The Debian Administrator's Handbook

Let us discuss the first case in greater detail with an example: all mail servers, such as postfix or sendmail are said to “provide” the mail-transport-agent virtual package. Thus, any package that needs this service to be functional (e.g. a mailing list manager, such as smartlist or sympa) simply states in its dependencies that it requires a mail-transport-agent instead of specifying a large yet incomplete list of possible solutions (e.g. postfix | sendmail | exim | …). Furthermore, it is useless to install two mail servers on the same machine, which is why each of these packages declares a conflict with the mail-transport-agent virtual package. The conflict with itself is ignored by the system, but this technique will prohibit the installation of two mail servers side by side.

DEBIAN POLICY List of virtual packages

For virtual packages to be useful, everyone must agree on their name. This is why they are standardized in the Debian Policy. The list includes among others mail-transport-agent for mail servers, c-compiler for C programming language compilers, www-browser for web browsers, httpd for web servers, ftp-server for FTP servers, x-terminal-emulator for terminal emulators in graphical mode (xterm ), and x-window-manager for window managers.

The full list can be found on the Web, at

→ http://www.debian.org/doc/packaging-manuals/virtual-package-names-list.txt

5.2.1.4.2. Interchangeability with Another Package

The Provides field is again interesting when the content of a package is included in a larger package. For example, the libdigest-md5-perl Perl module was an optional module in Perl 5.6, and has been integrated as standard in Perl 5.8 (and later versions, such as 5.10 present in Squeeze). As such, the package perl has since version 5.8 declared Provides: libdigest-md5-perl so that the dependencies on this package are met if the user has Perl 5.8 or 5.10. The libdigest-md5-perl package itself has eventually been deleted, since it no longer had any purpose when old Perl versions were removed.

Figure 5.1. Use of a Provides field in order to not break dependencies

This feature is very useful, since it is never possible to anticipate the vagaries of development, and it is necessary to be able to adjust to renaming, and other automatic replacement, of obsolete software.

BACK TO BASICS Perl, a programming language

Perl (Practical Extraction and Report Language) is a very popular programming language. It has many ready-to-use modules that cover a vast spectrum of applications, and that are distributed by the CPAN (Comprehensive Perl Archive Network) servers, an exhaustive network of Perl packages.

→ http://www.perl.org/

→ http://www.cpan.org/

Since it is an interpreted language, a program written in Perl does not require compilation prior to execution. This is why they are called “Perl scripts”.

5.2.1.4.3. Current Limitations

Virtual packages suffer from some troubling limitations, the most significant of which being the absence of a version number. To return to the previous example, a dependency such as Depends: libdigest-md5-perl (>= 1.6), despite the presence of Perl 5.10, will never be considered as satisfied by the packaging system — while in fact it most likely is satisfied. Unaware of this, the package system chooses the least risky option, assuming that the versions do not match.

GOING FURTHER Virtual package versions

Although today virtual packages can't have versions, this will not necessarily always be the case. Indeed, apt is already able to manage the versions of virtual packages and it is likely that dpkg eventually will too. We will then be able to write fields such as Provides: libstorable-perl (= 1.7) to indicate that a package provides the same functionality as libstorable-perl in its 1.7 version.

5.2.1.5. Replacing Files: The Replaces Field

The Replaces field indicates that the package contains files that are also present in another package, but that the package is legitimately entitled to replace them. Without this specification, dpkg fails, stating that it can not overwrite the files of another package (in fact, it is possible to force it to do so with the --force-overwrite option). This allows identification of potential problems and requires the maintainer to study the matter prior to choosing whether to add such a field.

The use of this field is justified when package names change or when a package is included in another. This also happens when the maintainer decides to distribute files differently among various binary packages produced from the same source package: a replaced file no longer belongs to the old package, but only to the new one.