Figure 3-1 “Milter Call Order Sequence” illustrates the
sequence in which the filter applications are called in a Milter
program.
A filter application cannot process any message untill it
registers its callbacks with Sendmail.
A filter application initiates a filter session using the smfi_setconn() API. The filter application initiates the session
once and before calling the smfi_main() API.
The smfi_setconn() API sets the socket through which the filter application
communicates with Sendmail. The filter application can optionally
call the smfi_opensocket() API, which attempts to open the specified socket
and ensures that the interface works properly. The filter application
can also make optional calls to smfi_settimeout(), smfi_setbacklog(), and smfi_setdbg() before passing control to the smfi_main() API. After initiating the session and calling
the optional APIs, the filter application must register with Sendmail,
using the smfi_register() API, which informs Sendmail about the filter callbacks
and the actual information the filter application requires. The filter
application then passes control to the smfi_main() API.
The smfi_main() API starts the listener for the filter application
and seeks for messages from Sendmail. The smfi_main() API makes respective calls to the callback functions
before validating the message. For example, during a HELO message, smfi_main() invokes the filter callback xxfi_helo().
