[ExchangeList] Fwd: Postfix/Exchange TLS interoperability

  • From: Danny <nocmonkey@xxxxxxxxx>
  • To: exchangelist@xxxxxxxxxxxxx
  • Date: Tue, 24 Oct 2006 14:38:14 -0400

Any Microsoft Exchange employees monitoring this list? If so, you may want to pay attention to this thread:
<http://archives.neohapsis.com/archives/postfix/2006-10/thread.html#1497 >

A preview of the latest response from Wietse...

---------- Forwarded message ----------
From: Wietse Venema
Date: Oct 24, 2006 2:27 PM
Subject: Re: Postfix/Exchange TLS interoperability
To: postfix-users@xxxxxxxxxxx

Victor Duchovni:
> On Mon, Oct 23, 2006 at 10:00:50PM -0400, Victor Duchovni wrote:
> > There was a recent report of interoperability issues between Exchange
> > acting as TLS client, and Postfix acting as a TLS server. I have just
> > experienced the converse problem, and the packet capture shows severe
> > breakage on the Exchange side...
> I have found that this happens when the Postfix SMTP client insists on
> "HIGH" grade ciphers, causing Exchange to negotiate DES-CBC3-SHA, which
> it then gets wrong.
> When the Postfix SMTP client proposes both "HIGH" and "MEDIUM" strength
> ciphers (in that order) the Exchange server overrides the client cipher
> preference and elects RC4-MD5, which does not exhibit the problem.
> So the trick with at least some versions of the Exchange software is to
> allow Exchange to select the RC4-MD5 combination despite the fact that
> both are deprecated (RC4 has a poor key-schedule, and MD5 is no longer
> collision resistant).

Is the Postfix default setting OK for Exchange?

- If yes, then we need to add a warning to Postfix documentation
so that people know bad things may happen when they make a change.

- If no, then we may have ti change the default. The primary purpose
of Postfix is to deliver mail, not to satisfy purists.


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