12.4.3.10 Target Host Choice

Keywords: daemon, multiple, single, single_sys

The interpretation and usage of the daemon keyword depends upon the type of channel to which it is applied.

The daemon keyword is used on SMTP channels to control the choice of a target host.

Normally, channels connect to whatever host is listed in the envelope address of the message being processed. The daemon keyword is used to tell the channel to instead connect to a specific remote system, generally a firewall or mail hub system, regardless of the envelope address. The actual remote system name should appear directly after the daemon keyword, as shown in the following example:

tcp_firewall smtp mx daemon firewall.acme.com
TCP-DAEMON

If the argument after the daemon keyword is not a fully qualified domain name, the argument will be ignored and the channel will connect to the channel's official host. The official host is the fully qualified hostname associated with the channel. This can be specified in the second line of a three line channel block:

tcp_firewall smtp mx daemon router
firewall.acme.com
TCP-DAEMON

The official host can also be specified after TCP-DAEMON in a two-line channel block so outbound connections will identify themselves as a particular host:

tcp_firewall smtp mx daemon router
TCP-DAEMON firewall.acme.com

When specifying the firewall or gateway system name as the official host name, the argument given to the daemon keyword is typically specified as router, as shown in the following example:

tcp_firewall smtp mx daemon router
firewall.acme.com
TCP-DAEMON

Messaging Server allows multiple destination addresses to appear in each queued message. Some channel programs, however, may only be able to process messages with one recipient, or with a limited number of recipients, or with a single destination system per message copy. For example, the SMTP client programs for the TCP/IP channels only establish a connection to a single remote host in a given transaction, so only addresses to that host can be processed (this despite the fact that a single channel is typically used for all TCP/IP traffic).

Other keywords of interest are multiple, single and single_sys. The multiple keyword, the default, creates a single copy of the message for the entire channel. The single keyword specifies that a separate copy of the message should be created for each destination address on the channel. The single_sys keyword creates a single copy of the message for each destination system used. Note that at least one copy of each message is created for each channel the message is queued to, regardless of the keywords used.