Ssl – How to force a own set of ciphers in Postfix 2.11

opensslpostfixssltls

I would like to force a own set of TLS cipher suites rather than use the Postfix built in ones.

My desired set of ciphers is (taken from nginx configuration):

ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:-DES:!RC4:!MD5:!PSK:!aECDH:EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA

Unfortunately I cannot find a reference to override the cipher suites. I found notices that it is possible but not how.

How would look like the equivalent Postfix configuration for smtp and smtpd?

Using Debian/7, Postfix/2.11.2, OpenSSL/1.0.1e

Best Answer

From Applied Crypto Hardening by bettercrypto.org:

smtpd_tls_security_level = may
smtp_tls_security_level = may
smtp_tls_loglevel = 1
# if you have authentication enabled, only offer it after STARTTLS
smtpd_tls_auth_only = yes
tls_ssl_options = NO_COMPRESSION
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_mandatory_ciphers=high
tls_high_cipherlist=EDH+CAMELLIA:EDH+aRSA:EECDH+aRSA+AESGCM:EECDH+aRSA+SHA384:EECDH+aRSA+SHA256:EECDH:+CAMELLIA256:+AES256:+CAMELLIA128:+AES128:+SSLv3:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!DSS:!RC4:!SEED:!ECDSA:CAMELLIA256-SHA:AES256-SHA:CAMELLIA128-SHA:AES128-SHA

[UPDATE: As the bettercrypto project, where I have the cipher-string from, is no longer active, make sure to chose your cipher-string from a current source.]

For master.cf you may want to configure the submission port to TLS only:

submission inet n - - - - smtpd
 -o smtpd_tls_security_level=encrypt
 -o tls_preempt_cipherlist=yes

However, this does not disallow usage of outdated ciphers for security level may, according to pull request #97, you can do this with:

smtpd_tls_protocols=!SSLv2,!SSLv3
smtp_tls_protocols=!SSLv2,!SSLv3

But this has not been merged with the following reason:

I am going to close this, SSLv3 makes sense here since it's better than good ol' plaintext.

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