Securing postfix with SSL/TLS on RHEL7

Updated -

Securing postfix (postfix-2.10.1-7.el7) 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
smtp_tls_security_level = may
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_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

Currently recommended ciphers, excluding DES-based ciphers to avoid SWEET32 attack

Follow given link to know how to generate DH Parameter file for option smtpd_tls_dh1024_param_file : Generate DH Parameter file

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

Disable everything except TLSv1.2

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_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

Currently recommended ciphers, excluding DES-based ciphers to avoid SWEET32 attack

Ciphers - Alternative Values

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:!SHA

smtp_tls_ciphers = high
smtpd_tls_ciphers = high

Remove SHA1-based ciphers, leaves SHA256 & SHA256 variations

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 = medium
smtpd_tls_ciphers = medium

Add RC4-SHA for older systems compatibility

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

7 Comments

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Jerry Williams's picture Active Contributor 219 points
16 September 2019 11:37 PM

I find this confusing. It is all mixed together. I would prefer to see a high security main.cf that supports FIPS. And then maybe a medium main.cf

CB Newbie 13 points
18 June 2020 6:17 PM

Were you able to get an update on this? What is the key format for the TLS keys.

Philip Perry's picture Active Contributor 173 points
3 October 2020 6:55 PM

Nice article - thank you.

A 2048-bit key for Diffie-Helman should be used to avoid the Logjam attack:

openssl dhparam -out /etc/pki/tls/private/postfix.dh.param.tmp 2048

TB Community Member 67 points
30 September 2021 4:04 PM

openssl dhparam parameter file creation fails when system is in FIPS enforcing mode. DH ciphers should be disabled in that case.

/etc/postfix/main.cf example:

smtpd_tls_ciphers = high
#smtpd_tls_dh1024_param_file = /etc/pki/tls/certs/postfix.dh.param
smtpd_tls_fingerprint_digest = sha256
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1, TLSv1.2
smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1, TLSv1.2
smtpd_use_tls = yes
smtp_tls_CAfile = /etc/pki/tls/certs/ca-bundle.crt
smtp_tls_ciphers = high
smtp_tls_fingerprint_digest = sha256
smtp_tls_mandatory_ciphers = high
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1, TLSv1.2
smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1, TLSv1.2
smtp_use_tls = yes
tls_high_cipherlist = FIPS@STRENGTH:!aNULL:!eNULL:!KRB5:!3DES:!DH
tls_preempt_cipherlist = yes
FA Active Contributor 270 points
17 February 2023 9:21 AM

How to renew /etc/pki/tls/certs/postfix.pem when it is expired? On RHEL 8 it lasts one year...

GS Community Member 44 points
30 May 2023 11:49 AM

we could do with this being updated for RHEL9 when time permits, thanks,

JJ Newbie 7 points
22 November 2023 3:54 PM

Totally agree with both Fabio Adamo (pem renewal steps) and Gary Spencer (include RHEL8 and RHEL9).