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spawn - Postfix external command spawner
spawn [generic Postfix daemon options] command_attributes...
The spawn daemon provides the Postfix equivalent of inetd. It listens on a port as speci-
fied 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.
COMMAND ATTRIBUTE SYNTAX
The external command attributes are given in the master.cf file at the end of a service
definition. The syntax is as follows:
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 of username.
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 charac-
ters by a shell command interpreter.
In order to enforce standard Postfix process resource controls, the spawn daemon runs only
one external command at a time. As such, it presents a noticeable overhead by wasting
precious process resources. The spawn daemon is expected to be replaced by a more struc-
The spawn 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 daemon does not talk to the
external command and thus is not vulnerable to data-driven attacks.
The following main.cf parameters are especially relevant to this program. See the Postfix
main.cf file for syntax details and for default values. Use the postfix reload command
after a configuration change.
List of names of environment parameters that can be exported to non-Postfix pro-
The process privileges used while not running an external command.
The amount of time the command is allowed to run before it is killed with force.
The service name is the name of the entry in the master.cf file. The default time
limit is given by the global command_time_limit configuration parameter.
master(8) process manager
syslogd(8) system logging
The Secure Mailer license must be distributed with this software.
IBM T.J. Watson Research
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