<html> <head> <title>Postfix Anatomy - Behind the Scenes</title> </head> <body> <h1><a href="big-picture.html"><img src="small-picture.gif" width="115" height="45"></a> Postfix Anatomy - Behind the Scenes</h1> <hr> <a href="index.html">Up one level</a> | <a href="receiving.html">Receiving Mail</a> | <a href="delivering.html">Delivering Mail</a> | Behind the Scenes | <a href="commands.html">Command-line Utilities</a> <p> The previous sections gave a simplified overview of how the Postfix system sends and receives mail. Several other things happen behind the scenes. Unfortunately, this is hard to visualize on a two-dimensional display, so this document has no illustration. <ul> <li>The <a href="master.8.html">master</a> daemon is the supervisor process that keeps an eye on the well-being of the mail system. It is typically started at system boot time by the <a href="postfix.1.html">postfix</a> command, and keeps running until the system goes down. The <a href="master.8.html">master</a> daemon is responsible for starting all other Postfix daemon processes on demand, and for restarting daemons that terminated prematurely because of some problem. The <a href="master.8.html">master</a> daemon is also responsible for enforcing the daemon process count limits as specified in the <b>master.cf</b> configuration file. <p> <li>The <a href="bounce.8.html">bounce or defer</a> daemon is called upon left and right by other daemon processes, in order to maintain per-message log files with non-delivery status information. <p> <li>The <a href="trivial-rewrite.8.html">trivial-rewrite</a> daemon is called upon left and right by other daemon processes, in order to rewrite an address to <i>user@fully.qualified.domain</i> form, or in order to resolve a destination. <p> <li>The <a href="showq.8.html">showq</a> daemon lists the Postfix queue status. This is the program behind the <a href="mailq.1.html">mailq</a> command. <p> <li>The <a href="flush.8.html">flush</a> daemon improves the performance of the SMTP <b>ETRN</b> request, and of its command-line equivalent, <b>sendmail -qR</b><i>destination</i>, for selected destinations. <p> <li>The <a href="proxymap.8.html">proxymap</a> daemon provides read-only lookup service to Postfix client processes. The purpose is to overcome chroot restrictions, and to consolidate the number of open lookup tables by sharing one open table among multiple processes. <p> <li>The <a href="spawn.8.html">spawn</a> daemon listens on a TCP port, UNIX-domain socket or FIFO, and runs non-Postfix commands on request, with the socket or FIFO connected to the standard input, output and error streams. It is currently used only in an example of the Postfix external content filtering system. </ul> <hr> <a href="index.html">Up one level</a> | <a href="receiving.html">Receiving Mail</a> | <a href="delivering.html">Delivering Mail</a> | Behind the Scenes | <a href="commands.html">Command-line Utilities</a> </body> </html>