list file, and then running apt-get update, will updates the ubuntu package repositories.

Now apt-get install php5-mcrypt works.