MTA preferences for use with MailScanner

[Mariano Absatz]

On Thu, 29 Jul 2004 14:48:37 +0200, David H. <dh at uptime.at> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: RIPEMD160
>
> Mariano Absatz wrote:
>
> | I only use ZMailer and wouln't recommend that to anyone not willing to
> | spend a LOT of time learning.
> |
> | I don't like Sendmail very much, it's a memory hog if you get lots of
> | simultaneous connections (but it's really fast when you don't).
> |
> Could you (maybe off list) tell me how you came to that conclusion? I am
> curious, because we do lots of high volume sendmail installs and with
> recent releases we cannot really relate to the issue you describe. I
> would be very interested in exchanging your experiences and integrating
> them into my knowlegde portfolio :)

No... (that is, you shouldn't integrate at least THIS comments in your
portfolio)... :-)

I thought I had said so and rereading my message I didn't... I used
Sendmail in pre-history and got away from it in prehistory (passed
thru smail and got to zmailer eons ago)...

However, at the time, and I think _this_ bit didn't change, sendmail
binary is just one large monolithic program that does most everything
an MTA is supposed to do:
1) smtp server
2) mail routing, including alias expansion, header rewriting and the like
3) smtp client
I don't know about the old protocols (uucp, bitnet, etc)... are they
still integrated in sendmail binary? I guess so, but I don't know.

Now, when a connection is received on port 25 the sendmail listening
there forks a (large) child that handles the smtp server part, then,
unless it's configured otherwise, routes the message and tries to send
it to its next hop... if it fails to do this last step, it lets... the
message queued for later retry...

In a MailScanner configuration, sendmail listening on port 25 is
configured only for receiving the message and queing it for later
processing (which will eventually happen AFTER MailScanner does its
job).

But, nevertheless, the full sendmail binary is spawned for every
connection received... when you get a couple of hundreds of
simultaneous connections, this gets kinda out of hand.

Most modern (and not so modern) MTAs (including zmailer, qmail, exim
and postfix, AFAIK) do have a very small 'smtp server' module that the
ONLY thing it knows how to do is to play the 'server' part of RFC821
and has a set of configurations or policies about what to accept and
what NOT to accept, but it usually doesn't know how to route a
message, how to expand an alias, how to deliver a local message and,
much less, how to play the 'client' part of RFC821 or how to handle
retries...

This usually scales much better when you get lots of simultaneous connections.

OTOH, sendmail approach is usually MUCH faster in processing a SINGLE
message when everything's fine (the message comes in and goes out or
is locally delivered in a single pass).

PLEASE correct me if sendmail changed its overall architecture and
this is no longer true (so I stop talking this nonsense)

As I said... this doesn't even accout as 2p :-)

Regards.


--
Mariano Absatz - El Baby
el (dot) baby (AT) gmail (dot) com
el (punto) baby (ARROBA:@) gmail (punto) com

-------------------------- MailScanner list ----------------------
To leave, send    leave mailscanner    to jiscmail at jiscmail.ac.uk
Before posting, please see the Most Asked Questions at
http://www.mailscanner.biz/maq/     and the archives at
http://www.jiscmail.ac.uk/lists/mailscanner.html