Configure a Postfix server to Allow SASL Authentication

Postfix version used in this documentation is 2.9.6-2. All change made to /etc/postfix/main.cf and /etc/postfix/master.cf need a Postfix service reload. You should adjust your setup with your own domain and SASL user database.

An in-depth details can be found here.

Table of Content

• Install SASL libaries
• Configure Postfix to use SASL and saslauthd
• Configure Postfix service
• Enable saslauthd
• Create sasldb2 with sasl users and password
• Test send email
• Use an alternate SMTP port
• Configure email client

Install SASL libraries

# apt-get update
# apt-get install sasl2-bin libsasl2-2 libsasl2-modules

Configure Postfix to use SASL and saslauthd

In /etc/postfix/sasl/smtpd.conf:

pwcheck_method: saslauthd
mech_list: PLAIN LOGIN

Copy the file to /usr/lib/sasl2 directory because libssal uses that location

# cp /etc/postfix/sasl/smtpd.conf  /usr/lib/sasl2/smtpd.conf

Configure Postfix service

Enable SASL in Postfix, enforce TLS. In /etc/postfix/main.cf file:

smtpd_sasl_auth_enable = yes
smtpd_sasl_path = smtpd
smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_local_domain =

...
############################################################
# TLS smtpd daemon configuration
############################################################

tls_random_exchange_name = /var/run/postfix/prng_exch

smtpd_tls_CApath                    = /etc/ssl/certs
smtpd_tls_cert_file                 = /etc/ssl/certs/server.pem
smtpd_tls_key_file                  = /etc/ssl/private/server.key
smtpd_tls_mandatory_exclude_ciphers = aNULL

smtpd_tls_loglevel               = 1
smtpd_tls_received_header        = yes
smtpd_tls_security_level         = encrypt
smtpd_tls_session_cache_database = btree:/var/run/postfix/smtpd_scache

Allow authenticated users to relay. In /etc/postfix/main.cf:

smtpd_recipient_restrictions =
    reject_unknown_recipient_domain,
    permit_sasl_authenticated,
    reject_non_fqdn_recipient,
    reject_unauth_destination

Enbable saslauthd

Edit /etc/default/saslauthd to use saslauthd. The last option depends on if Postfix is runnign in chroot. Read the comments in the file for details.

START=yes
NAME="saslauthd"
MECHANISMS="sasldb"
#OPTIONS="-c -m /var/run/saslauthd"
OPTIONS="-c -m /var/spool/postfix/var/run/saslauthd"

Start saslauthd

# service saslauthd start

Create sasldb2 with users and password

# saslpasswd2 -c -u somedomain.com -a smtpauth username

Verify:

# sasldblistusers2
mail@somedomain.com: userPassword

Make sure Postfix user can read sasldb:

# chown postfix:postfix /etc/sasldb2

Test send email

Generate base64 authentication string, for example, to use plain authentication method:

$ gen-auth plain
Username: username@somedomain.com
Password: password
Auth String:  AFVzZXJuYW1lOiB1c2VybmFtZUBzb21lZG9tYWluLmNvbQBwYXNzd29yZA==

You can also use login method. You will have two base64 encoded authentication strings, one for username, one for password, and in the test below you use "AUTH LOGIN" for the authentication dialog.

Test smtp over TLS:

$ openssl s_client -connect somedomain.com:25 -starttls smtp
EHLO somedomain.com
250-somedomain.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-AUTH PLAIN LOGIN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250 DSN
AUTH PLAIN  AFVzZXJuYW1lOiB1c2VybmFtZUBzb21lZG9tYWluLmNvbQBwYXNzd29yZA==
235 2.7.0 Authentication successful
...
mail from: <someuser@somdomain.com>
rcpt to: <someuser@example.com>
data
some message
.

Use an alternate SMTP port

When cloud or ISP providers block outgoing port SMTP port 25, or other well known smtp ports, you can configure Postfix to run on a differrent port. Make sure you open firewall rule to allow that port.

Edit /etc/postfix/master.cf and add a line (keep your smtp line if you still want port 25 open):

2525 inet  n  -       -       -       -      smtpd

Configure email client

Configurations for email clients depends on what software you use. In general you should use these settings:

• SMTP_HOST: <your Postifix server>
• SMTP_PORT: <your Postfix port>
• SMTP_USERNAME: <sasl username, e.g. username@somedomain.com>
• SMTP_USERPASS: <sasl user password>
• SMTP_STARTTLS: true
• SMTP_AUTH: login or plain