Re: recommend TLS settings

[Jonathan Sélea]

Thanks for very good information and some good criticism!
I have taken those into consideration and adjusted it accordingly.





Thanks again Viktor



On 12/04/2017 03:24 PM, Viktor Dukhovni wrote:

Regarding "smtpd_tls_mandatory_ciphers=high", I use port 587 alot. I dont know if that makes any diffrence. Otherwhise, I do agree with you that tools like "hardenize" is made by "encryption zealots" (I like that name btw) that "does not understand who don't understand opportunistic TLS".

On Dec 4, 2017, at 8:22 AM, Jonathan Sélea <jonat...@selea.se> wrote:

I recently stumbled upon hardinze too, and came up with this config that makes the checks 
"all green".

Green per some poorly designed checklist is not necessarily better.

smtpd_tls_protocols = !SSLv2 !SSLv3
smtpd_tls_mandatory_protocols = !SSLv2 !SSLv3
smtp_tls_protocols = !SSLv2 !SSLv3
smtp_tls_mandatory_protocols = !SSLv2 !SSLv3
lmtp_tls_protocols = !SSLv2 !SSLv3
lmtp_tls_mandatory_protocols = !SSLv2 !SSLv3

These are fine and default in all supported Postfix versions.

smtpd_tls_mandatory_ciphers=high

This may be counter-productive.  You're forcing peers that
only do RC4 to send in the clear instead.  Probably not a
win, and with peers that can do HIGH ciphers, you get HIGH
anyway.  On the other hand support for only RC4 or 3DES
(now medium in some newer OpenSSL versions) is rare, most
peers will support AES, and yet "medium" is still a better
choice for opportunistic TLS.

tls_high_cipherlist=EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:AES256-SHA:CAMELLIA128-SHA:AES128-SHA

Don't hand-order your ciphers.  Why CAMELLIA above AES for example?
And what happens when newer ciphers show up?

smtpd_tls_exclude_ciphers = aNULL, eNULL, EXPORT, DES, RC4, MD5, PSK, aECDH, 
EDH-DSS-DES-CBC3-SHA, EDH-RSA-DES-CDC3-SHA, KRB5-DE5, CBC3-SHA, CAMELLIA, SEED, 
3DES, AES128-GCM-SHA256, AES256-GCM-SHA384, AES128-SHA256, AES256-SHA256, 
AES256-SHA, AES128-SHA

Excluding aNULL on a port 25 server is mostly counter-productive:

    https://tools.ietf.org/html/rfc7672#section-8.2

ditto with DES3 and RC4, just put them at the end.  Funny you put
CAMELLIA first on your cipher list, and then exclude it!  And
then exclude a bunch of strong AES ciphers.  This clearly makes the
point that such guides and efforts to get to "green" are a bad idea.

smtpd_tls_eecdh_grade=ultra

Actually "auto" is a much better setting in Postfix versions that
support it.

tls_preempt_cipherlist = yes

This is typically OK, though in your case, the client's settings
might be more sensible. :-(

tls_eecdh_strong_curve = prime256v1
tls_eecdh_ultra_curve = secp384r1

These are defaults.

smtpd_tls_dh512_param_file = /etc/ssl/dh512_param.pem
smtpd_tls_dh1024_param_file = /etc/ssl/dh2048_param.pem

This is fine, provided the names match the content.  Note that
with EXPORT ciphers disabled the 512-bit parameters will never
be used.

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