[Mimedefang] Reverse DNS Testing

[David F. Skoll]

On Wed, 25 Jun 2003, Richard Laager wrote:

> Has anyone here tried setting up MIMEDefang to enforce reverse DNS entries
> for the MAIL FROM: domain, and/or the $RelayAddr?

Sendmail itself normally verifies that the domain has either an MX or an A
record.

You can enforce reverse DNS like this:

sub filter_relay {
	my($hostip, $hostname, $helo) = @_;
	if ($hostname eq "[$hostip]") {
		# NOTE: If lookup failed temporarily, we reject!
		# We should really check for that.
		return('REJECT', "Sorry: $hostip has no reverse DNS");
	}
	return ('CONTINUE', "ok");
}

> Any thoughts on if this
> causes a lot of legitimate mail to be blocked?

Probably.  However, AOL has implemented something like this, so I expect
DNS maintainers to clean up their act, and I'd expect most hosts to
have good reverse DNS within a few months to a year.

--
David.