Flashplugin-nonfree di Debian Lenny

When doing

$sudo apt-get install flashplugin-nonfree

in Lenny, it will return:

Package flashplugin-nonfree is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package flashplugin-nonfree has no installation candidate

That's because Lenny (stable) don't list the package anymore. Detailed policy here says the package go to testing. Below is the summary taken from the site.

The decision made on 2007-12-21 is, starting with Lenny, that
flashplugin-nonfree is to be maintained in "unstable" for users of
"unstable" and "testing", and is to be maintained at "backports.org" for
users of "stable".

I think that's because Debian concern alot on the term GNU/Linux.

Comprehending the policy, the package can be installed via adding backport repository into /etc/apt/source.list. Add the line below into the file:

deb http://www.backports.org/debian lenny-backports main contrib non-free
$ sudo apt-get update
$ sudo apt-get -t lenny-backports install flashplugin-nonfree

Detailed instruction can be found in backports site here.