User service daemon and client specification - chapter 4
Service-side configuration
Which services may be run by whom and under what conditions is
controlled by configuration files.
The daemon will read these files in order. Certain directives in the
files modify the daemon's execution settings for invoking the service,
for example allowing certain file descriptors to be specified by the
client or specifying which program to execute to provide the service.
The last instance of each such setting will take effect. The
directives which specify which program to execute will not stop the
configuration file from being read; they will be remembered and will
only take effect if they are not overridden by a later directive.
The daemon will first read /etc/userv/system.default
. Then, by
default (this behaviour may be modified), it will read a per-user file
~/.userv/rc
, if it exists and the service user's shell is in
/etc/shells
. Finally it will read
/etc/userv/system.override
.
When it has read all of these files it will act according to the
currently values of of the execution settings.
The configuration file is a series of directives, usually one per
line. The portion of a line following a hash character #
is
taken as a comment and ignored. Each directive consists of a series
of tokens separated by linear whitespace (spaces and tabs); tokens may
be words consisting of non-space characters, or, where a string is
required, a string in double quotes. Double-quoted strings may
contain the following backslash escapes:
\n
- newline
\t
- tab
\r
- carriage return
\
OOO
- character whose octal code is OOO
\x
XX
- character whose hex code is XX
\
punctuation
- literal punctuation character (eg
\\
, \"
) \
newline
(ie, backslash at end of line)- string continues on next line
Relative pathnames in directives are relative to the service program's
current directory (usually the service user's home directory).
Pathnames starting with the two characters ~/
are taken to be
relative to the service user's home directory.
The following directives take effect immediately:
cd
pathname
- Change directory in the service program. cd is cumulative. It
is an error if the directory cannot be changed to.
cd should not be used between execute-from-directory and
the invocation of the service program, as the test for the
availability of the service program would be done with the old current
directory and the actual execution with the new (probably causing an
error).
eof
- Stop reading the configuration file in question, as if end of file had
been reached. Any control constructs (if, catch-quit or
errors-push) which were started in that file will be considered
finished. Parsing will continue in the file which caused the file
containing the eof to be read.
quit
- Stop reading configuration files and act immediately on the current
settings. The behaviour of quit is subject to the
catch-quit control construct.
include
filename
include-ifexist
filename
- Read the configuration file filename, and then return to this
file and continue parsing it with the next directive. It is an error
if the file cannot be opened and read, unless include-ifexist
is used and the file does not exist, in which case the directive is
silently ignored.
include-lookup
parameter
directory
include-lookup-all
parameter
directory
- Read the configuration file in directory whose name is the value
of parameter (see the description of if, Control structure directives, subsection 4.2.3). If parameter has several values they will
be tried in order; with include-lookup this search will stop
when one is found, but with include-lookup-all the search will
continue and any files appropriate to other values will be read too.
If none of the parameter's values had a corresponding file then the
file :default
will be read, if it exists. If parameter's
list of values was empty then the file :none
will be tried first
and read if it exists, otherwise :default
will be tried.
It is not an error for any of the files (including :default
) not
to exist, but it is an error if a file exists and cannot be read or if
the directory cannot be accessed.
A translation will be applied to values before they are used to
construct a filename, so that the lookup cannot access dotfiles or
files in other directories: values starting with full stops will have
a colon prepended (making :.
), colons will be doubled, and each
slash will be replaced with a colon followed by a hyphen :-
. A
parameter value which is the empty string will be replaced with
:empty
(note that this is different from a parameter not having
any values).
include-directory
directory
- Read configuration from all files in directory directory which
are plain files whose names consist only of alphanumerics and hyphens
and start with an alphanumeric. They will be read in lexical order.
It is an error for the directory not to exist or for it or any of the
files found not to be read successfully, or for anything with an
appropriate name not to be a plain file or a symbolic link to a plain
file.
error
text ...
- Causes an error whose message includes the descriptive string
text. text may consist of several tokens with intervening
whitespace. The whitespace will be included in the message as found
in the configuration file: all the characters until the end of the
line will be included verbatim, unless they are part of a
double-quoted string, in which case the usual meaning of the string
(i.e., after backslash escape processing) will be used. Comments and
linear whitespace at the end of the line (or just before the comment)
will still be ignored.
message
text ...
- Causes a message including the descriptive string text to be
delivered as if it were an error message, but does not actually cause
an error.
The following directives have no immediate effect, but are remembered
and have an effect on later processing of the configuration files.
user-rcfile
filename
- Specifies that the file filename should be read instead of the
user's
~/.userv/rc
. This does not happen immediately;
instead, the setting is remembered and used after the
system.default configuration file has been read. This
directive has no effect in a user's configuration file or in the
system.override file, as the user's configuration file has
already been found and read by then and will not be re-read.
errors-to-stderr
- Causes error messages to be delivered to the client's stderr.
errors-to-file
filename- Error messages will be written to filename, which will be opened
in the context of and with the privileges of the service user.
errors-to-syslog
[facility [level]]- Error messages will be delivered using syslog. The default
facility is
user
; the default level is error
.
The following directives are used to create control structures. If
the end of the file is encountered before the end of any control
structure which was started inside it then that control structure is
considered finished. This is not an error.
if
condition
elif
condition
else
fi
- Lines following if are interpreted only if the condition is
true. Many conditions are properties of parameter values. Most
parameters have a single string as a value; however, some may yield
zero or several strings, in which case the condition is true if it is
true of any of the strings individually. Parameters are described
below.
The conditions are:
glob
parameter
glob-pattern ...
- The value of the parameter whose name is given matches one of the glob
patterns (anchored at both ends; backslashes can be used to escape
metacharacters).
range
parameter
min
max
- The value of the parameter is a nonnegative integer and lies within
the range specified. min or max may be
$
to indicate
no lower or upper limit, respectively.
grep
parameter
filename
- The filename refers to a file one of whose lines is the value of
the parameter (leading or trailing whitespace on each line and empty
lines in the file are ignored). It is an error for the file not to be
opened and read.
!
condition
- The condition is not true.
- Conjunctions:
&
and |
( condition
& condition
& condition
...
)
is true if all the listed conditions are true; where |
is used it
is true if any of them is true. Newlines must be used to separate one
condition from the next, as shown, and the parentheses are mandatory.
These conjunctions do not do lazy evaluation.
The parameters are:
service
- The service name specified when the client was called.
calling-user
- Two strings: the login name of the calling user (determined as for
USERV_USER, above) and the calling uid (represented in
decimal).
calling-group
- Several strings: the primary and supplementary group names and gids
(in decimal) of the calling process. All the group names come first,
and then the gids. If the first supplementary group is the same as
the primary group then it is elided.
calling-user-shell
- The calling user's shell, as listed in the password entry for the
calling login name (as determined for USERV_USER, above).
service-user
- Two strings: the name of the service user (as specified to the client)
and their uid (represented in decimal).
service-group
- Several strings: the primary and supplementary group names and gids
(in decimal) of the service user.
service-user-shell
- The service user's shell, as listed in their password entry.
u-
name
- The value of the user-defined variable name passed by the caller
using the --defvar command-line option to the client. If the
variable was not defined then this parameter is an empty list of
strings; in this case any condition which tests it will be false, and
include-lookup
on it will read the :none
file, or
:default
if :none
is not found.
errors-push
filenamesrorre
- Stacks the error handling behaviour currently in effect. Any changes
to error handling will take effect only between errors-push and
srorre.
catch-quit
hctac
- Any use of quit inside catch-quit will merely cause the
parsing to continue at hctac instead. Any control constructs
started since the catch-quit will be considered finished if a
quit is found.
If an error occurs inside catch-quit the execution settings
will be reset (as if by the reset directive) and parsing will
likewise continue at hctac.
If a lexical or syntax error is detected in the same configuration
file as the catch-quit, while looking for the hctac
after an error or quit, that new error will not be caught.
The following directives modify the execution settings; the server
will remember the fact that the directive was encountered and act on
it only after all the configuration has been parsed. The last
directive which modifies any particuar setting will take effect.
reject
- Reject the request. execute, execute-from-directory and
execute-from-path will change this setting.
execute
program [
argument ...]
- Execute the program program, with the arguments as specified,
followed by any arguments given to the client if
no-suppress-args is in effect. It is an error for the
execution to fail when it is attempted (after all the configuration
has been parsed). If program does not contain a slash it will
be searched for on the service user's path.
execute-from-directory
pathname [
argument ...]
- Take all the characters after the last slash of the service name
specified when the client was called, and execute that program in the
directory named by pathname as if it had been specified for
execute. The part of the service name used may contain only
alphanumerics and hyphens and must start with an alphanumeric (and it
must be non-empty), otherwise it is an error.
This directive is ignored if the relevant program does not exist in
the directory specified; in this case the program to execute is left
at its previous setting (or unset, if it was not set before).
It is an error for the test for the existence of the program to fail
other than with a `no such file or directory' indication. It is also
an error for the execution to fail if and when it is attempted (after
all the configuration has been parsed).
execute-from-path
- service is interpreted as a program on the default PATH
(or as a pathname of an executable, if it contains a
/
). This
directive is very dangerous, and is only provided to make the
--override options effective. It should not normally be used.
It is an error for the execution to fail when it is attempted (after
all the configuration has been parsed).
execute-builtin
service-name
service-arguments
- Executes the builtin service service-name. These builtin
services display information about the server and/or the request, and
ignore any arguments passed from the service side except possibly to
print them as part of their output. They write their results to their
standard output (i.e., wherever file descriptor 1 is directed). The
builtin services are:
execute
- Displays the execution settings, defined variables,
arguments, etc. with which the builtin service was invoked.
environment
- Displays the environment variable settings with which the builtin
service was invoked.
parameter
parameter
- Displays the values of the service configuration language parameter
specified.
version
- Displays the version string and compilation details of the uservd
server program.
reset
- Displays the default reset configuration (evaluated when reset
is found in a configuration file, or when an error is caught by
catch-quit).
toplevel
- Displays the top-level default configuration (the configuration data,
evaluated by the server, which calls all the other configuration
files).
override
- Displays the top-level override configuration (the configuration data,
evaluated by the server, which causes all the other configuration data
to be parsed).
help
- Displays a list of the understood builtin service names and arguments.
In the future other builtin services may be defined which do more than
just print information.
set-environment
no-set-environment
- Runs
/etc/environment
to set the service user's environment.
This adds the overhead of invoking a shell, but doesn't cause any
shell (de)mangling of the service's arguments. This is achieved by
invoking
.../program arg arg arg ...
as
/bin/sh -c '. /etc/environment; exec "$@"' - .../program arg arg arg ...
no-set-environment cancels the effect of
set-environment.
no-suppress-args
suppress-args
- Include any arguments given to the client as arguments to the program
invoked as a result of an execute,
execute-from-directory or execute-from-path directive.
suppress-args undoes the effect of no-suppress-args.
require-fd
fd-range read|write
- Insist that the filedescriptor(s) be opened for reading resp. writing.
It is an error if any descriptor marked as required when the service
is about to be invoked (after the configuration has been parsed) was
not specified when the client was invoked. Each file descriptor has a
separate setting, and the last one of require-fd,
allow-fd, ignore-fd, null-fd or reject-fd
which affected a particular file descriptor will take effect.
fd-range may be a single number, two numbers separated by a
hyphen, or one number followed by a hyphen (indicating all descriptors
from that number onwards). It may also be one of the words
stdin
, stdout
or stderr
. Open-ended file descriptor
rangers are allowed only with reject-fd and ignore-fd,
as otherwise the service program would find itself with a very large
number of file descriptors open.
When the configuration has been parsed, and before the service is
about to be executed, stderr (fd 2) must be required or allowed
(require-fd or allow-fd) for writing; this is so that
the error message printed by the server's child process if it cannot
exec the service program is not lost.
allow-fd
fd-range [read|write]
- Allow the descriptor(s) to be opened for reading resp. writing, or
either if neither
read
nor write
is specified. If a
particular descriptor not specified by the client then it will be open
onto /dev/null
(for reading, writing, or both, depending on
whether read
, write
or neither was specified).
null-fd
fd-range [read|write]
- Specify that the descriptor(s) be opened onto /dev/null for
reading resp. writing, or both if neither
read
nor write
is specified. Any specification of these file descriptors by the
client will be silently ignored; the client will see its ends of the
descriptors being closed immediately.
reject-fd
fd-range
- Do not allow the descriptor(s) to be specified by the client. It is
an error if any descriptor(s) marked for rejection are specified when
the service is about to be invoked (after the configuration has been
parsed).
ignore-fd
fd-range
- Silently ignore any specification by the client of those
descriptor(s). The pipes corresponding to these descriptors will be
closed just before the service is invoked.
disconnect-hup
no-disconnect-hup
- Causes the service's process group to get a SIGHUP if the
client disconnects before the main service process terminates.
no-disconnect-hup cancels disconnect-hup.
If one of the reading descriptors specified when the client is called
gets a read error, or if the service is disconnected for some other
reason, then the SIGHUP will be delivered before the
writing end(s) of the service's reading pipe(s) are closed, so that
the client can distinguish disconnection from reading EOF on a pipe.
reset
- Resets the execution settings to the default. This is equivalent to:
cd ~/
reject
no-set-environment
suppress-args
allow-fd 0 read
allow-fd 1-2 write
reject-fd 3-
disconnect-hup
If no execute, execute-from-path,
execute-from-directory or builtin is interpreted before
all the files are read then the request is rejected.
If a syntax error or other problem occurs when processing a
configuration file then a diagnostic will be issued, to wherever the
error messages are currently being sent (see the errors- family
of directives, above).
The error will cause processing of the configuration files to cease at
that point, unless the error was inside a catch-quit construct.
In this case the settings controlling the program's execution will be
reset to the defaults as if a reset directive had been issued,
and parsing continues after hctac.
The default configuration processing is as if the daemon were parsing
an overall configuration file whose contents were as follows:
reset
user-rcfile ~/.userv/rc
errors-to-stderr
include /etc/userv/system.default
if grep service-user-shell /etc/shells
errors-push
catch-quit
include-ifexist file specified by most recent user-rcfile directive
hctac
srorre
fi
include /etc/userv/system.override
quit
If one of the --override options to the client is used then it
will instead be as if the daemon were parsing an overall configuration
as follows:
reset
errors-to-stderr
include file containing configuration data sent by client
quit
User service daemon and client specification
- userv is Copyright 1996-1999 Ian Jackson.
Contents; abstract; next; back.
0.62
Ian Jackson ian@davenant.greenend.org.uk