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debconf(7) - phpMan
DEBCONF(7) Miscellaneous Information Manual 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 pre‐
vious 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 fron‐
tend 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_FRON‐
TEND 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 impor‐
tance:
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-preconfig‐
ure(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/deb‐
conf/
.
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 DEB‐
CONF_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 con‐
figure 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 DEB‐
CONF_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 ver‐
biage as much as possible.
DEBCONF_DB_FALLBACK
Stack a database after the normally used databases, so that it is used as a fall‐
back to get configuration information from. See "Unattended Package Installation"
above. If the value of the variable is the name of an existing database in deb‐
conf.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 DEB‐
CONF_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 deb‐
conf.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 whip‐
tail 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 noninter‐
active 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 DEB‐
CONF_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 AT debian.org>
DEBCONF(7)
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