A member of these forums PM'd me to ask how to install Flash player, so I thought I'd make a HowTo out of my answer.
First, remember that there are Free/Open-Source alternatives to Adobe's Flash Player, such as Gnash and Swfdec. Personally, I use the latest version of Gnash 0.8.3. It plays YouTube videos and a lot of other SWF videos, but it's not a complete replacement for Adobe Flash in the way that OpenOffice is a complete replacement for MS Office.
The Swfdec and Gnash instructions below assume that you are running Debian Lenny. There are versions of Swfdec and Gnash for Debian Etch, but they are pretty poor substitutes for Adobe Flash. I encourage you to use Free/Open-Source Software, but if you're running Debian Etch, please do NOT jeopardize the stability of your system by installing the versions of Swfdec or Gnash for Debian Lenny/Sid.
Swfdec instructions
=============
To install Swfdec on Debian Lenny, you would run:
# apt-get remove --purge flashplugin-nonfree
# apt-get install swfdec-mozilla libswfdec-0.6-90 libsidplay1 gstreamer0.10-ffmpeg gstreamer0.10-plugins-base gstreamer0.10-plugins-good gstreamer0.10-plugins-ugly
Restart Iceweasel and enjoy.
Gnash instructions
============
To install the latest version of Gnash (0.8.3) in Debian Lenny, you'll have to choose whether you want to use AGG (Antigrain) or OpenGL as the rendering backend. If in doubt, choose AGG, but if you have hardware-acceleration you may want to use OpenGL.
I'll come back to that point in a minute, but first run:
# apt-get remove --purge flashplugin-nonfree
# apt-get install gstreamer0.10-alsa gstreamer0.10-ffmpeg gstreamer0.10-fluendo-mp3 gstreamer0.10-gnomevfs gstreamer0.10-plugins-base gstreamer0.10-plugins-good
Now, if you want AGG to do the rendering, run:
# apt-get install gnash gnash-common gnash-common gnash-cygnal gnash-tools konqueror-plugin-gnash klash mozilla-plugin-gnash
Alternatively, if you want OpenGL to do the rendering, run:
# apt-get install gnash gnash-common-opengl gnash-common gnash-cygnal gnash-tools konqueror-plugin-gnash klash mozilla-plugin-gnash
In either case, finish by restarting Iceweasel. Enjoy.
Adobe Flash instructions
================
As I mentioned above, Gnash or Swfdec is not as compatible with Adobe's proprietary format as Adobe's own software. It has taken a lot of time, but I have learned not to care when a SWF video doesn't play. In fact, I even block most of them with the Mozilla NoScript plugin. Nonetheless, there are occasions when I want to watch Flash videos and I am tempted to go back to Adobe.
So far the bug hasn't gotten me, but if it gets you, you can install Adobe Flash either the Sid repository or the Etch-Backports repository. I recommend Etch-Backports.
To install from Etch-Backports, add the following line to your /etc/apt/sources.list file:
deb http://www.backports.org/debian etch-backports main contrib non-free
and run:
# wget -O - http://backports.org/debian/archive.key | apt-key add -
# apt-get update
# apt-get -t etch-backports install flashplugin-nonfree
Restart Iceweasel and enjoy.
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