2.2.4 Preservation of local configuration
Files that are intended to be changeable by the local administrator are kept in
/etc/. Debian policy dictates that all changes to locally
configurable files be preserved across package upgrades.
If a default version of a locally configurable file is shipped in the package
itself then the file is listed as a "conffile". The package
management system does not upgrade conffiles that have been changed by the
administrator since the package was last installed without getting the
administrator's permission. On the other hand, if the conffile has not been
changed by the administrator then the conffile will be upgraded along with the
rest of the package. This is almost always desirable and so it is advantageous
to minimize changes to conffiles.
To list the conffiles belonging to a package run the following command:
dpkg --status package
The list follows the "Conffiles:" line.
For more information about conffiles you can read the section of the Debian
Policy Manual entitled "Configuration files". (See References, Section 15.1).