Channel definitions appear in the lower half of the MTA configuration file, imta.cnf, following the rewrite rules (see 10.2 The MTA Configuration File rules section and the start of the channel definitions.
A channel definition contains the name of the channel followed by an optional list of keywords that define the configuration of the channel, and a unique channel tag, which is used in rewrite rules to route messages to the channel. Channel definitions are separated by single blank lines. Comments, but no blank lines, may appear inside a channel definition.
[blank line] ! sample channel definition Channel_Name keyword1 keyword2 Channel_Tag [blank line]
Collectively, the channel definitions are referred to as the channel host table. An individual channel definition is called a channel block. For example, in the example below, the channel host table contains three channel definitions or blocks.
! test.cnf - An example configuration file. ! ! Rewrite Rules . . . ! BEGIN CHANNEL DEFINITIONS ! FIRST CHANNEL BLOCK l local-host ! SECOND CHANNEL BLOCK a_channel defragment charset7 usascii a-daemon ! THIRD CHANNEL BLOCK b_channel noreverse notices 1 2 3 b-daemon
A typical channel entry will look something like this:
tcp_intranet smtp mx single_sys subdirs 20 noreverse maxjobs 7 SMTP_POOL maytlsserver allowswitchchannel saslswitchchannel tcp_auth tcp_intranet-daemon
The first word, in this case tcp_intranet, is the channel name. The last word, in this case tcp_intranet-daemon, is called the channel tag. The channel tag is the name used by rewrite rules to direct messages. The words in between the channel name and channel tag are called channel keywords and specify how the message is to be processed. Hundreds of different keywords allow messages to processed in many ways. A complete listing of channel keywords is listed and described in Chapter 12, Configuring Channel Definitions
The channel host table defines the channels Messaging Server can use and the names of the systems associated with each channel.
On UNIX systems, the first channel block in the file always describes the local channel, l. (An exception is a defaults channel, which can appear before the local channel.) The local channel is used to make routing decisions and for sending mail sent by UNIX mail tools.
You can also set global options for channels in the MTA Option file, option.dat, or set options for a specific channel in a channel option file. For more information on the option files, see 10.4.6 Option File, and 10.4.2 TCP/IP (SMTP) Channel Option Files. For details on configuring channels, see Chapter 12, Configuring Channel Definitions. For more information about creating MTA channels, see 10.2 The MTA Configuration File.