When I try to install/update any plugin from Help -> Install New Software I receive error messages like this:
No repository found at http://download.eclipse.org/releases/galileo.
or
Unable to connect to repository http://pydev.org/updates/site.xml
Invalid argument
This happens with all Eclipse versions.
I use Eclipse on Debian Sid.
I had the same problem, try running eclipse with these options:
-vmargs -Djava.net.preferIPv4Stack=true
It worked for me
(see https://lists.launchpad.net/openjdk/msg03123.html)