On Thu, Oct 03, 2013 at 02:48:37PM -0400, micah wrote:
> Regarding tighter mandatory parameters on the submission port - any idea
> what these could reasonably be? For example, if I disable SSLv2/v3 am I
> going to cut off Outlook users?
With Postfix SSLv2 is off by default in the SMTP and LMTP clients.
The Postfix SMTP server accepts SSLv2 by default. Other clients
should likewise disable SSLv2. If you want, you can disable it in
the server. This almost certainly makes no difference.
> I looked at some of my logs and found the following from a small sample
> over the last day:
No SSLv2 as expected, but a good deal of SSLv3. You should break
this down by submission vs. port 25 traffic. There is negligible
benefit from disabling SSLv3 at this point, you gain TLSv1 extensions
on the first client SSL HELLO, but at this time these don't yield
substantive additional security.
If you have opportunistic TLS enabled in both directions, you're
ahead of the pack. Beyond that, secure your end-point systems,
routers, firewalls, ...
> but...the way this works: the server gets offered a list of ciphersuites
> from the client, and then the server picks a ciphersuite, so without
> knowing how the server picks its ciphersuites from the client, these
> results are not clear.
By default the server picks the client's most preferred cipher that
is also available on the server. You can set "tls_preempt_cipherlist
= yes" to have the server use its most preferred cipher supported
by the client. This could break some fragile clients that offer
ciphers (at a low preference) whose implementation is broken.
> So that leaves us with the unanswered question of what does that
> translate into for restricting those paramters on the server?
Postfix defaults are chosen carefully, and you'll get very little
mileage from changing them. You're likely spending time on the
wrong problem. The real issues are not the crypto settings, rather:
- Most SMTP clients don't do TLS and sent plaintext.
- Even when TLS is used, it is typically unauthenticated.
All the crypto algorithm issues pale in comparison. Therefore, if
you want more secure SMTP, migrate to DNSSEC and publish TLSA RRs.
Then wait for the world to migrate to DANE-capable SMTP clients.
Postfix is leading the pack, Exim will likely be a year behind, and
perhaps if we're lucky one of the larger cloud providers will adopt
DANE at some point.
> On a slightly other subject, I know that the smtps port 465 has already
> been reallocated as a port number, because it is considered deprecated,
> but I dont understand why.
Because that's the standard. With "smtpd_tls_security_level =
encrypt" set for the submission service, it really does not matter
which protocol wraps which.
--
Viktor.
