On Fri, May 05, 2023 at 01:57:19PM +1000, Sean Gallagher via Postfix-users 
wrote:

> > This is rarely what you want.  I'd be inclined to require that the
> > "inet_interfaces" parameter be non-empty (though it could still be
> > effectively empty as a list by setting it to be a mixture of spaces and
> > at least one comma).
>
> You need to be careful what "empty" means. If inet_interfaces has only 
> IPv4 addresses and inet_protocols includes "ipv6", then it is 
> effectively "empty" from the ipv6 point of view, but it is clearly not 
> "empty"

That's a non-issue.  With that, Postfix will only listen on IPv4 as
specified, when the "inet" endpoint only specifies the port.

In other words:

    - The *primary* purpose of "inet_interfaces" is to specify the
      listen IP address for services with no explicit hostname or
      ip address specified.

    - A secondary, convenience function, is that the same addresses,
      when exactly one non-loopback, per address family, are also
      used as the default bind address for the associated family.

An empty setting is fine for outbound connections (no fixed address),
but not so good for inbound services unless you really mean to always
use explicit hostname/ip prefixes for all "inet" services and want
"postfix start" to fail if any are lax.

-- 
    Viktor.
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