debconf(7)
NAME
debconf - Debian package configuration system
DESCRIPTION
Debconf is a configuration system for Debian packages. There is a
rarely-used command named debconf, documented in debconf(1)
Debconf provides a consistent interface for configuring packages,
allowing you to choose from several user interface frontends. It
supports preconfiguring packages before they are installed, which
allows large installs and upgrades to ask you for all the necessary
information up front, and then go do the work while you do something
else. It lets you, if you're in a hurry, skip over less important
questions and information while installing a package (and revisit it
later).
Preconfiguring packages
Debconf can configure packages before they are even installed onto your
system. This is useful because it lets all the questions the packages
are going to ask be asked at the beginning of an install, so the rest
of the install can proceed while you are away getting a cup of coffee.
If you use apt (version 0.5 or above), and you have apt-utils
installed, each package apt installs will be automatically
preconfigured. This is controlled via /etc/apt/apt.conf.d/70debconf
Sometimes you might want to preconfigure a package by hand, when you're
not installing it with apt. You can use dpkg-preconfigure(8) to do
that, just pass it the filenames of the packages you want to
preconfigure. You will need apt-utils installed for that to work.
Reconfiguring packages
Suppose you installed the package, and answered debconf's questions,
but now that you've used it awhile, you realize you want to go back and
change some of your answers. In the past, reinstalling a package was
often the thing to do when you got in this situation, but when you
reinstall the package, debconf seems to remember you have answered the
questions, and doesn't ask them again (this is a feature).
Luckily, debconf makes it easy to reconfigure any package that uses it.
Suppose you want to reconfigure debconf itself. Just run, as root:
dpkg-reconfigure debconf
This will ask all the questions you saw when debconf was first
installed. It may ask you other questions as well, since it asks even
low priority questions that may have been skipped when the package was
installed. You can use it on any other package that uses debconf, as
well.
Frontends
One of debconf's unique features is that the interface it presents to
you is only one of many, that can be swapped in at will. There are many
debconf frontends available:
dialog The default frontend, this uses the whiptail(1) or dialog(1)
programs to display questions to you. It works in text mode.
readline
The most traditional frontend, this looks quite similar to how
Debian configuration always has been: a series of questions,
printed out at the console using plain text, and prompts done
using the readline library. It even supports tab completion. The
libterm-readline-gnu-perl package is strongly recommended if you
chose to use this frontend; the default readline module does not
support prompting with default values. At the minimum, you'll
need the perl-modules package installed to use this frontend.
This frontend has some special hotkeys. Pageup (or ctrl-u) will
go back to the previous question (if that is supported by the
package that is using debconf), and pagedown (or ctrl-v) will
skip forward to the next question.
This is the best frontend for remote admin work over a slow
connection, or for those who are comfortable with unix.
noninteractive
This is the anti-frontend. It never interacts with you at all,
and makes the default answers be used for all questions. It
might mail error messages to root, but that's it; otherwise it
is completely silent and unobtrusive, a perfect frontend for
automatic installs. If you are using this front-end, and require
non-default answers to questions, you will need to preseed the
debconf database; see the section below on Unattended Package
Installation for more details.
gnome This is a modern X GUI using the gtk and gnome libraries. Of
course, it requires a valid DISPLAY to work; debconf will fall
back to other frontends if it can't work. Note that this
frontend requires you have the libgnome2-perl package installed.
kde This frontend provides a simple X GUI written with the Qt
library. It fits well the KDE desktop. You of course need a
DISPLAY to use this frontend and must install libqt-perl. The
frontend will fall back to dialog if some of the prerequisites
are not met.
editor This is for those fanatics who have to do everything in a text
editor. It runs your editor on a file that looks something like
a typical unix config file, and you edit the file to communicate
with debconf. Debconf's author prefers to not comment regarding
the circumstances that led to this frontend being written.
web
This frontend acts as a web server, that you connect to with
your web browser, to browse the questions and answer them. It
has a lot of promise, but is a little rough so far. When this
frontend starts up, it will print out the location you should
point your web browser to. You have to run the web browser on
the same machine you are configuring, for security reasons.
Do keep in mind that this is not a very secure frontend. Anyone
who has access to the computer being configured can currently
access the web server and configure things while this frontend
is running. So this is more of a proof of concept than anything.
You can change the default frontend debconf uses by reconfiguring
debconf. On the other hand, if you just want to change the frontend for
a minute, you can set the DEBIAN_FRONTEND environment variable to the
name of the frontend to use. For example:
DEBIAN_FRONTEND=readline apt-get install slrn
The dpkg-reconfigure(8) and dpkg-preconfigure(8) commands also let you
pass --frontend= to them, followed by the frontend you want them to
use.
Note that not all frontends will work in all circumstances. If a
frontend fails to start up for some reason, debconf will print out a
message explaining why, and fall back to the next-most similar
frontend.
Priorities
Another nice feature of debconf is that the questions it asks you are
prioritized. If you don't want to be bothered about every little thing,
you can set up debconf to only ask you the most important questions.
On the other hand, if you are a control freak, you can make it show you
all questions. Each question has a priority. In increasing order of
importance:
low Very trivial questions that have defaults that will work in the
vast majority of cases.
medium Normal questions that have reasonable defaults.
high Questions that don't have a reasonable default.
critical
Questions that you really, really need to see (or else).
Only questions with a priority equal to or greater than the priority
you choose will be shown to you. You can set the priority value by
reconfiguring debconf, or temporarily by passing --priority= followed
by the value to the dpkg-reconfigure(8) and dpkg-preconfigure(8)
commands, or by setting the DEBIAN_PRIORITY environment variable.
Backend Database
Debconf uses a rather flexible and potentially complicated backend
database for storing data such as the answers to questions. The file
/etc/debconf.conf is used to configure this database. If you need to
set up something complicated, like make debconf read a remote database
to get defaults, with local overrides, read the debconf.conf(5) man
page for all the gory details. Generally, the backend database is
located in /var/cache/debconf/
.
Unattended Package Installation
If you have many machines to manage you will sometimes find yourself in
the position of needing to perform an unattended installation or
upgrade of packages on many systems, when the default answers to some
configuration questions are not acceptable. There are many ways to
approach this; all involve setting up a database and making debconf use
it to get the answers you want.
You should really read debconf.conf(5) before this section, as you need
to understand how debconf's databases work.
The easiest way to set up the database is to install the packages on
one machine and answer their questions as usual. Or you might just use
dpkg-preconfigure(8) to configure a set of packages without actually
installing them. Or you might even decide to write a plain text debconf
database by hand or something.
Once you have the database, you need to figure out how to make the
remote systems use it. This depends of course on the configuration of
those systems and what database types they are set up to use.
If you are using the LDAP debconf database, an entire network of debian
machines can also have any or all package installation questions
answered automatically by a single LDAP server.
But perhaps you're using something a little bit easier to set up like,
say, the default debconf database configuration, or you just don't want
your remote systems to use LDAP all the time. In this case the best
approach is to temporarily configure the remote systems to stack your
database underneath their own existing databases, so they pull default
values out of it. Debconf offers two environment variables,
DEBCONF_DB_FALLBACK and DEBCONF_DB_OVERRIDE, to make it easy to do this
on the fly. Here is a sample use:
cat /var/cache/debconf/config.dat | \
ssh root@target "DEBIAN_FRONTEND=noninteractive \
DEBCONF_DB_FALLBACK=Pipe apt-get upgrade"
This makes the debconf on the remote host read in the data that is
piped across the ssh connection and interpret it as a plain text format
debconf database. It then uses that database as a fallback database --
a read-only database that is queried for answers to questions if the
system's main debconf database lacks answers.
Here's another way to use the DEBCONF_DB_FALLBACK environment variable:
ssh -R 389:ldap:389 root@target \
"DEBCONF_DB_FALLBACK='LDAP{host:localhost}' apt-get upgrade"
Here ssh is used to set up a tunneled LDAP connection and run debconf.
Debconf is told to use the LDAP server as the fallback database. Note
the use of "{host:localhost}" to configure how debconf accesses the
LDAP database by providing the "host" field with a value of
"localhost".
Here's another method:
scp config.dat root@target:
ssh root@target "DEBCONF_DB_FALLBACK='File{/root/config.dat}' apt-get
upgrade
Here you copy the database over with scp, and then ssh over and make
debconf use the file you copied over. This illustrates a shorthand you
can use in the DEBCONF_DB_FALLBACK parameters -- if a field name is
left off, it defaults to "filename".
There is only one problem with these uses of the DEBCONF_DB_FALLBACK
parameter: While the fallback database can provide answers to questions
the other debconf databases have never seen, it is only queried as a
fallback; after the other databases. If you need to instead temporarily
override an existing value on the remote host, you should instead use
the DEBCONF_DB_OVERRIDE variable. Like DEBCONF_DB_FALLBACK, it sets up
a temporary database, but this database is consulted before any others,
and can be used to override existing values.
Developing for Debconf
Package developers and others who want to develop packages that use
debconf should read debconf-devel(7)
.
Briefly, debconf communicates with maintainer scripts or other programs
via standard input and output, using a simple line-oriented command
language similar to that used by common internet protocols such as
SMTP. Programs use this protocol to ask debconf to display questions to
the user, and retrieve the user's answers. The questions themselves are
defined in a separate file, called the "templates file", which has a
format not unlike a debian control file.
Debian packages which use debconf typically provide both a templates
file and a "config" script (run to preconfigure the package) in the
control metadata section of the package.
ENVIRONMENT
DEBIAN_FRONTEND
Used to temporarily change the frontend debconf uses. See above.
DEBIAN_PRIORITY
Used to temporarily change the minimum priority of question
debconf will display. See above.
DEBCONF_DEBUG
Turns on debugging output on standard error. May be set to a
facility name or a regular expression which matches a facility
name (such as '.*' to output all debug info). The facility names
include:
user Debugging info of interest to a debconf user.
developer
Debugging info of interest to a package developer.
db Debugging info about the backend database.
DEBCONF_NOWARNINGS
Set to "yes" to disable some warnings that debconf may display.
Does not suppress display of fatal errors.
DEBCONF_TERSE
Set to "yes" to enable terse mode, in which debconf frontends
cut down on the verbiage as much as possible.
DEBCONF_DB_FALLBACK
Stack a database after the normally used databases, so that it
is used as a fallback to get configuration information from. See
"Unattended Package Installation" above. If the value of the
variable is the name of an existing database in debconf.conf,
then that database will be used. Otherwise, the environment
variable can be used to configure a database on the fly, by
telling the type of database, and optionally passing field:value
settings, inside curly braces after the type. Spaces are used to
separate fields, so you cannot specify a field value containing
whitespace.
Thus, this uses the fallbackdb in debconf.conf:
DEBCONF_DB_FALLBACK=fallbackdb
While this sets up a new database of type File, and tells it a filename
to use and turns off backups:
DEBCONF_DB_FALLBACK=File{Filename:/root/config.dat Backup:no}
And as a shorthand, this sets up a database of type File with a
filename:
DEBCONF_DB_FALLBACK=File{/root/config.dat}
Note that if a fallback database is set up on the fly, it will be read-
only by default.
DEBCONF_DB_OVERRIDE
Stack a database before the normally used databases, so that it
can override values from them. The value of the variable works
the same as does the value of DEBCONF_DB_FALLBACK.
DEBCONF_DB_REPLACE
Use a given database instead of the normally used databases.
This may be useful for testing with a separate database without
having to create a separate debconf.conf, or to avoid locking
the normal databases.
DEBCONF_SYSTEMRC
If this environment variable is set, debconf will ignore a
user's ~/.debconfrc file, and use the system one instead. If it
is set to the name of a regular file, debconf will use that file
in preference to the system configuration files.
DEBCONF_FORCE_DIALOG
If this environment variable is set, debconf will use dialog in
preference to whiptail for the dialog frontend.
DEBCONF_FORCE_XDIALOG
If this environment variable is set, debconf will use Xdialog in
preference to whiptail for the dialog frontend.
DEBCONF_NONINTERACTIVE_SEEN
Set to "true" to cause the seen flag to be set for questions
asked in the noninteractive frontend.
BUGS
Probably quite a few, there's a lot of code here.
If you do file a bug report, be sure to include the following
information:
* The debconf frontend you were using when the problem occurred
* What you did to trigger the problem.
* The full text of any error messages. If you can reproduce the
bug, do so with DEBCONF_DEBUG='.*' set and exported. This speeds
up debugging a lot.
SEE ALSO
debconf.conf(5), debconf-devel(7), dpkg-preconfigure(8), dpkg-
reconfigure(8), debconf(1),
AUTHOR
Joey Hess <joeyh@debian.org>
DEBCONF(7)
Free and Open Source Software