5. Configuration file handling (from old Packaging Manual)

dpkg can do a certain amount of automatic handling of package configuration files.

Whether this mechanism is appropriate depends on a number of factors, but basically there are two approaches to any particular configuration file.

The easy method is to ship a best-effort configuration in the package, and use dpkg’s conffile mechanism to handle updates. If the user is unlikely to want to edit the file, but you need them to be able to without losing their changes, and a new package with a changed version of the file is only released infrequently, this is a good approach.

The hard method is to build the configuration file from scratch in the postinst script, and to take the responsibility for fixing any mistakes made in earlier versions of the package automatically. This will be appropriate if the file is likely to need to be different on each system.

5.1. Automatic handling of configuration files by dpkg

A package may contain a control information file called conffiles. This file should be a list of filenames of configuration files needing automatic handling, separated by newlines. The filenames should be absolute pathnames, and the files referred to should actually exist in the package.

When a package is upgraded dpkg will process the configuration files during the configuration stage, shortly before it runs the package’s postinst script,

For each file it checks to see whether the version of the file included in the package is the same as the one that was included in the last version of the package (the one that is being upgraded from); it also compares the version currently installed on the system with the one shipped with the last version.

If neither the user nor the package maintainer has changed the file, it is left alone. If one or the other has changed their version, then the changed version is preferred - i.e., if the user edits their file, but the package maintainer doesn’t ship a different version, the user’s changes will stay, silently, but if the maintainer ships a new version and the user hasn’t edited it the new version will be installed (with an informative message). If both have changed their version the user is prompted about the problem and must resolve the differences themselves.

The comparisons are done by calculating the MD5 message digests of the files, and storing the MD5 of the file as it was included in the most recent version of the package.

When a package is installed for the first time dpkg will install the file that comes with it, unless that would mean overwriting a file already on the file system.

However, note that dpkg will not replace a conffile that was removed by the user (or by a script). This is necessary because with some programs a missing file produces an effect hard or impossible to achieve in another way, so that a missing file needs to be kept that way if the user did it.

Note that a package should not modify a dpkg-handled conffile in its maintainer scripts. Doing this will lead to dpkg giving the user confusing and possibly dangerous options for conffile update when the package is upgraded.

5.2. Fully-featured maintainer script configuration handling

For files which contain site-specific information such as the hostname and networking details and so forth, it is better to create the file in the package’s postinst script.

This will typically involve examining the state of the rest of the system to determine values and other information, and may involve prompting the user for some information which can’t be obtained some other way.

When using this method there are a couple of important issues which should be considered:

If you discover a bug in the program which generates the configuration file, or if the format of the file changes from one version to the next, you will have to arrange for the postinst script to do something sensible - usually this will mean editing the installed configuration file to remove the problem or change the syntax. You will have to do this very carefully, since the user may have changed the file, perhaps to fix the very problem that your script is trying to deal with - you will have to detect these situations and deal with them correctly.

If you do go down this route it’s probably a good idea to make the program that generates the configuration file(s) a separate program in /usr/sbin, by convention called packageconfig and then run that if appropriate from the post-installation script. The packageconfig program should not unquestioningly overwrite an existing configuration - if its mode of operation is geared towards setting up a package for the first time (rather than any arbitrary reconfiguration later) you should have it check whether the configuration already exists, and require a --force flag to overwrite it.