How to enable user authentication for a Postfix SMTP server with SASL

I had originally written this tutorial for xmodulo.com

Every mail server administrator dreads his or her server becoming compromised by spammers. A lot of effort, time and even money is spent on securing mail servers and making sure that the servers do not become open relay.
To combat against spambots in an SMTP server, Postfix in general uses the mynetworks parameter to specify the trusted sender network i.e., LAN. In a typical scenario, the users stationed in the internal LAN are legitimate users, and Postfix will happily accept SMTP requests from them, and forward the emails towards destination. Although this used to be the standard practice in the past, today's users want mobility. Everyone wants to be able to send/receive emails in their phones/tablets/laptops at work, home, on the go, or even from their favorite coffee shop around the corner. For people who are in the fields for critical services, a simple email alert could save a lot of time, effort and money.
To cope up with the mobility need, Postfix started to support another method of validating users. Simple Authentication and Security Layer (SASL) is a framework that can be used by many connection-oriented Internet protocols for securing data, servers and users. With SASL enabled, Postfix will not accept any incoming SMTP connections without proper authentication. As smart spammer can imitate a legitimate email account, no SMTP from even internal users are accepted without authentication.
This tutorial will focus on setting up a Postfix SMTP server to use Dovecot SASL for user authentication. As Dovecot provides mechanisms for user authentication, Postfix will simply ask Dovecot to do the work for it. That way, there is no need to re-invent the wheel.

Prerequisites

  1. A working mail server running on postfix and dovecot2
  2. SSL/TLS support for the mail server3

Preparing Dovecot

Backing up configuration files prior to modification is always a good idea.
Since Dovecot will be the one doing most of the work, we will start configuration with Dovecot.
First of all, a listener is added to Dovecot. Postfix will use this listener to communicate with Dovecot.
root@mail:~# vim /etc/dovecot/conf.d/10-master.conf
## The listener is added under the service auth section ##
service auth {
 unix_listener /var/spool/postfix/private/auth {
  mode = 0660
         user = postfix
         group = postfix
   } ##end listener
} ## end service auth
The above definition places the socket to be used by Postfix at /var/spool/postfix/private/auth with permission 0660 for Postfix only.
root@mail:~# vim /etc/dovecot/conf.d/10-auth.conf
auth_mechanisms = plain login
The above parameter provides the plain login authentication mechanisms for Postfix.
Finally, for the changes to take effect, we restart the Dovecot service as follows.
root@mail:~# service dovecot restart

Preparing Postfix

Necessary SST/TLS and SASL parameters are added in the configuration file main.cf.
root@mail:~# vim /etc/postfix/main.cf
#### SASL ####
## specify SASL type ##
smtpd_sasl_type = dovecot

## path to the SASL socket relative to postfix spool directory i.e. /var/spool/postfix ##
smtpd_sasl_path = private/auth

## postfix appends the domain name for SASL logins that do not have the domain part ##
smtpd_sasl_local_domain = example.tst

## SASL default policy ##
smtpd_sasl_security_options = noanonymous

## for legacy application compatibility ##
broken_sasl_auth_clients = yes

## enable SMTP auth ##
smtpd_sasl_auth_enable = yes

## smtp checks ##
## these checks are based on first match, so sequence is important ##
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
The official guideline can be consulted for more details on available parameters and their function.
SSL/TLS specific parameters are added to the server as well.
root@mail:~# vim /etc/postfix/main.cf
#### SSL/TLS parameters ####

## 'encrypt' will enforce SSL. Not recommended for live servers ##
smtpd_tls_security_level = may 
#smtpd_tls_security_level = encrypt 

smtpd_tls_received_header = yes 
smtpd_tls_auth_only = no 

## loglevel 3 or 4 can be used during troubleshooting ##
smtpd_tls_loglevel = 1 

## path to certificate and key file ##
smtpd_tls_cert_file = /etc/ssl/certs/postfixcert.pem 
smtpd_tls_key_file = /etc/ssl/private/postfixkey.pem 
smtpd_use_tls=yes 

## server will announce STARTTLS ##
smtp_tls_note_starttls_offer = yes 

smtpd_tls_session_cache_timeout = 3600s
Now Postfix is reloaded with updated settings.
root@mail:~# service postfix restart
At this point, Postfix will not allow SMTP connections without authentication.

Mail User Agent Configuration

Your mail client is configured with mandatory authentication for SMTP as shown below.

Troubleshooting

If SASL is not working correctly, the following troubleshooting may help.

Enabling Verbose Postfix Logs

To increase the level of output in Postfix log, the "-v" parameter can be added in the following file.
root@mail:/etc/postfix# vim /etc/postfix/master.cf
smtp      inet  n       -       -       -       -       smtpd -v
Now there should be more verbose information the log file at /var/log/mail.log, which can help with the troubleshooting process.

Telnet to port 25

telnet connection to port 25 should be successful.
$ telnet mail.example.tst 25
ehlo  mail.example.tst
250- mail.example.tst
250-PIPELINING 
250-SIZE 10240000 
250-VRFY 
250-ETRN 
250-STARTTLS 
250-AUTH PLAIN LOGIN 
250-AUTH=PLAIN LOGIN 
250-ENHANCEDSTATUSCODES 
250-8BITMIME 
250 DSN
Amongst other features that the SMTP server advertises, the STARTTLS and AUTH features should be available.
Sending mails using telnet should fail, and no authentication information should be sent to the server.
$ telnet mail.example.tst 25
ehlo  mail.example.tst
mail from:sarmed@example.tst
250 2.1.0 Ok 
rcpt to:sarmed@example.tst
554 5.7.1 : Relay access denied

Tuning parameter – mynetworks

Earlier in the tutorial, the Postfix server was configured to allow SMTP connections originated in the trusted network i.e., mynetworks, as shown in /etc/postfix/main.cf.
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination
To make sure that mails originating from mynetworks do not pass through unauthenticated, /etc/postfix/main.cf can be modified as follows.
smtpd_recipient_restrictions = permit_sasl_authenticated, reject_unauth_destination
Based on the requirements, permit_mynetworks can be allowed or denied later on.
To sum up, SASL can provide additional security to a mail server by enforcing mandatory authentication to users for SMTP requests. As users may use a mail server from anywhere, SASL can meet with the security requirements that do not conflict with the mobility of users.
Hope this helps.


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