Sendmail v8.12 is the first release of Sendmail to draw a distinction
between the Message Submission Process (MSP) and the Mail Transfer
Agent (MTA) portions of the email equation. Simply put, the MSP is
the procedure used by local processes when they wish to emit email
from the local machine. The MTA is the process which handles receiving
email from other systems and either relaying it onwards or delivering
it locally. In other words, the MTA is the process that listens on
25/tcp and the thing we want to shut off.
The problem is that the default behavior of the MSP is to attempt to
deliver outgoing email by talking to the MTA over the system's
internal loopback interface. This means that if we shut off the local
MTA, then the default MSP will be unable to send email out of the
system! This is not a feature.
It turns out that the MSP doesn't have to emit email by talking to
the local MTA over the loopback interface. It can talk to any mail
server on the network that the administrator desires. In this mode,
the MSP is acting very much like the nullclient configuration that
we discussed in the previous section. The macro configuration file
is only slightly more complicated:
include(`cf/m4/cf.m4')
define(`confCF_VERSION', `Submit')
define(`__OSTYPE__', `')
define(`confTIME_ZONE', `USE_TZ')
define(`confDONT_INIT_GROUPS', `True')
FEATURE(`msp', `mailhost')
Again, set the pathname on the first line and the name of the machine
on the last line as appropriate for your site. The configuration
file produced from the above macros should be installed as
/etc/mail/submit.cf on your systems.
Note that you can also simply hack the
submit.cf file
provided by your vendor. Just look for the line that reads
D{MTAHost}[127.0.0.1]
Just replace
[127.0.0.1] with the name of your new
relay host. For example,
D{MTAHost}mailhost
Save your changes, and you're done!
The split between MTA and MSP is also reflected in how the Sendmail
daemon is started at boot time. When using Sendmail v8.12, two daemons
are normally started by the system boot scripts:
/usr/sbin/sendmail -bd -q15m
/usr/sbin/sendmail -Ac -q15m
The first line looks identical to the normal Sendmail invocation for
Sendmail v8.11 and earlier-- this is the MTA process that is listening
on 25/tcp for incoming email, and which processes a queue of messages
received by this process. The second line invokes a daemon for the MSP.
This daemon doesn't listen on any port for incoming messages: its sole
duty is to process a separate MSP queue for messages generated on the
local machine but which could not be delivered immediately because the
MSP's mail relay was unavailable for some reason.
So in v8.12 Sendmail, the MSP has its own message queue and its own
daemon for flushing that queue on a regular basis. This means that on
email "client" type machines the MTA daemon is completely unnecessary
and can be shut off completely. In order to get this to happen,
however, it will probably be necessary to hack the default boot script
provided by your vendor. I have yet to find a vendor who has a simple
configuration switch for disabling the MTA process without disabling
the MSP. In fact, most vendors in my experience don't even realize
that you can run the MSP without a local MTA.
