<book>
<title>User service daemon and client specification
<author>Ian Jackson <email>ian@davenant.greenend.org.uk
-<version>0.61.4</version>
+<version>1.0.3</version>
<abstract>
This is a specification for a Unix system facility to allow one
between them.
<copyright>
-<prgn/userv/ is Copyright 1996-1999 Ian Jackson.
+<prgn/userv/ is Copyright 1996-2003 Ian Jackson.
<p>
<prgn/userv/ is free software; you can redistribute it and/or modify
<chapt id="notes">Applications and notes on use
<p>
+<sect id="examples">Examples
+<p>
+
+The companion package, <prgn/userv-utils/, contains a selection of
+example services, some of which are useful tools in their own right.
+See the <prgn/README/ in its top-level directory for details.
+
<sect id="standards">Standard services and directory management
<p>
In later versions of this specification standard service names and
interfaces for common services such as mail delivery and WWW CGI
-scripts will be specified.
+scripts may be specified.
<p>
<prgn/userv/-using applications and system services which hide
<prgn/userv/ behind wrapper scripts may need to store information in
the user's filespace to preserve the correct placement of the security
perimiters. Such applications should usually do so in a directory
-(created by them) <tt>~/.userv/.servdata/<var/service/</>, where
-<var/service/ is the service name or application in question.
+(created by them) <tt>~/.userv/<var/service/</>, where <var/service/
+is the service name or application in question.
<p>
-The use of a dot-directory inside <tt>~/.userv</> will hopefully avoid
-the user becoming confused by finding parts of a semi-privileged
-application's internal state in their filespace, and or discourage
-them from fiddling with and thus corrupting it. (Note that such
-applications should of course not rely for their global integrity on
-the integrity of the data on the user's side of the security
-boundary.)
+If desired, a dot-directory inside <tt>~/.userv</> may be used to
+avoid the user becoming confused by finding parts of a semi-privileged
+application's internal state in their filespace, and/or discourage
+them from fiddling with and thus corrupting it.
+<p>
+
+However, <prgn/userv/ applications should of course not rely for their
+global integrity and security on the integrity of the data on the
+user's side of the security boundary.
<sect id="reducepriv">Reducing the number of absolutely privileged subsystems
<p>
trusted with the security of the system.
<p>
-Using <prgn/userv/ many of these subsystems no longer need any unusual
-privilege.
-<p>
+If they were to use <prgn/userv/, many of these subsystems would no
+longer need any unusual privilege. <p>
<prgn/cron/ and <prgn/at/, <prgn/lpr/ and the system's mail transfer
agent (<prgn/sendmail/, <prgn/smail/, <prgn/exim/ or the like) all
-fall into this category.
+fall into this category, though <prgn/userv/-based versions of these
+programs are not currently available.
<sect id="noexcess">Do not give away excessive privilege to <prgn/userv/-using facilities
<p>
It is debatable whether the user-controlled state should be kept in
the user's filespace (in dotfiles, say) or kept in a separate area set
aside for the purpose; however, using the user's home directory (and
-probably creating a separate subdirectory of it as a dotfile to
-contain many subsystems' state) has fewer implications for the rest of
-the system and makes it entirely clear where the security boundaries
-lie.
+possibly creating a separate subdirectory of it as a dotfile to
+contain subsystem state) has fewer implications for the rest of the
+system and makes it entirely clear where the security boundaries lie.
-<sect id="notreally"><prgn/userv/ is not a replacement for <prgn/really/ and <prgn/sudo/
+<sect id="notreally"><prgn/userv/ can often replace <prgn/sudo/, but not <prgn/really/
<p>
<prgn/userv/ is not intended as a general-purpose system
administration tool with which system administrators can execute
-privileged programs when they need to. It is unsuitable for this
-purpose precisely because it enforces a strong separation between the
-calling and the called program, which is undesirable in this context.
+arbitrary programs like text editors as root (or other system users)
+when they need to. It is unsuitable for this purpose precisely
+because it enforces a strong separation between the calling and the
+called program, which is undesirable in this context.
<p>
-Its facilities for restricting activities to running certain programs
-may at first glance seem to provide similar functionality to
+However, its use when restricted to running particular programs in
+particular ways is very similar to many common uses of
<prgn/sudo/<footnote><prgn/sudo/ is a program which allows users to
execute certain programs as root, according to configuration files
-specified by the system administrator.</footnote>. However, the
-separation mentioned above is a problem here too, particular for
-interaction - it can be hard for a <prgn/userv/ service program to
-interact with its real caller or the user in question.
+specified by the system administrator.</footnote>. <prgn/userv/ is
+generally much better than restricted <prgn/sudo/, because it protects
+the called program much more strongly from bad environmental
+conditions set up by the caller. Most programs that one might want to
+run via restricted <prgn/sudo/, have not been designed to run in a
+partially hostile environment. <prgn/userv/ allows these programs to
+be run in a safer environment and should be used instead.
+
+<sect id="stdinerr">Error handling and input streams (eg stdin)
+<p>
+
+When the service program is reading from a file descriptor connected
+to the calling side, the fd that the service program refers to a pipe
+set up by <prgn/userv/ and not to the same object as was presented by
+the caller.
+<p>
+
+Therefore if there is some kind of error it is possible for the
+service-side fd to give premature end of file. If it is important to
+tell whether all of the intended data has been received by the service
+program, the datastream must contain an explicit end-of-file
+indication of some kind.
+<p>
+
+For example, consider a <prgn/userv/ service for submitting a mail
+message, where message is supplied on the service's stdin. However,
+if the calling process is interrupted before it has written all of the
+message, the service program will get EOF on the message data. In a
+naive arrangement this would cause a half-complete message to be
+sent. To prevent this, it is necessary to adopt some kind of explicit
+end indication; for example, the end of the message could be signalled
+by a dot on a line by itself, and dots doubled, as in SMTP. Then the
+service program would know when the entire message had been received,
+and could avoid queueing incomplete messages.
<sect id="nogeneral">Don't give access to general-purpose utilities
<p>
~ian / userv.git / blobdiff