User service daemon and client specification - Chapter 5
Information passed through the client/daemon combination
The information described below is the only information which passes
between the caller and the service.
- The service name supplied by the caller is available in the
configuration language for deciding whether and which service program
to invoke, in the
service parameter, and is used by the
configuration directives. It is usually used to select which service
program to invoke. It is also passed to the service program in the
USERV_SERVICE environment variable.
- File descriptors specified by the client and allowed according to the
configuration language will be connected. Each file descriptor is
opened for reading or writing. Communication is via pipes, one end of
each pipe being open on the appropriate file descriptor in the service
program (when it is invoked) and the other end being held by the
client process, which will read and write files it opens on behalf of
its caller or file descriptors it is passed by its caller.
Data may be passed into the service through reading pipes and out of
it through writing pipes. These pipes can remain open only until the
service and client have terminated, or can be made to stay open after
the client has terminated and (if the service program forks) the main
service process has exited; the behaviour is controlled by options
passed to the client by its caller.
The caller can arrange that a writing pipe be connected to a pipe or
similar object and cause attempts to write to that descriptor by the
service to generate a
SIGPIPE is caught or ignored) in the service.
Likewise, the service can close filedescriptors specified for reading,
which will cause the corresponding filedescriptors passed by the
caller to be closed, so that if these are pipes processes which write
to them will receive
no-suppress-args is set then arguments passed to the client
by its caller will be passed on, verbatim, to the service.
- Fatal signals and system call failures experienced by the client will
result in the disconnection of the service from the client and
possibly some of the communication file descriptors described above;
disconnect-hup is set then the service will also be sent a
- The value of the
USER) environment variable
as passed to the client will be used as the login name of the calling
user if the uid of the calling process matches the uid corresponding
to that login name. Otherwise the calling uid's password entry will
be used to determine the calling user's login name.
This login name and the calling uid are available in the configuration
language in the
calling-user parameter and are passed to the
service program in environment variables
The shell corresponding to that login name (according to the password
entry) is available as in the configuration language's
If no relevant password entry can be found then no service will be
- The numeric values and textual names for calling gid and supplementary
group list are available in the configuration language in the
calling-group parameter and are passed to the service in
If no name can be found for a numeric group to which the calling
process belongs then no service will be invoked.
- The name of the current working directory in which the client was
invoked is passed, if available and not hidden using
to the service program in the
USERV_CWD variable. This grants no
special access to that directory unless it is a subdirectory of a
directory which is executable (searchable) but not readable by the
- Settings specified by the caller using the --defvar
name=value option to the client are available in the
configuration language as the corresponding u-name
parameters and are passed to the service program in environment
- If the calling user is root or the same as the service user then
options may be given to the client which bypass the usual security
features; in this case other information may pass between the caller
and the service.
User service daemon and client specification
Ian Jackson email@example.com