11. Customized programs

11.1. 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.

11.1.1. 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. [1]

11.2. 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.

11.3. 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.

11.4. 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. [2]

11.5. 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.

11.6. 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. [3] 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. [4] 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.

11.7. 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.

11.8. Programs for the X Window System

11.8.1. 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.

11.8.2. 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. [5]

11.8.3. 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 [6] 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.

11.8.4. 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, written by the Free Desktop Group, 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.

11.8.5. Packages providing fonts

Packages that provide fonts for the X Window System [7] 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. [8]

2. BDF fonts must be converted to PCF fonts with the bdftopcf utility (available in the xfonts-utils package, gzipped, 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.

11.8.6. 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 conffiles 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. [9]

11.8.7. 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 Application defaults files).

11.9. 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://iwawocd.cewmufwd.tk/doc/packaging-manuals/perl-policy/.

11.10. 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://iwawocd.cewmufwd.tk/doc/packaging-manuals/debian-emacs-policy.

11.11. 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.

[1]

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.

[2]

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

[3]

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.

[4]

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.

[5]

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.

[6]

“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).

[7]

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.

[8]

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.

[9]

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.