Network
interfaces can have one or more addresses associated with each
interface, and each address will have a hostname associated with it.
For example, on a machine with two interfaces, the one connected to
the outside world might have the name
host.your.domain, whereas the interface that is
connected to the internal network might have the name
host.sub.your.domain.
When sendmail first starts up as a listening
daemon, it binds to a port on all interfaces or on a particular
interface (See this section). It then waits to accept
connections from hosts or programs (clients) that wish to route mail
through it.
When a client connects to the local machine,
sendmail records the local IP address of the
connected-to interface in the ${if_addr} macro
(${if_addr}), the family of that address in the
${if_family} macro (${if_family}),
and the name associated with the interface over which the connection
was made in this ${if_name} macro. If the
connection is on the local host's
loopback interface, the
${if_name} macro is undefined.
The ${if_name} macro can be useful when you are
set up to do virtual hosting. You can have
sendmail give its greeting message in a form
that makes it appear to be the host that is associated with the
interface:
LOCAL_CONFIG
define(`confSMTP_LOGIN_MSG', `$?{if_name}${if_name}$|$j$. ESMTP MTA')
Here, we define sendmail's
initial greeting message with the
SmtpGreetingMessage option (SmtpGreetingMessage). It has one of two forms, depending on
whether the ${if_name} contains a value. The
conditional macro $? looks up the value in
${if_name}. If that value is non-NULL, the value
in ${if_name} is printed. Otherwise, (the
$|) the canonical local hostname is printed (the
$j). The $. terminates the if
test, and a literal ESMTP MTA is always printed:
220 virtual.domain ESMTP MTA 
the outside interface
220 host.your.domain ESMTP MTA the loopback interface
${if_name} is transient. If it is defined in the
configuration file or in the command line, that definition can be
ignored by sendmail. Note that a
$& prefix is necessary when you reference this
macro in rules (that is, use $&{if_name}, not
${if_name}).
