Preseeding provides a way to set answers to questions asked during the installation process, without having to manually enter the answers while the installation is running. This makes it possible to fully automate most types of installation and even offers some features not available during normal installations.
Preseeding is not required. If you use an empty preseed file, the installer will behave just the same way as in a normal manual installation. Each question you preseed will (if you got it right!) modify the installation in some way from that baseline.
There are three methods that can be used for preseeding: initrd, file and network. Initrd preseeding will work with any installation method and supports preseeding of more things, but it requires the most preparation. File and network preseeding each can be used with different installation methods.
The following table shows which preseeding methods can be used with which installation methods.
|hd-media (including usb-stick)||yes||yes||yes[a]|
but only if you have network access, and set
An important difference between the preseeding methods is the point at which the preconfiguration file is loaded and processed. For initrd preseeding this is right at the start of the installation, before the first question is even asked. Preseeding from the kernel command line happens just after. It is thus possible to override configuration set in the initrd by editing the kernel command line (either in the bootloader configuration or manually at boot time for bootloaders that allow it). For file preseeding this is after the CD or CD image has been loaded. For network preseeding it is only after the network has been configured.
Obviously, any questions that have been processed before the preconfiguration file is loaded cannot be preseeded (this will include questions that are only displayed at medium or low priority, like the first hardware detection run). A not so convenient way to avoid these questions from being asked is to preseed them through the boot parameters, as described in Section B.2.2, “Using boot parameters to preseed questions”.
In order to easily avoid the questions that would normally appear before the preseeding occurs, you can start the installer in “auto” mode. This delays questions that would normally be asked too early for preseeding (i.e. language, country and keyboard selection) until after the network comes up, thus allowing them to be preseeded. It also runs the installation at critical priority, which avoids many unimportant questions. See Section B.2.3, “Auto mode” for details.
Although most questions used by
debian-installer can be preseeded using this method,
there are some notable exceptions. You must (re)partition an entire disk
or use available free space on a disk; it is not possible to use existing
Preseeding makes use of the
debconf framework. This
framework is the preferred mechanism used in Ubuntu to interact with the user
when configuring packages and also forms the heart of
debconf framework questions or dialogs are
based on templates. There are different types of
templates for different types of questions. The actual questions are
“generated” from templates at runtime; multiple questions can
use the same template.
The following types of templates are relevant for preseeding.
string: allows the user to type any value
password: similar to string but the value typed is not displayed
boolean: for yes/no or true/false type of questions
select: allows the user to select one option from a list
multiselect: allows the user to select zero, one or more options from a list
note: used to display a message
debian-installer templates are stored in a readable file
/var/cache/debconf/templates.dat. This file contains all fixed
text and all translations. It can also contain a default value for the
template. The fixed text can include variables that will be replaced at
Another readable file
is used to store the values for variables and the answers given to questions.
A question always refers to the template used to ask it. For obvious
security reasons the values for questions of type “password”
are stored in a separate, non-readable file in the same directory.