Tutorials Point

  Unix for Beginners
  Unix Shell Programming
  Advanced Unix
  Unix Useful References
  Unix Useful Resources
  Selected Reading

Copyright © 2014 by tutorialspoint

  Home     References     Discussion Forums     About TP  

spawn - Unix, Linux Command

previous next AddThis Social Bookmark Button



spawn - Postfix external command spawner


spawn [generic Postfix daemon options] command_attributes...


The spawn(8) daemon provides the Postfix equivalent of inetd. It listens on a port as specified in the Postfix master.cf file and spawns an external command whenever a connection is established. The connection can be made over local IPC (such as UNIX-domain sockets) or over non-local IPC (such as TCP sockets). The command's standard input, output and error streams are connected directly to the communication endpoint.

This daemon expects to be run from the master(8) process manager.


The external command attributes are given in the master.cf file at the end of a service definition. The syntax is as follows:
user=username (required)
user=username:groupname The external command is executed with the rights of the specified username. The software refuses to execute commands with root privileges, or with the privileges of the mail system owner. If groupname is specified, the corresponding group ID is used instead of the group ID of username.
argv=command... (required) The command to be executed. This must be specified as the last command attribute. The command is executed directly, i.e. without interpretation of shell meta characters by a shell command interpreter.


In order to enforce standard Postfix process resource controls, the spawn(8) daemon runs only one external command at a time. As such, it presents a noticeable overhead by wasting precious process resources. The spawn(8) daemon is expected to be replaced by a more structural solution.


The spawn(8) daemon reports abnormal child exits. Problems are logged to syslogd(8).


This program needs root privilege in order to execute external commands as the specified user. It is therefore security sensitive. However the spawn(8) daemon does not talk to the external command and thus is not vulnerable to data-driven attacks.


Changes to main.cf are picked up automatically as spawn(8) processes run for only a limited amount of time. Use the command "postfix reload" to speed up a change.

The text below provides only a parameter summary. See postconf(5) for more details including examples.

In the text below, transport is the first field of the entry in the master.cf file.


transport_time_limit ($command_time_limit) The amount of time the command is allowed to run before it is terminated.


config_directory (see ’postconf -d’ output) The default location of the Postfix main.cf and master.cf configuration files.
daemon_timeout (18000s) How much time a Postfix daemon process may take to handle a request before it is terminated by a built-in watchdog timer.
export_environment (see ’postconf -d’ output) The list of environment variables that a Postfix process will export to non-Postfix processes.
ipc_timeout (3600s) The time limit for sending or receiving information over an internal communication channel.
mail_owner (postfix) The UNIX system account that owns the Postfix queue and most Postfix daemon processes.
max_idle (100s) The maximum amount of time that an idle Postfix daemon process waits for the next service request before exiting.
max_use (100) The maximal number of connection requests before a Postfix daemon process terminates.
process_id (read-only) The process ID of a Postfix command or daemon process.
process_name (read-only) The process name of a Postfix command or daemon process.
queue_directory (see ’postconf -d’ output) The location of the Postfix top-level queue directory.
syslog_facility (mail) The syslog facility of Postfix logging.
syslog_name (postfix) The mail system name that is prepended to the process name in syslog records, so that "smtpd" becomes, for example, "postfix/smtpd".


postconf(5), configuration parameters
master(8), process manager
syslogd(8), system logging


The Secure Mailer license must be distributed with this software.


Wietse Venema
IBM T.J. Watson Research
P.O. Box 704
Yorktown Heights, NY 10598, USA

previous next Printer Friendly