Customized programs
===================

.. _s-arch-spec:

Architecture specification strings
----------------------------------

If a program needs to specify an *architecture specification string* in
some place, it should select one of the strings provided by
``dpkg-architecture -L``. The strings are in the format ``os-arch``, though the OS
part is sometimes elided, as when the OS is Linux.

Note that we don't want to use ``arch-debian-linux`` to apply to the
rule ``architecture-vendor-os`` since this would make our programs
incompatible with other Linux distributions. We also don't use something
like ``arch-unknown-linux``, since the ``unknown`` does not look very
good.

.. _s-arch-wildcard-spec:

Architecture wildcards
~~~~~~~~~~~~~~~~~~~~~~

A package may specify an architecture wildcard. Architecture wildcards
are in the format ``any`` (which matches every architecture),
``os``-any, or any-\ ``cpu``.  [#]_

.. _s11.2:

Daemons
-------

The configuration files ``/etc/services``, ``/etc/protocols``, and
``/etc/rpc`` are managed by the ``netbase`` package and must not be
modified by other packages.

If a package requires a new entry in one of these files, the maintainer
should get in contact with the ``netbase`` maintainer, who will add the
entries and release a new version of the ``netbase`` package.

The configuration file ``/etc/inetd.conf`` must not be modified by the
package's scripts except via the ``update-inetd`` script or the
``DebianNet.pm`` Perl module. See their documentation for details on how
to add entries.

If a package wants to install an example entry into ``/etc/inetd.conf``,
the entry must be preceded with exactly one hash character (``#``). Such
lines are treated as "commented out by user" by the ``update-inetd``
script and are not changed or activated during package updates.

.. _s11.3:

Using pseudo-ttys and modifying wtmp, utmp and lastlog
------------------------------------------------------

Some programs need to create pseudo-ttys. This should be done using
Unix98 ptys if the C library supports it. The resulting program must not
be installed setuid root, unless that is required for other
functionality.

The files ``/var/run/utmp``, ``/var/log/wtmp`` and ``/var/log/lastlog``
must be installed writable by group ``utmp``. Programs which need to
modify those files must be installed setgid ``utmp``.

.. _s11.4:

Editors and pagers
------------------

Some programs have the ability to launch an editor or pager program to
edit or display a text document. Since there are lots of different
editors and pagers available in the Debian distribution, the system
administrator and each user should have the possibility to choose their
preferred editor and pager.

In addition, every program should choose a good default editor/pager if
none is selected by the user or system administrator.

Thus, every program that launches an editor or pager must use the EDITOR
or PAGER environment variable to determine the editor or pager the user
wishes to use. If these variables are not set, the programs
``/usr/bin/editor`` and ``/usr/bin/pager`` should be used, respectively.
These commands may be invoked explicitly (e.g., as ``/usr/bin/editor``) or
via a PATH search (e.g., as ``editor``).

These two files are managed through the ``dpkg`` "alternatives"
mechanism. Every package providing an editor or pager must call the
``update-alternatives`` script to register as an alternative for
``/usr/bin/editor`` or ``/usr/bin/pager`` as appropriate. The
alternative should have a slave alternative for
``/usr/share/man/man1/editor.1.gz`` or
``/usr/share/man/man1/pager.1.gz`` pointing to the corresponding manual
page.

If it is very hard to adapt a program to make use of the EDITOR or PAGER
variables, that program may be configured to use
``/usr/bin/sensible-editor`` and ``/usr/bin/sensible-pager`` as the
editor or pager program respectively. These are two scripts provided in
the sensible-utils package that check the EDITOR and PAGER variables and
launch the appropriate program, and fall back to ``/usr/bin/editor`` and
``/usr/bin/pager`` if the variable is not set.

A program may also use the VISUAL environment variable to determine the
user's choice of editor. If it exists, it should take precedence over
EDITOR. This is in fact what ``/usr/bin/sensible-editor`` does.

It is not required for a package to depend on ``editor`` and ``pager``,
nor is it required for a package to provide such virtual
packages. [#]_

.. _s-web-appl:

Web servers and applications
----------------------------

This section describes the locations and URLs that should be used by all
web servers and web applications in the Debian system.

1. Cgi-bin executable files are installed in the directory

   ::

       /usr/lib/cgi-bin

   or a subdirectory of that directory, and the script

   ::

       /usr/lib/cgi-bin/.../cgi-bin-name

   should be referred to as

   ::

       http://localhost/cgi-bin/.../cgi-bin-name

2. (Deleted)

3. Access to images

   Images for a package should be stored in ``/usr/share/images/package``
   and referred to through an alias ``/images/`` as::

       http://localhost/images/package/filename

4. Web Document Root

   Web Applications should try to avoid storing files in the Web
   Document Root. Instead they should use the /usr/share/doc/package
   directory for documents. If access to the web document root is
   unavoidable then use

   ::

       /var/www/html

   as the Document Root. This might be just a symbolic link to the
   location where the system administrator has put the real document
   root.

5. Providing httpd and/or httpd-cgi

   All web servers should provide the virtual package ``httpd``. If a
   web server has CGI support it should provide ``httpd-cgi``
   additionally.

   All web applications which do not contain CGI scripts should depend
   on ``httpd``, all those web applications which ``do`` contain CGI
   scripts, should depend on ``httpd-cgi``.

.. _s-mail-transport-agents:

Mail transport, delivery and user agents
----------------------------------------

Debian packages which process electronic mail, whether mail user agents
(MUAs) or mail transport agents (MTAs), must ensure that they are
compatible with the configuration decisions below. Failure to do this
may result in lost mail, broken ``From:`` lines, and other serious brain
damage!

The mail spool is ``/var/mail`` and the interface to send a mail message
is ``/usr/sbin/sendmail`` (as per the FHS). On older systems, the mail
spool may be physically located in ``/var/spool/mail``, but all access
to the mail spool should be via the ``/var/mail`` symlink. The mail
spool is part of the base system and not part of the MTA package.

All Debian MUAs, MTAs, MDAs and other mailbox accessing programs (such
as IMAP daemons) must lock the mailbox in an NFS-safe way. This means
that ``fcntl()`` locking must be combined with dot locking. To avoid
deadlocks, a program should use ``fcntl()`` first and dot locking after
this, or alternatively implement the two locking methods in a non
blocking way.  [#]_ Using the functions ``maillock`` and
``mailunlock`` provided by the ``liblockfile*`` packages is the
recommended way to accomplish this.

Mailboxes are generally either mode 600 and owned by user or mode 660
and owned by ``user:mail``.  [#]_ The local system administrator may
choose a different permission scheme; packages should not make
assumptions about the permission and ownership of mailboxes unless
required (such as when creating a new mailbox). A MUA may remove a
mailbox (unless it has nonstandard permissions) in which case the MTA or
another MUA must recreate it if needed.

The mail spool is 2775 ``root:mail``, and MUAs should be setgid mail to
do the locking mentioned above (and must obviously avoid accessing other
users' mailboxes using this privilege).

``/etc/aliases`` is the source file for the system mail aliases (e.g.,
postmaster, usenet, etc.), it is the one which the sysadmin and
``postinst`` scripts may edit. After ``/etc/aliases`` is edited the
program or human editing it must call ``newaliases``. All MTA packages
must come with a ``newaliases`` program, even if it does nothing, but
older MTA packages did not do this so programs should not fail if
``newaliases`` cannot be found. Note that because of this, all MTA
packages must have ``Provides``, ``Conflicts`` and
``Replaces:  mail-transport-agent`` control fields.

The convention of writing ``forward to address`` in the mailbox itself is not supported. Use a
``.forward`` file instead.

The ``rmail`` program used by UUCP for incoming mail should be
``/usr/sbin/rmail``. Likewise, ``rsmtp``, for receiving
batch-SMTP-over-UUCP, should be ``/usr/sbin/rsmtp`` if it is supported.

If your package needs to know what hostname to use on (for example)
outgoing news and mail messages which are generated locally, you should
use the file ``/etc/mailname``. It will contain the portion after the
username and ``@`` (at) sign for email addresses of users on the machine
(followed by a newline).

Such a package should check for the existence of this file when it is
being configured. If it exists, it should be used without comment,
although an MTA's configuration script may wish to prompt the user even
if it finds that this file exists. If the file does not exist, the
package should prompt the user for the value (preferably using
``debconf``) and store it in ``/etc/mailname`` as well as using it in
the package's configuration. The prompt should make it clear that the
name will not just be used by that package. For example, in this
situation the ``inn`` package could say something like:

::

    Please enter the "mail name" of your system.  This is the hostname portion
    of the address to be shown on outgoing news and mail messages.  The
    default is syshostname, your system's host name.

    Mail name ["syshostname"]:

where syshostname is the output of ``hostname --fqdn``.

.. _s11.7:

News system configuration
-------------------------

All the configuration files related to the NNTP (news) servers and
clients should be located under ``/etc/news``.

There are some configuration issues that apply to a number of news
clients and server packages on the machine. These are:

``/etc/news/organization``
    A string which should appear as the organization header for all
    messages posted by NNTP clients on the machine

``/etc/news/server``
    Contains the FQDN of the upstream NNTP server, or localhost if the
    local machine is an NNTP server.

Other global files may be added as required for cross-package news
configuration.

.. _s11.8:

Programs for the X Window System
--------------------------------

.. _s11.8.1:

Providing X support and package priorities
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Programs that can be configured with support for the X Window System
must be configured to do so and must declare any package dependencies
necessary to satisfy their runtime requirements when using the X Window
System. If such a package is of higher priority than the X packages on
which it depends, it is required that either the X-specific components
be split into a separate package, or that an alternative version of the
package, which includes X support, be provided, or that the package's
priority be lowered.

.. _s11.8.2:

Packages providing an X server
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Packages that provide an X server that, directly or indirectly,
communicates with real input and display hardware should declare in
their ``Provides`` control field that they provide the virtual package
``xserver``.  [#]_

.. _s11.8.3:

Packages providing a terminal emulator
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Packages that provide a terminal emulator for the X Window System which
meet the criteria listed below should declare in their ``Provides``
control field that they provide the virtual package
``x-terminal-emulator``. They should also register themselves as an
alternative for ``/usr/bin/x-terminal-emulator``, with a priority of 20.
That alternative should have a slave alternative for
``/usr/share/man/man1/x-terminal-emulator.1.gz`` pointing to the
corresponding manual page.

To be an ``x-terminal-emulator``, a program must:

- Be able to emulate a DEC VT100 terminal, or a compatible terminal.

- Support the command-line option ``-e command``, which creates a new
  terminal window [#]_ and runs the specified command.  <command> may
  be multiple arguments, which form the argument list to the executed
  program.  In other words, the behavior is as though the arguments
  were passed directly to ``execvp``, bypassing the shell.
  (``xterm``'s behavior of falling back on using the shell if ``-e``
  had a single argument and exec failed is permissible but not
  required.)

- Support the command-line option ``-T title``, which creates a new
  terminal window with the window title title.

.. _s11.8.4:

Packages providing a window manager
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Packages that provide a window manager should declare in their
``Provides`` control field that they provide the virtual package
``x-window-manager``. They should also register themselves as an
alternative for ``/usr/bin/x-window-manager``, with a priority
calculated as follows:

-  Start with a priority of 40.

-  If the window manager complies with `The Window Manager Specification
   Project <https://www.freedesktop.org/wiki/Specifications/wm-spec>`_,
   written by the `Free Desktop
   Group <https://www.freedesktop.org/wiki/>`_, add 40 points.

-  If the window manager permits the X session to be restarted using a
   *different* window manager (without killing the X server) in its
   default configuration, add 10 points; otherwise add none.

That alternative should have a slave alternative for
``/usr/share/man/man1/x-window-manager.1.gz`` pointing to the
corresponding manual page.

.. _s11.8.5:

Packages providing fonts
~~~~~~~~~~~~~~~~~~~~~~~~

Packages that provide fonts for the X Window System  [#]_ must do a
number of things to ensure that they are both available without
modification of the X or font server configuration, and that they do not
corrupt files used by other font packages to register information about
themselves.

1.  Fonts of any type supported by the X Window System must be in a
    separate binary package from any executables, libraries, or
    documentation (except that specific to the fonts shipped, such as
    their license information). If one or more of the fonts so packaged
    are necessary for proper operation of the package with which they
    are associated the font package may be Recommended; if the fonts
    merely provide an enhancement, a Suggests relationship may be used.
    Packages must not Depend on font packages.  [#]_

2.  BDF fonts must be converted to PCF fonts with the ``bdftopcf``
    utility (available in the ``xfonts-utils`` package, ``gzip``\ ped,
    and placed in a directory that corresponds to their resolution:

    -  100 dpi fonts must be placed in ``/usr/share/fonts/X11/100dpi/``.

    -  75 dpi fonts must be placed in ``/usr/share/fonts/X11/75dpi/``.

    -  Character-cell fonts, cursor fonts, and other low-resolution
       fonts must be placed in ``/usr/share/fonts/X11/misc/``.

3.  Type 1 fonts must be placed in ``/usr/share/fonts/X11/Type1/``. If
    font metric files are available, they must be placed here as well.

4.  Subdirectories of ``/usr/share/fonts/X11/`` other than those listed
    above must be neither created nor used. (The ``PEX``, ``CID``,
    ``Speedo``, and ``cyrillic`` directories are excepted for historical
    reasons, but installation of files into these directories remains
    discouraged.)

5.  Font packages may, instead of placing files directly in the X font
    directories listed above, provide symbolic links in that font
    directory pointing to the files' actual location in the filesystem.
    Such a location must comply with the FHS.

6.  Font packages should not contain both 75dpi and 100dpi versions of a
    font. If both are available, they should be provided in separate
    binary packages with ``-75dpi`` or ``-100dpi`` appended to the names
    of the packages containing the corresponding fonts.

7.  Fonts destined for the ``misc`` subdirectory should not be included
    in the same package as 75dpi or 100dpi fonts; instead, they should
    be provided in a separate package with ``-misc`` appended to its
    name.

8.  Font packages must not provide the files ``fonts.dir``,
    ``fonts.alias``, or ``fonts.scale`` in a font directory:

    -  ``fonts.dir`` files must not be provided at all.

    -  ``fonts.alias`` and ``fonts.scale`` files, if needed, should be
       provided in the directory
       ``/etc/X11/fonts/fontdir/package.extension``, where fontdir is
       the name of the subdirectory of ``/usr/share/fonts/X11/`` where
       the package's corresponding fonts are stored (e.g., ``75dpi`` or
       ``misc``), package is the name of the package that provides these
       fonts, and extension is either ``scale`` or ``alias``, whichever
       corresponds to the file contents.

9.  Font packages must declare a dependency on ``xfonts-utils`` in their
    ``Depends`` or ``Pre-Depends`` control field.

10. Font packages that provide one or more ``fonts.scale`` files as
    described above must invoke ``update-fonts-scale`` on each directory
    into which they installed fonts *before* invoking
    ``update-fonts-dir`` on that directory. This invocation must occur
    in both the ``postinst`` (for all arguments) and ``postrm`` (for all
    arguments except ``upgrade``) scripts.

11. Font packages that provide one or more ``fonts.alias`` files as
    described above must invoke ``update-fonts-alias`` on each directory
    into which they installed fonts. This invocation must occur in both
    the ``postinst`` (for all arguments) and ``postrm`` (for all
    arguments except ``upgrade``) scripts.

12. Font packages must invoke ``update-fonts-dir`` on each directory
    into which they installed fonts. This invocation must occur in both
    the ``postinst`` (for all arguments) and ``postrm`` (for all
    arguments except ``upgrade``) scripts.

13. Font packages must not provide alias names for the fonts they
    include which collide with alias names already in use by fonts
    already packaged.

14. Font packages must not provide fonts with the same XLFD registry
    name as another font already packaged.

.. _s-appdefaults:

Application defaults files
~~~~~~~~~~~~~~~~~~~~~~~~~~

Application defaults files must be installed in the directory
``/etc/X11/app-defaults/`` (use of a localized subdirectory of
``/etc/X11/`` as described in the *X Toolkit Intrinsics - C Language
Interface* manual is also permitted). They must be registered as
``conffile``\ s or handled as configuration files.

Customization of programs' X resources may also be supported with the
provision of a file with the same name as that of the package placed in
the ``/etc/X11/Xresources/`` directory, which must be registered as a
``conffile`` or handled as a configuration file.  [#]_

.. _s11.8.7:

Installation directory issues
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Historically, packages using the X Window System used a separate set of
installation directories from other packages. This practice has been
discontinued and packages using the X Window System should now generally
be installed in the same directories as any other package. Specifically,
packages must not install files under the ``/usr/X11R6/`` directory and
the ``/usr/X11R6/`` directory hierarchy should be regarded as obsolete.

Include files previously installed under ``/usr/X11R6/include/X11/``
should be installed into ``/usr/include/X11/``. For files previously
installed into subdirectories of ``/usr/X11R6/lib/X11/``, package
maintainers should determine if subdirectories of ``/usr/lib/`` and
``/usr/share/`` can be used. If not, a subdirectory of ``/usr/lib/X11/``
should be used.

Configuration files for window, display, or session managers or other
applications that are tightly integrated with the X Window System may be
placed in a subdirectory of ``/etc/X11/`` corresponding to the package
name. Other X Window System applications should use the ``/etc/``
directory unless otherwise mandated by policy (such as for
:ref:`s-appdefaults`).

.. _s-perl:

Perl programs and modules
-------------------------

Perl programs and modules should follow the current Perl policy.

The Perl policy can be found in the ``perl-policy`` files in the
``debian-policy`` package. It is also available from the Debian web
mirrors at https://www.debian.org/doc/packaging-manuals/perl-policy/.

.. _s-emacs:

Emacs lisp programs
-------------------

Please refer to the "Debian Emacs Policy" for details of how to package
emacs lisp programs.

The Emacs policy is available in ``debian-emacs-policy.gz`` of the
emacsen-common package. It is also available from the Debian web mirrors
at https://www.debian.org/doc/packaging-manuals/debian-emacs-policy.

.. _s11.11:

Games
-----

The permissions on ``/var/games`` are mode 755, owner ``root`` and group
``root``.

Each game decides on its own security policy.

Games which require protected, privileged access to high-score files,
saved games, etc., may be made set-\ *group*-id (mode 2755) and owned by
``root:games``, and use files and directories with appropriate
permissions (770 ``root:games``, for example). They must not be made
set-\ *user*-id, as this causes security problems. (If an attacker can
subvert any set-user-id game they can overwrite the executable of any
other, causing other players of these games to run a Trojan horse
program. With a set-group-id game the attacker only gets access to less
important game data, and if they can get at the other players' accounts
at all it will take considerably more effort.)

Some packages, for example some fortune cookie programs, are configured
by the upstream authors to install with their data files or other static
information made unreadable so that they can only be accessed through
set-id programs provided. You should not do this in a Debian package:
anyone can download the ``.deb`` file and read the data from it, so
there is no point making the files unreadable. Not making the files
unreadable also means that you don't have to make so many programs
set-id, which reduces the risk of a security hole.

As described in the FHS, binaries of games should be installed in the
directory ``/usr/games``. This also applies to games that use the X
Window System. Manual pages for games (X and non-X games) should be
installed in ``/usr/share/man/man6``.

.. [#]
   Internally, the package system normalizes the GNU triplets and the
   Debian arches into Debian arch triplets (which are kind of inverted
   GNU triplets), with the first component of the triplet representing
   the libc and ABI in use, and then does matching against those
   triplets. However, such triplets are an internal implementation
   detail that should not be used by packages directly. The libc and ABI
   portion is handled internally by the package system based on the os
   and cpu.

.. [#]
   The Debian base system already provides an editor and a pager
   program.

.. [#]
   If it is not possible to establish both locks, the system shouldn't
   wait for the second lock to be established, but remove the first
   lock, wait a (random) time, and start over locking again.

.. [#]
   There are two traditional permission schemes for mail spools: mode
   600 with all mail delivery done by processes running as the
   destination user, or mode 660 and owned by group mail with mail
   delivery done by a process running as a system user in group mail.
   Historically, Debian required mode 660 mail spools to enable the
   latter model, but that model has become increasingly uncommon and the
   principle of least privilege indicates that mail systems that use the
   first model should use permissions of 600. If delivery to programs is
   permitted, it's easier to keep the mail system secure if the delivery
   agent runs as the destination user. Debian Policy therefore permits
   either scheme.

.. [#]
   This implements current practice, and provides an actual policy for
   usage of the ``xserver`` virtual package which appears in the virtual
   packages list. In a nutshell, X servers that interface directly with
   the display and input hardware or via another subsystem (e.g., GGI)
   should provide ``xserver``. Things like ``Xvfb``, ``Xnest``, and
   ``Xprt`` should not.

.. [#]
   "New terminal window" does not necessarily mean a new top-level X
   window directly parented by the window manager; it could, if the
   terminal emulator application were so coded, be a new "view" in a
   multiple-document interface (MDI).

.. [#]
   For the purposes of Debian Policy, a "font for the X Window System"
   is one which is accessed via X protocol requests. Fonts for the Linux
   console, for PostScript renderer, or any other purpose, do not fit
   this definition. Any tool which makes such fonts available to the X
   Window System, however, must abide by this font policy.

.. [#]
   This is because an X client may be displayed by a remote X server,
   in which case X fonts are provided by the remote X server, not
   retrieved locally; the Debian package system is empowered to deal
   only with the local file system.

.. [#]
   Note that this mechanism is not the same as using app-defaults;
   app-defaults are tied to the client binary on the local file system,
   whereas X resources are stored in the X server and affect all
   connecting clients.
