Install the Server

sudo apt-get install tftpd-hpa

The default boot TFTP directory is /var/lib/tftpboot.

Download an Ubuntu Server ISO

wget http://releases.ubuntu.com/10.04/ubuntu-10.04-server-amd64.iso

Mount the ISO, and copy the netboot directory

sudo mount -o loop ubuntu-10.04-server-amd64.iso /mnt
sudo rsync -a --delete /mnt/install/netboot/ /var/lib/tftpboot/
sudo umount /mnt