User service daemon and client specification - Information passed through the client/daemon combination

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# 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 `execute-from-directory` and `execute-from-path` 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` (or `EPIPE` if `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 `SIGPIPE` or `EPIPE`.

- If `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; if `disconnect-hup` is set then the service will also be sent a `SIGHUP`.

- The value of the `LOGNAME` (or `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 `USERV_USER` and `USERV_UID`. The shell corresponding to that login name (according to the password entry) is available as in the configuration language's `calling-user-shell` parameter. If no relevant password entry can be found then no service will be invoked.

- 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 environment variables. 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 `--hidecwd`, 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 service user.

- 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 variables `USERV_U_name`.

- 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.

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User service daemon and client specification