Postfix SMTP Authentication for Mail servers

These informations are layed down in /etc/postfix/sasl_passwd:

[root@mail postfix]# less /etc/postfix/sasl_passwd
# foo.com         username:password
# bar.com            username:password

	Using the hostname Postfix can identify the correct username:password when there are multiple entries in sasl_passwd
	username:password are entered in plaintext format. They are separated by a single colon “:”

The mail server that we want to relay through in this example is mail.my-isp.org; username is test and it’s password is testpass. We open /etc/postfix/sasl_passwd and add our credentials. When we are done it looks like this:

[root@mail postfix]# cat /etc/postfix/sasl_passwd
mail.my-isp.org      test:testpass

As you have noticed, the credentials in sasl_passwd are entered plaintext. That means that anybody who can open the file will be able to read this sensitive information. Therefore we change ownership and permission to root and r/w only.

[root@mail postfix]# chown root:root /etc/postfix/sasl_passwd && chmod 600 /etc/postfix/sasl_passwd

After these commands ownership and permissions read like this:

[root@mail postfix]# ls -all /etc/postfix/sasl_passwd
-rw-------    1 root     root           79 Dec 30 23:50 /etc/postfix/sasl_passwd

	Note
	You wonder why Postfix running as user postfix can read this file? Postfix will start as user root, read all files that need root permission and switch to user postfix after that.

Now that we have set correct ownership and permissions there is one more thing to do. A plaintext file can’t be read as fast as database. Postfix requires this file to be a database, because it doesn’t want to spend a lot of time looking the credentials up when it needs to get it’s job done. We create a sasl_passwd.db with the help of postmap:

[root@mail postfix]# postmap hash:/etc/postfix/sasl_passwd

After that there will be a new file sasl_passwd.db in /etc/postfix/.

[root@mail postfix]# ls -all /etc/postfix/sasl_passwd.db
-rw-------    1 root     root        12288 Mar 13 23:13 /etc/postfix/sasl_passwd.db

From the onwership and permissions you can see that postmap applied the same as in the source file. That’s it for sasl_passwd; you only need to get back when the informations need an update.

	Note
	Don’t forget to postmap the file, when you change credentials. Postfix will tell you anyway by claiming that sasl_passwd is newer than sasl_passwd.db in the maillog.

The first thing we do is enabling SMTP AUTH for the smtp daemon. We open main.cf and enter some documentation first and then we set smtp_sasl_auth_enable to yes.

# SASL SUPPORT FOR SERVERS
#
# The following options set parameters needed by Postfix to enable
# Cyrus-SASL support for authentication of mail servers.
#
smtp_sasl_auth_enable = yes

Then we tell Postfix where to find sasl_passwd by adding smtp_sasl_password_maps = hash:/path/to/sasl_passwd to the configuration.

smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd

Finally we set security options. In our scenario we will allow Postfix to use anonymous and plaintext authentication. That’s why we set the paramter, but leave it empty:

smtp_sasl_security_options =

All settings together will give this listing in main.cf.

# SASL SUPPORT FOR SERVERS
#
# The following options set parameters needed by Postfix to enable
# Cyrus-SASL support for authentication of mail servers.
#
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =

All that you need to do now is to reload Postfix and you’re ready to use your ISPs mail server to relay mail.

[root@mail postfix]# postfix reload
postfix/postfix-script: refreshing the Postfix mail system

Have fun!