Postfix: The Definitive Guide

Unix Topics

There’s no question that the more familiar you are with Unix, the better a Postfix administrator you’ll be. Postfix is very much a Unix program working in conjunction with the underlying operating system for many of its functions. If you’re new to Unix, you should study an introductory text. In the meantime, this section presents some fundamental concepts that you will need to understand to follow explanations in the book.

Login Names and UID Numbers

The list of recognized users on a system is stored in the /etc/passwd file. Every user should have a unique login name and user ID number (commonly written as uid or UID). The UID, not the user’s login name, is the important attribute for identity and ownership checks. The login name is a convenience for humans, and the system uses it primarily to determine what the UID is. Some Postfix configuration parameters require UIDs rather than login names when referring to user accounts. Postfix sometimes takes on the identify of different users. A process is said to be using the rights or privileges of that account when assuming its identity.

Pseudo-Accounts

A pseudo-account is a normal Unix system account except that it does not permit logins. These accounts are used to perform administrative functions or to run programs under specific user privileges. Your system most likely came installed with several pseudo-accounts. Account names such as bin and daemon are common ones. Generally, these accounts prevent logins by using an ...