FAI is an automated installation tool to install or deploy Debian GNU/Linux and other distributions on a bunch of different hosts or a Cluster. Its more flexible than other tools like kickstart for Red Hat, autoyast and alice for SuSE or Jumpstart for SUN Solaris.
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Whereas preseeding d-i (Debian-Installer) is one approach of installing Debian systems the advantage of FAI is that you arent limited to Debian systems, youre more flexible in regards to configuration and you can also manage your systems after installing them as well – for example using softupdates for system updates. Find official documentation at the FAI homepage and in the FAI-wiki.