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At this point, we should have a solid understanding of the different types of 
packages available and how to manage them. However, it's sometimes the case that 
we need to run software on our server for which there is no package available within 
the standard repositories. Therefore, in the next section, we'll learn how to add 
additional repositories from which to install software.
Managing package repositories
Often, the repositories that 
come pre-installed with Ubuntu will suffice for the 
majority of the Debian packages you'll install via APT. Every now and then, though, 
you may need to install an additional repository in order to take advantage of 
software not normally provided by Ubuntu, or versions of packages newer than 
what you would normally have available. Adding additional repositories allows you 
to subscribe to additional sources of software and install packages from them the 
same as you would from any other source.
Adding additional repositories should be considered a last resort, however. When 
you install an additional repository, you're effectively trusting the author of that 
repository with your organization's server. Although I haven't ever seen this happen 
first-hand, it's theoretically possible for authors of software to include back doors 
or malware in software packages (intentionally or unintentionally), and then make 
them available for others via a software repository. Therefore, you should only add 
repositories from sources that you have reason to trust.
In addition, it sometimes happens that a maintainer of a repository simply gives 
up on it and disappears. This I have seen happen first-hand. In this situation, the 
repository may go offline (which would show errors during 
apt
 transactions, 
indicating that it's not able to connect to the repository), or worse, the repository 
stays online, but security updates are never made available, causing your server to 
become vulnerable to attack. Sometimes, you just don't have a way around it. You 
need a specific application and Ubuntu doesn't offer it by default. Your only
 option 
may be to compile an application from source or add a repository. The decision 
is yours, but just keep security in mind whenever possible. When in doubt, avoid 
adding a repository unless it's the only way to obtain what you're looking for.