The -bs command-line
switch causes sendmail to run a single SMTP
session in the foreground over its standard input and output, and
then exit. The SMTP session is exactly like a network SMTP session.
Usually, one or more messages are submitted to
sendmail for delivery.
This mode is intended for use at sites that wish to run
sendmail with the inetd(8)
daemon. To implement this, place an entry such as the following in
your inetd.conf(5) file, then restart
inetd(8) by killing it with a SIGHUP signal:
smtp stream tcp nowait root /usr/sbin/sendmail sendmail -bs
With this scheme it is important to either use
cron(3) to run sendmail
periodically to process its queue:
0 * * * * /usr/sbin/sendmail -q
or run sendmail in the background to process the
queue periodically by specifying an interval to the
-q command-line switch's interval
(Section 11.8.1):
/usr/sbin/sendmail -q1h
There are advantages and disadvantages to using
inetd(8) instead of the -bd
daemon mode to listen for and process incoming SMTP messages. The
advantages are the following:
At security-conscious sites, sendmail can be
hidden behind a tcpd(8) or
miscd(8) wrapper that can selectively accept or
reject connections. (But see TCPWRAPPERS in TCPWRAPPERS for a way to include this support directly
inside sendmail.)
At hosts that receive few incoming mail messages, this mode avoids
the need to run a daemon.
The disadvantages are the following:
At sites that receive many incoming mail messages, this mode causes a
new sendmail process to be started for each
connection. Compared to daemon mode, this can adversely affect system
performance.
At highly loaded sites, with older versions of
inetd(8), this mode circumvents the
sendmail program options that are intended to
avoid overloading the system with too many concurrent
sendmail processes.
In general, the inetd(8) approach should be used
only on lightly loaded machines that receive few SMTP connections.
The -bs switch is also useful for MUAs that prefer
to use SMTP rather than a pipe to transfer a mail message to
sendmail. Depending on how it is configured,
mh(1) can use this feature.