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41.

To set a value to be used inside &d-i;, just pass <userinput><replaceable>path/to/variable</replaceable>=<replaceable>value</replaceable></userinput> for any of the preseed variables listed in the examples in this appendix. If a value is to be used to configure packages for the target system, you will need to prepend the <firstterm>owner</firstterm><footnote> <para> The owner of a debconf variable (or template) is normally the name of the package that contains the corresponding debconf template. For variables used in the installer itself the owner is <quote>d-i</quote>. Templates and variables can have more than one owner which helps to determine whether they can be removed from the debconf database if the package is purged. </para> </footnote> of the variable as in <userinput><replaceable>owner</replaceable>:<replaceable>path/to/variable</replaceable>=<replaceable>value</replaceable></userinput>. If you don't specify the owner, the value for the variable will not be copied to the debconf database in the target system and thus remain unused during the configuration of the relevant package.

49.

There are several features of Debian Installer that combine to allow fairly simple command lines at the boot prompt to result in arbitrarily complex customized automatic installs. To illustrate this, here are some examples that can be used at the boot prompt: <informalexample><screen>
auto url=autoserver
</screen></informalexample> This relies on there being a DHCP server that will get the machine to the point where <literal>autoserver</literal> can be resolved by DNS, perhaps after adding the local domain if that was provided by DHCP. If this was done at a site where the domain is <literal>example.com</literal>, and they have a reasonably sane DHCP setup, it would result in the preseed file being retrieved from <literal>http://autoserver.example.com/d-i/&releasename;/./preseed.cfg</literal>.

50.

The last part of that url (<literal>d-i/&releasename;/./preseed.cfg</literal>) is taken from <literal>auto-install/defaultroot</literal>. By default this includes the directory <literal>&releasename;</literal> to allow future versions to specify their own codename and let people migrate forwards in a controlled manner. The <literal>/./</literal> bit is used to indicate a root, relative to which subsequent paths can be anchored (for use in preseed/include and preseed/run). This allows files to be specified either as full URLs, paths starting with / that are thus anchored, or even paths relative to the location where the last preseed file was found. This can be used to construct more portable scripts where an entire hierarchy of scripts can be moved to a new location without breaking it, for example copying the files onto a USB stick when they started out on a web server. In this example, if the preseed file sets <literal>preseed/run</literal> to <literal>/scripts/late_command.sh</literal> then the file will be fetched from <literal>http://autoserver.example.com/d-i/&releasename;/./scripts/late_command.sh</literal>.

51.

If there is no local DHCP or DNS infrastructure, or if you do not want to use the default path to <filename>preseed.cfg</filename>, you can still use an explicit url, and if you don't use the <literal>/./</literal> element it will be anchored to the start of the path (i.e. the third <literal>/</literal> in the URL). Here is an example that requires minimal support from the local network infrastructure: <informalexample><screen>
auto url=<replaceable>http://192.168.1.2/path/to/mypreseed.file</replaceable>
</screen></informalexample> The way this works is that:

55.

In addition to specifying the url, you can also specify settings that do not directly affect the behavior of &d-i; itself, but can be passed through to scripts specified using <literal>preseed/run</literal> in the loaded preseed file. At present, the only example of this is <literal>auto-install/classes</literal>, which has an alias <literal>classes</literal>. This can be used thus: <informalexample><screen>
auto url=<replaceable>example.com</replaceable> classes=<replaceable>class_A;class_B</replaceable>
</screen></informalexample> The classes could for example denote the type of system to be installed, or the localization to be used.

56.

It is of course possible to extend this concept, and if you do, it is reasonable to use the auto-install namespace for this. So one might have something like <literal>auto-install/style</literal> which is then used in your scripts. If you feel the need to do this, please mention it on the <email>debian-boot@lists.debian.org</email> mailing list so that we can avoid namespace conflicts, and perhaps add an alias for the parameter for you.

57.

The <literal>auto</literal> boot label is not yet defined on all architectures. The same effect may be achieved by simply adding the two parameters <literal>auto=true priority=critical</literal> to the kernel command line. The <literal>auto</literal> parameter is an alias for <literal>auto-install/enable</literal> and controls the delay of the locale and keyboard questions until after there has been a chance to preseed them, while <literal>priority</literal> is an alias for <literal>debconf/priority</literal> and setting it to <literal>critical</literal> stops any questions with a lower priority from being asked.

59.

An extensive example of how to use this framework, including example scripts and classes, can be found on the <ulink url="http://hands.com/d-i/">website of its developer</ulink>. The examples available there also show many other nice effects that can be achieved by creative use of preconfiguration.

87.

if substring (option vendor-class-identifier, 0, 3) = "d-i" {
filename "http://host/preseed.cfg";
}

89.

A good way to use the DHCP preseeding is to only preseed values specific to your network, such as the Debian mirror to use. This way installs on your network will automatically get a good mirror selected, but the rest of the installation can be performed interactively. Using DHCP preseeding to fully automate Debian installs should only be done with care.