To run Hadoop it is necessary (at least that is what I know) to be able to ssh to the localhost without a passphrase. The Hadoop quick start guide has simple instructions on how to do that. The first time I installed and ran Hadoop was for a class project and I worked in a pc having Ubuntu 8.04. I could follow all the instructions of that guide without any problem. But this time when I wanted to run it on my laptop having Fedora 10, the passphraseless ssh was not working. I searched the internet for a solution and I found an excellent one here, thanks to Daniel Owen for the article. The trick is in the permissions. The authorized_keys file must have the permission 644. Anything other than that does not work. The known_host file created by ssh client also has the same permission.