debootstrap is a tool which will install a Debian base system into a subdirectory of another, already installed system. It doesn't require an installation CD, just access to a Debian repository. It can also be installed and run from another operating system, so, for instance, you can use debootstrap to install Debian onto an unused partition from a running Gentoo install.
Since DebianTesting and DebianUnstable change rapidly, if you are going to attempt to install DebianTesting or DebianUnstable, it is recommended that you download the latest version of debootstrap from the package page and use that. Normally it would not be wise to install a package from unstable onto stable, but in this case it's safe, since debootstrap is really just a shell script and its dependencies are minimal.
To locate a mirror near you, see the list of Debian worldwide mirror sites.
To setup a Lenny system:
# debootstrap lenny ./lenny-chroot http://ftp.us.debian.org/debian
Full process to create a complete Debian installation of sid (unstable):
main # cd / ; mkdir /sid-root main # debootstrap sid /sid-root http://ftp.us.debian.org/debian/ [ ... watch it download the whole system ] main # echo "proc /sid-root/proc proc defaults 0 0" >> /etc/fstab main # mount proc /sid-root/proc -t proc main # echo "sysfs /sid-root/sys sysfs defaults 0 0" >> /etc/fstab main # mount sysfs /sid-root/sys -t sysfs main # cp /etc/hosts /sid-root/etc/hosts main # chroot /sid-root /bin/bash chroot # dselect [ you may use aptitude, install mc and vim ... ] main # echo "8:23:respawn:/usr/sbin/chroot /sid-root " \ "/sbin/getty 38400 tty8" >> /etc/inittab [ define a login tty that will use this system ] main # init q [ reload init ]
Debian GNU/Linux Installation Guide : Installing Debian GNU/Linux from a Unix/Linux System