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burnersk
Asked: 2015-02-22 01:31:11 +0800 CST

How to force a own set of ciphers in Postfix 2.11?

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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

ssl

3 Answers

  1. 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    [UPDATE: As the bettercrypto project, where I have the cipher-string from, is no longer active, make sure to chose your cipher-string from a current source.]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    However, this does not disallow usage of outdated ciphers for security level may, according to pull request #97, you can do this with: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.

    • 10

  2. man postconf says "You are strongly encouraged to not change this setting."

    Nevertheless, you can, like this:

    smtp_tls_security_level = encrypt
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_mandatory_ciphers=high
smtpd_tls_security_level = encrypt
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_mandatory_ciphers=high
tls_high_cipherlist=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

    I assume this is experimental and you're not worried about mail flow from most MTAs. Check for "handshake failure" in the postfix logs. I'd suggest testing with smtp_ outgoing first so you can see what's in the queue and any local nginx-generated SMTP sessions shouldn't fail.

    • 3

  3. It is well documented. From http://www.postfix.org/TLS_README.html#server_cipher

    smtpd_tls_mandatory_ciphers = high
smtpd_tls_mandatory_exclude_ciphers = aNULL, MD5
    • 0