On Fri, Dec 28, 2001 at 09:58:06AM +0800, Suresh Ramasubramanian wrote:
> +++ Matthew Byng-Maddick [exim-users] <27/12/01 22:51 +0000>:
> > My argument is that, just because a host says in the EHLO banner line
> > 250-STARTTLS that you shouldn't assume you can do TLS to it. The reason
> > for this is that TLS includes all sorts of other requirements which cannot
> > be communicated in-band in SMTP, for example client certificates. Therefor
> > an outbound mailer *SHOULD NOT* be configured to do TLS to any host that
> > advertises the STARTTLS feature.
> I disagree. You mean that
> 1. Either your host should be psychic
Of course not. Don't be ridiculous. In the rest of my message, I talked
about agreement.
> or
> 2. You compile an (ever growing) list of hosts to which you _should_ do TLS
This is exactly what I mean, and, in this case, presumably with what
connection parameters, i.e. client certificates or equivalent.
> Simpler to specifically exempt those which do offer STARTTLS but implement it
> brokenly, as exim does now.
Well, so you're going to compile an exemption list which includes every
exim compiled with TLS? This is a fantastic idea, I don't know why I
didn't think of it. The point about the rest of the network still stands,
however. This technique of using STARTTLS doesn't ensure privacy, and
anyone who claims it does is, IMNSHO, being a complete and utter clueless
dork.
If your mailserver offers TLS, does it check the CA on the certificates?
If not, how does it stop an MitM attack?
> > Your argument, that any host which advertises TLS should allow me to
> > establish a TLS connection is rather like saying "just because a host
> > allows me to do SMTP AUTH, I should send it my username and password,
> > just in case".
> So, taking that argument ahead, even if a host advertises ESMTP in its
> banner, you'd send a HELO and talk SMTP to it?
If you read up about ESMTP I think you may be surprised to find that that
is not how it works. As, I'm sure, you well know, the correct way to do
ESMTP is to try the EHLO and see if you get back a command unrecognised
response. If you do, then you say HELO and you fallback to normal SMTP.
The EHLO response line is quite specific, but the fact remains that you
can't communicate, what is, essential out-of-band data about establishing
a TLS connection, so just knowing that the host implements TLS and the
STARTTLS command isn't enough.
As I've said before, if you want to capture the mail, there are generally
enough other ways, that it doesn't guarantee any sort of privacy.
MBM
--
Matthew Byng-Maddick <mbm@???> http://colondot.net/
