12.3.2.1. Using a Preseed File
There are several places where the installer can get a preseeding file:
in the initrd used to start the machine; in this case, preseeding happens at the very beginning of the installation, and all questions can be avoided. The file just needs to be called preseed.cfg and stored in the initrd root.
on the boot media (CD or USB key); preseeding then happens as soon as the media is mounted, which means right after the questions about language and keyboard layout. The preseed/file boot parameter can be used to indicate the location of the preseeding file (for instance, /cdrom/preseed.cfg when the installation is done off a CD-ROM, or /hd-media/preseed.cfg in the USB-key case).
from the network; preseeding then only happens after the network is (automatically) configured; the relevant boot parameter is then preseed/url=http://server/preseed.cfg.
At a glance, including the preseeding file in the initrd looks like the most interesting solution; however, it is rarely used in practice, because generating an installer initrd is rather complex. The other two solutions are much more common, especially since boot parameters provide another way to preseed the answers to the first questions of the installation process. The usual way to save the bother of typing these boot parameters by hand at each installation is to save them into the configuration for isolinux (in the CD-ROM case) or syslinux (USB key).
12.3.2.2. Creating a Preseed File
A preseed file is a plain text file, where each line contains the answer to one Debconf question. A line is split across four fields separated by whitespace (spaces or tabs), as in, for instance, d-i mirror/suite string stable:
the first field is the “owner” of the question; “d-i” is used for questions relevant to the installer, but it can also be a package name for questions coming from Debian packages;
the second field is an identifier for the question;
third, the type of question;
the fourth and last field contains the value for the answer; note that it must be separated from the third field with a single space, so that the value can start with whitespace.
The simplest way to write a preseed file is to install a system by hand. Then debconf-get-selections --installer will provide the answers concerning the installer. Answers about other packages can be obtained with debconf-get-selections. However, a cleaner solution is to write the preseed file by hand, starting from an example and the reference documentation: with such an approach, only questions where the default answer needs to be overridden can be preseeded; using the priority=critical boot parameter will instruct Debconf to only ask critical questions, and use the default answer for others.
DOCUMENTATION Installation guide appendix
The installation guide, available online, includes detailed documentation on the use of a preseed file in an appendix. It also includes a detailed and commented sample file, which can serve as a base for local customizations.
→ http://www.debian.org/releases/squeeze/i386/apb.html
→ http://www.debian.org/releases/squeeze/example-preseed.txt
12.3.2.3. Creating a Customized Boot Media
Knowing where to store the preseed file is all very well, but the location isn't everything: one must, one way or another, alter the installation boot media to change the boot parameters and add the preseed file.
12.3.2.3.1. Booting From the Network
When a computer is booted from the network, the server sending the initialization elements also defines the boot parameters. Thus, the change needs to be made in the PXE configuration for the boot server; more specifically, in its /tftpboot/pxelinux.cfg/default configuration file. Setting up network boot is a prerequisite; see the Installation Guide for details.
→ http://www.debian.org/releases/squeeze/i386/ch04s05.html
12.3.2.3.2. Preparing a Bootable USB Key
Once a bootable key has been prepared (see Section 4.1.2, “Booting from a USB Key”), a few extra operations are needed. Assuming the key contents are available under /media/usbdisk/:
copy the preseed file to /media/usbdisk/preseed.cfg
edit /media/usbdisk/syslinux.cfg and add required boot parameters (see example below).
Example 12.2. syslinux.cfg file and preseeding parameters
default vmlinuz
append preseed/file=/hd-media/preseed.cfg locale=en_US console-keymaps-at/keymap=us languagechooser/language-name=English countrychooser/shortlist=US vga=normal initrd=initrd.gz --
12.3.2.3.3. Creating a CD-ROM Image
A USB key is a read-write media, so it was easy for us to add a file there and change a few parameters. In the CD-ROM case, the operation is more complex, since we need to regenerate a full ISO image. This task is handled by debian-cd, but this tool is rather awkward to use: it needs a local mirror, and it requires an understanding of all the options provided by /usr/share/debian-cd/CONF.sh; even then, make must be invoked several times. /usr/share/debian-cd/README is therefore a very recommended read.
Having said that, debian-cd always operates in a similar way: an “image” directory with the exact contents of the CD-ROM is generated, then converted to an ISO file with a tool such as genisoimage, mkisofs or xorriso. The image directory is finalized after debian-cd's make image-trees step. At that point, we insert the preseed file into the approriate directory (usually $TDIR/squeeze/CD1/, $TDIR being one of the parameters defined by the CONF.sh configuration file). The CD-ROM uses isolinux as its bootloader, and its configuration file must be adapted from what debian-cd generated, in order to insert the required boot parameters (the specific file is $TDIR/squeeze/boot1/isolinux/isolinux.cfg). Then the “normal” process can be resumed, and we can go on to generating the ISO image with make image CD=1 (or make images if several CD-ROMs are generated).
12.3.3. Simple-CDD: The All-In-One Solution
Simply using a preseed file is not enough to fulfill all the requirements that may appear for large deployments. Even though it is possible to execute a few scripts at the end of the normal installation process, the selection of the set of packages to install is still not quite flexible (basically, only “tasks” can be selected); more important, this only allows installing official Debian packages, and precludes locally-generated ones.
On the other hand, debian-cd is able to integrate external packages, and debian-installer can be extended by inserting new steps in the installation process. By combining these capabilities, it should be possible to create a customized installer that fulfills our needs; it should even be able to configure some services after unpacking the required packages. Fortunately, this is not a mere hypothesis, since this is exactly what Simple-CDD (in the simple-cdd package) does.
The purpose of Simple-CDD is to allow anyone to easily create a distribution derived from Debian, by selecting a subset of the available packages, preconfiguring them with Debconf, adding specific software, and executing custom scripts at the end of the installation process. This matches the “universal operating system” philosophy, since anyone can adapt it to their own needs.