Linux Postfix SMTP (Mail Server) SSL Certificate Installations and Configuration


In this tutorial you will learn about Installing SSL Certificate (Secure Server Certificate) to secure communication between Postfix SMTP server and mail client such as Outlook or Thunderbird.

You need to generate a CSR certificate for CA, to use with your Postfix mail server. This tutorial instuctions are tested under:

  1. Redhat enterprise Linux 5
  2. CentOS 5 Server
  3. FreeBSD 7 server

Procedure for creating a CSR on postfix MTA is just like web server. You need to use OpenSSL which is a cryptography toolkit implementing the Secure Sockets Layer (SSL v2/v3) and Transport Layer Security (TLS v1) network protocols and related cryptography standards under Linux / UNIX. To configure postfix SSL SMTP you need 3 files

  • The private key generated using step #1
  • Your .crt certificate file (it will be send by CA)
  • CA certificate (CA: Certification Autority)

Let us see how to create certificate for Postfix smtp server called smtp.theos.in.

Step # 1: Generating a CSR and private key for Postfix SMTP

Type the command to create a SSL CSR for a mail server called smtp.theos.in:
# mkdir /etc/postfix/ssl
# cd /etc/postfix/ssl
# openssl req -new -nodes -keyout smtp.theos.in.key -out smtp.theos.in.csr

Most important is Common Name, in our example it is set to smtp.theos.in. For the common name, you should enter the full mail server address of your site.

Sample output:

Generating a 1024 bit RSA private key

………..++++++

……………………++++++

writing new private key to ‘smtp.theos.in.key’

—–

You are about to be asked to enter information that will be incorporated

into your certificate request.

What you are about to enter is what is called a Distinguished Name or a DN.

There are quite a few fields but you can leave some blank

For some fields there will be a default value,

If you enter ‘.’, the field will be left blank.

—–

Country Name (2 letter code) [AU]:IN

State or Province Name (full name) [Some-State]:MHA

Locality Name (eg, city) []:Pune

Organization Name (eg, company) [Internet Widgits Pty Ltd]:NIXCRAFT LTD

Organizational Unit Name (eg, section) []:ITDEPT

Common Name (eg, YOUR name) []:smtp.theos.in

Email Address []:ssladmin@staff.theos.in

Please enter the following ‘extra’ attributes

to be sent with your certificate request

A challenge password []:myPassword

An optional company name []:NIXCRAFT LTD

Step # 2: Submit CSR to CA

Now a CSR is generated. All you have to do is copy and paste the contents of the CSR file into the SSL certificate providers (aka CA) account. Never ever give out your private key or certificate to anyone. After verification you should receive a zip file with certificates.

Step # 3 : Install your SSL certificate

Unzip file and upload certificates to /etc/postfix/ssl directory.

Step # 4: Configure Postfix SMTP for SSL certificate

Open postfix smtp configuration file and append following directive:
smtpd_use_tls = yes
smtpd_tls_auth_only = yes
smtpd_tls_key_file = /etc/postfix/ssl/smtp.theos.in.key
smtpd_tls_cert_file = /etc/postfix/ssl/smtp.theos.in.crt
smtpd_tls_CAfile = /etc/postfix/ssl/caroot.crt
smtpd_tls_loglevel = 1

smtpd_tls_received_header = yes
smtpd_tls_session_cache_timeout = 3600s
tls_random_source = dev:/dev/urandom

Save and close the file. Restart or reload postfix service
# postfix reload.
OR
# /etc/init.d/postfix restart

Note I have SASL configured as follows in main.cf:
smtpd_sasl_auth_enable = yes
smtpd_sasl_local_domain =
smtpd_sasl_security_options=noanonymous
smtpd_recipient_restrictions = permit_sasl_authenticated, reject_unauth_destination, permit_mynetworks check_relay_domains
smtpd_delay_reject = yes
broken_sasl_auth_clients = yes

Test Postfix TLS (SSL)

In order to test the TLS, just telnet smpt.theos.in on port 25 (you must see STARTTLS and AUTH lines):
$ telnet smpt.theos.in 25
Output:

Trying 202.54.221.5…

Connected to smtp.theos.in.

Escape character is ‘^]’.

220 smtp.theos.in ESMTP Postfix

ehlo smtp.theos.in

250-smtp.theos.in

250-PIPELINING

250-SIZE 10240000

250-ETRN

250-STARTTLS

250-AUTH PLAIN LOGIN

250-AUTH=PLAIN LOGIN

250-ENHANCEDSTATUSCODES

250-8BITMIME

250 DSN

And mail log file…
# tail -f /var/log/maillog
Output:

Jul 12 14:25:10 smtp postfix/smtpd[28817]: connect from unknown[84.167.114.61]

Jul 12 14:25:11 smtp postfix/smtpd[28817]: setting up TLS connection from unknown[84.167.114.61]

Jul 12 14:25:11 smtp postfix/smtpd[28817]: TLS connection established from unknown[84.167.114.61]: TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)

Jul 12 14:25:12 smtp postfix/smtpd[28817]: B3A0A9D8443: client=unknown[84.167.114.61], sasl_method=PLAIN, sasl_username=user1@theos.in

Jul 12 14:25:13 smtp postfix/cleanup[28807]: B3A0A9D8443: message-id=<46968015.50400@theos.in>

Jul 12 14:25:13 smtp postfix/qmgr[28806]: B3A0A9D8443: from=, size=632, nrcpt=1 (queue active)

Jul 12 14:25:14 smtp postfix/smtpd[28817]: disconnect from unknown[122.167.114.61]

Jul 12 14:25:14 smtp postfix/smtp[28821]: B3A0A9D8443: to=, relay=aspmx.l.google.com[209.85.163.27]:25, delay=2.1, delays=1.5/0/0.13/0.49, dsn=2.0.0, status=sent (250 2.0.0 OK 1184268314 n29si21297786elf)

Jul 12 14:25:14 smtp postfix/qmgr[28806]: B3A0A9D8443: removed

You can use same SSL security certificates with dovecot secure IMAPS / POP3S server.

By vichhaiy Posted in Linux

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