Securing postfix with SSL/TLS on RHEL6

Updated -

Securing postfix (postfix-2.6.6-8.el6) that uses openssl

This article is part of the Securing Applications Collection

Configuration File

   /etc/postfix/main.cf

shortform


smtp_use_tls = yes smtpd_use_tls = yes smtpd_tls_security_level = encrypt smtpd_tls_auth_only = yes smtp_tls_note_starttls_offer = yes smtpd_tls_key_file = /etc/pki/tls/private/postfix.key smtpd_tls_cert_file = /etc/pki/tls/certs/postfix.pem smtpd_tls_dh1024_param_file = /etc/pki/tls/private/postfix.dh.param smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt smtpd_tls_loglevel = 1 smtpd_tls_session_cache_timeout = 3600s smtpd_tls_session_cache_database = btree:/var/lib/postfix/smtpd_tls_cache tls_random_source = dev:/dev/urandom smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3 smtpd_tls_protocols = !SSLv2, !SSLv3 smtp_tls_mandatory_protocols = !SSLv2, !SSLv3 smtp_tls_protocols = !SSLv2, !SSLv3 smtp_tls_mandatory_ciphers = high smtpd_tls_mandatory_ciphers = high smtp_tls_exclude_ciphers = EXP, MEDIUM, LOW, DES, 3DES, SSLv2 smtpd_tls_exclude_ciphers = EXP, MEDIUM, LOW, DES, 3DES, SSLv2 tls_high_cipherlist = kEECDH:+kEECDH+SHA:kEDH:+kEDH+SHA:+kEDH+CAMELLIA:kECDH:+kECDH+SHA:kRSA:+kRSA+SHA:+kRSA+CAMELLIA:!aNULL:!eNULL:!SSLv2:!RC4:!MD5:!DES:!EXP:!SEED:!IDEA:!3DES tls_medium_cipherlist = kEECDH:+kEECDH+SHA:kEDH:+kEDH+SHA:+kEDH+CAMELLIA:kECDH:+kECDH+SHA:kRSA:+kRSA+SHA:+kRSA+CAMELLIA:!aNULL:!eNULL:!SSLv2:!MD5:!DES:!EXP:!SEED:!IDEA:!3DES smtp_tls_ciphers = high smtpd_tls_ciphers = high

Warning

postfix on RHEL6 only allows control of the protocol and/or ciphers when

     smtpd_tls_security_level = encrypt

If the setting is

    smtpd_tls_security_level = may

then all protocols are allowed, and export grade or better ciphers are used.

Protocols

smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3
smtpd_tls_protocols = !SSLv2, !SSLv3
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3
smtp_tls_protocols = !SSLv2, !SSLv3

TLSv1 or better

Protocol - Alternative Values

smtpd_tls_mandatory_protocols = TLSv1.2, !TLSv1, !TLSv1.1, !SSLv3, !SSLv2
smtpd_tls_protocols = TLSv1.2, !TLSv1.1, !TLSv1, !SSLv3, !SSLv2
smtp_tls_mandatory_protocols =  TLSv1.2, !TLSv1.1, !TLSv1, !SSLv3, !SSLv2
smtp_tls_protocols = TLSv1.2, !TLSv1.1, !TLSv1, !SSLv3, !SSLv2

Disable everything except TLSv1.2. Note that this will currently result in rejection of some encrypted traffic in anything other than a closed environment.

smtpd_tls_mandatory_protocols = !SSLv2
smtpd_tls_protocols = !SSLv2
smtp_tls_mandatory_protocols = !SSLv2
smtp_tls_protocols = !SSLv2

Allow SSLv3 or better

Ciphers

smtp_tls_mandatory_ciphers = high
smtpd_tls_mandatory_ciphers = high
smtp_tls_exclude_ciphers = EXP, MEDIUM, LOW, DES, 3DES
smtpd_tls_exclude_ciphers = EXP, MEDIUM, LOW, DES, 3DES

ciphers provided in openssl's HIGH category, excluding DES-based ciphers to avoid SWEET32 attack

Ciphers - Alternative Values

smtp_tls_mandatory_ciphers = medium
smtpd_tls_mandatory_ciphers = medium

ciphers provided in openssl's MEDIUM or better category

Certificate Handling

postfix uses a key file and certificates file.

Key File

smtpd_tls_key_file = /etc/pki/tls/private/postfix.key

key should be readable only by root

Certificate File

smtpd_tls_cert_file = /etc/pki/tls/certs/postfix.pem

Should contain the server certificate followed by any intermediate certificates and then the root certificate.

Diffie-Helman Parameter File

smtpd_tls_dh1024_param_file = /etc/pki/tls/private/postfix.dh.param

The use of stronger ciphers can be enabled by ensuring there is a Diffie-Helman parameter file available
This file should be renewed on a periodic (weekly) basis.

openssl dhparam -out /etc/pki/tls/private/postfix.dh.param.tmp 1024
mv /etc/pki/tls/private/postfix.dh.param.tmp  /etc/pki/tls/private/postfix.dh.param