controlling STARTTLS by IP address

[M. Balridge]

Quoting Michael Fox <news at mefox.org>:

> > Seems like your firewall could redirect to a different port that doesn't
> > offer starttls.
> 
> Yes, of course.  But that would require multiple ports, making the client
> configuration cumbersome and error-prone.

It looks like there's an internal Dovecot solution, so all's well. 

I just thought to remind people that with some firewalls, there's always a way
to perform "silent" redirections using the DNAT target in the PREROUTING
table, i.e.,:

-t nat -A PREROUTING -i ${EXTIF} -s ${NOTLSSOURCES} -p tcp --dport 110 \
 --syn -j DNAT --to-destination ${DOVECOT}:${NOTLSPOP3PORT}

If you're using a Linux iptables firewall, you wouldn't need to expose the
different port to the client, but would make use of the NAT subsystem to
redirect the connection from certain IP#s->POP3 to the service port where
you've denied TLS.

No client would need to be made aware of the "secret" ${NOTLSPOP3PORT}, and in
fact, the firewall would continue to DROP packets sent to it from elsewhere if
you have a default-deny policy in effect.

=R=