An Introduction to Debconf

Introduction

In this document, I'll explain how maintainer scripts in debian packages have interacted with the user so far, the problems this causes, and the things it prevents us from doing. Then I'll show how debconf can address these problems.

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The status quo

Currently, if a package needs to prompt the user for input, it just does, using standard input and standard output to communicate with them. A few packages use things like dialog for a user interface, while most use bare-bones textual interfaces. There is little consistency between these interfaces, since they are each written from scratch. They use different methods to indicate default values, different ways to present lists of choices, and even prompt in different ways when the user just needs to hit enter after being shown a message. Many packages ask the user a series of questions with no way to go back to a previous question, or to start over.

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Problems with the status quo

Most packages that prompt do so in the postinst, and so the user has to baby-sit an install, answering questions as they come up, and then waiting for some more packages to be installed and some more questions to arrive. There is no way to answer all the questions first and then let dpkg install everything unattended, and so installs are a long, drawn out process.

Upgrades often take a long time as well, because many packages ask the same questions over and over each time they are installed. Those that don't have to store the user's last response somewhere, and they do this in a variety of different, inconsistent, ways.

Moreover, there is no way to simply use default answers for all the questions asked, if you're in a hurry or don't want to be bothered with them, which has historically made the debian install a barrier to new users.

The traditional way of asking questions has made some specialized uses of debian harder as well. Many experienced debian users would like to put "apt-get update; apt-get upgrade" in a cron job, and have their system upgraded periodically to unstable. People working on clusters or other large-scale debian installations can't afford to answer the same questions over and over again on each machine, and have hacked together various ways around this.

Finally, several distributions have appeared recently, based on debian but catering to inexperienced users. None of them want the user to see any questions at all when they install, and they have used various hacks to suppress the questions.

It seems clear that we need something better to replace the traditional method of querying the user from a maintainer script. It needs to present a consistent user interface to the user. It needs to be able to prioritize questions so non-interactive installs are possible, as well as installs with all questions asked, or only the most important ones. It needs to be able to ask questions only once. And it would be nice if the questions could be stored in a database on a central machine and sent out to other machines in a cluster, so they need only be asked once for a whole cluster.

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Debconf antecedents

In fall of 1998, Wichert Akkerman came up with a specification for a configuration management system that would address these needs. It was refined on the debian lists over the next several months. The current version of the specification is in Debian policy.

The specification is very flexible, allowing for multiple different back-end databases (using arbitrary database formats from SQL to plain text), that can be layered together and accessed over the network or locally. It also allows for a variety of user interfaces to be presented to the user. The maintainer scripts communicate with the back-end and front-end using a simple language.

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Debconf

Debconf is an implementation of this specification. At this point, it implements almost the entire spec. It is already fully usable as a consistent way to configure packages.

As it stands now, debconf is usable along-side traditional packages, and packages that use debconf will behave just like normal packages except they use a consistent UI. Debconf also hooks into apt so if you use apt to install several debconf-aware packages at once, the packages will prompt for configuration information before they are installed. An example is in order -- this is using debconf's plain text front-end. This really can't show off all of debconf's features, but it should give you the idea.

root@gumdrop:~> apt-get install sash lynx
Reading Package Lists... Done
Building Dependency Tree... Done
The following NEW packages will be installed:
  lynx sash 
0 packages upgraded, 2 newly installed, 0 to remove and 26 not upgraded.
Need to get 0B/1089kB of archives. After unpacking 2480kB will be used.
Lynx Configuration
------------------

Please enter the default URL to use if none is given when invoking lynx. 

You must enter either a full URL (including the "http://", etc.) or an
absolute pathname. 

What is lynx's homepage? http://debian.org/       

Sash Configuration
------------------

The best way to use sash is to install it as root's login shell. Currently
root's login shell is set to /usr/bin/zsh. 

Should sash be root's login shell? [yN] n

Selecting previously deselected package lynx.
(Reading database ... 41813 files and directories currently installed.)
Unpacking lynx (from .../lynx_2.8.2-3.1_i386.deb) ...
Selecting previously deselected package sash.
Unpacking sash (from .../archives/sash_2.1-5.1_i386.deb) ...
Setting up lynx (2.8.2-3.1) ...
Update-menus[31963]: further output (if any) will appear in /tmp/update-menus.31963

Setting up sash (2.1-5.1) ...

root@gumdrop:~>