Beginning with Red Hat Linux 7.3, which was the first version to offer the alternatives feature,
two major services were configured to use alternatives: mail transport and printing services.
The alternatives facility let system administrators choose the following, related to mail transport and printing:
- Mail Transport Agent (MTA) : If the sendmail, exim and postfix mail transport agents are installed, as an administrator you can choose which of those services is the default for sending and receiving e-mail.
- Printing: If both LPRng and CUPS printing services are installed, you can choose which service is the default for printing documents.
As an administrator, you still need to configure each alternative service to work.
LPRng is no longer delivered with Fedora or RHEL, although it is still available from sites such as http://rpmfind.net/linux/RPM/.
Likewise, the feature for switching the printing service described below is not included with Fedora or RHEL, but can still be found in earlier Red Hat Linux systems.)
In terms of setting up the alternatives side of mail services, much of the work of creating links so that the services can be chosen has already been done. Links relating to the default services are set up in the /etc/alternatives directory. Definitions that identify the alternative
components of sendmail, exim, and postfix mail servers are contained in the
/var/lib/alternatives directory.
Because much of the configuration has been done in advance, the first step in switching
between the different mail services installed on your computer is only a couple of clicks away.
