From: Wietse Venema (wietseporcupine.org)
Date: Fri Dec 01 2006 - 08:03:01 CST

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Joey:
> If I put 100 for maxproc under smtp, this will control the MAX connections
> to the smtp port at any given time right?
>
> # ==========================================================================
> # service type private unpriv chroot wakeup maxproc command + args
> # (yes) (yes) (yes) (never) (100)
> # ==========================================================================
> 26 inet n - n - - smtpd
> smtp inet n - n - 100 smtpd
>
> What happens when the 101st connection is attempted, does it get server
> busy, and then retry?

Different things happen at different protocol levels.

At the TCP level, there is no difference. The connection
completes as usual.

At the SMTP level, things do change. There is no server "220 my.host.name"
greeting until an smtpd process becomes available.

However, if you have 100s or 1000s of clients, the TCP behavior
changes too. The connection no longer completes; the clients sees
a TCP timeout. I haven't yet implemened an "overflow responder"
that you can specify in master.cf.

> How can I tell what the peak amount of connections are over the course of a
> day?

The anvil service logs some peak information.

Postfix 2.3 and later log a warning when all ports are busy.

    warning: service "smtp" (25) has reached its process limit
    "100": new clients may experience noticeable delays

> Does this 100 consider BOTH incoming smtp as well as outgoing smtp?

This seting is for smtpd, which is used for receiving mail.

        Wietse

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