Hey there all,
I'm in the process of vetting historical postfix configs -- comparing so
many things in a historic config where prior employees overrode the
defaults and perhaps why.
Wietse, let me say thank you for making it easier than it would be with
sendmail.cf :)
I see sites like cipherlist.eu suggesting overriding the "medium" cipher
set to only be:
smtpd_use_tls = yes
smtpd_tls_security_level = may
smtpd_tls_mandatory_protocols = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1
smtpd_tls_protocols = !SSLv2,!SSLv3,!TLSv1,!TLSv1.1
smtpd_tls_mandatory_ciphers = medium
tls_medium_cipherlist = EECDH+AESGCM:EDH+AESGCM
tls_preempt_cipherlist = yes
WHich feels like it would break anyone with an older mail server. We
*want* to receive mail from everyone -- since we're saying "may" we
basically default to no cipher if we can't do one. If someone else thinks
ECDHE-RSA-AES128-GCM-SHA256 is enough, I'd rather that be used than go
plaintext.
It also feels like overriding "medium" is a really poor idea here. Like,
if you want optimal security, why wouldn't you override high?
When I look at postconf -d, I see:
tls_export_cipherlist = aNULL:-aNULL:HIGH:MEDIUM:LOW:EXPORT:+RC4:@STRENGTH
tls_high_cipherlist = aNULL:-aNULL:HIGH:@STRENGTH
tls_low_cipherlist = aNULL:-aNULL:HIGH:MEDIUM:LOW:+RC4:@STRENGTH
tls_medium_cipherlist = aNULL:-aNULL:HIGH:MEDIUM:+RC4:@STRENGTH
But can't find how @STRENGTH is evaluated.
So the questions (all inter-related):
1) What does @STRENGTH mean in this context? Where is it defined? It is
some kind of bitstrength equivalence?
2) Will postfix allow you to specify a "custom" cipherlist while leaving
the defaults in, like tls_ourorg_cipherlist?
3) Will the builtin values of these ciphers change over time, for
example to exclude known-broken or known-deprecated ciphers, inline with
RFC's perhaps? (i.e. will "High" change)
4) Is there an equivalent target for tls_protocols, i.e that only reprents
"high" (as in non-broken) protocols? For example, it looks like at some
point postfix stopped doing sslv2 and sslv3 (so the above
smtpd_tls_protocols is already wrong for a modern postfix?), but with
tls10 also known-problematic, will that drop out at some point?
Best,
-Dan
--
--------Dan Mahoney--------
Techie, Sysadmin, WebGeek
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