Postfix SMTP Authentication for Mail servers

16. SMTP Authentication for Mail servers


SMTP AUTH for mail server is a feature that is often required to relay mail through other mail servers. To enable SMTP AUTH for Postfix, acting as mail client in this scenario, you need to do the following steps:

Procedure 10. Configure SMTP AUTH for mail servers

  1. Provide a file, which will holds necessary information about credentials
  2. Configure Postfix to enable SMTP AUTH for the smtp daemon
  3. Configure Postfix to use the file with the SASL credentials.

16.1. Add credentials to sasl_passwd

Postfix, acting as mail client in this scenario, will need to be able to

  1. know when to provide a username and password
  2. pick the right credentials when there is more than one mail server who requires Postfix to SMTP AUTH

16.1.1. Enter credentials

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

[[email protected] postfix]# less /etc/postfix/sasl_passwd
# foo.com1         username:password2
#            username:password
1 Using the hostname Postfix can identify the correct username:password when there are multiple entries in sasl_passwd
2 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; 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:

[[email protected] postfix]# cat /etc/postfix/sasl_passwd      test:testpass

16.1.2. Secure sasl_passwd

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.

[[email protected] postfix]# chown root:root /etc/postfix/sasl_passwd && chmod 600 /etc/postfix/sasl_passwd

After these commands ownership and permissions read like this:

[[email protected] postfix]# ls -all /etc/postfix/sasl_passwd
-rw-------    1 root     root           79 Dec 30 23:50 /etc/postfix/sasl_passwd
[Note] 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.

16.1.3. Create sasl_passwd DB file

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:

[[email protected] postfix]# postmap hash:/etc/postfix/sasl_passwd

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

[[email protected] 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] 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.

16.2. Enable SMTP AUTH

There are only three options that you must set to enable SMTP AUTH for mail servers in Postfix.

[Note] Note
You can easily tell that these parameters are settings for the smtp daemon. They all begin with smtp_.

16.2.1. Enable SMTP AUTH

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

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

16.2.2. Set path to sasl_passwd

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

16.2.3. Set security options

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

# 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 =

16.2.4. Reload Postfix

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

[[email protected] postfix]# postfix reload
postfix/postfix-script: refreshing the Postfix mail system

Have fun!

Configure DomainKeys (OpenDKIM) with Postfix on CentOS 7

Configure DomainKeys (OpenDKIM) with Postfix on CentOS 7

Very useful guide

In this post we will demonstrate how to install & configure DomainKeys with postfix (MTA) on CentOS 7 , i am assuming Postfix is already installed with following domain and hostname.

Hostname =

Domain =

Step:1 Set EPEL Repository using below rpm command

OpenDKIM package is not available in the default yum repositories but available in CentOS 7 EPEL repositories.

[[email protected] ~]# rpm -Uvh

Step:2 Install OpenDKIM Package using yum

[[email protected] ~]# yum install -y opendkim

Step:3 Run below Command to create keys

Execute the below command to create public & private keys under folder “/etc/opendkim/keys

[[email protected] ~]# opendkim-default-keygen
Generating default DKIM keys:
Default DKIM keys for created in /etc/opendkim/keys.
[[email protected] ~]#
[[email protected] ~]# cd /etc/opendkim/keys/
[[email protected] keys]# ll
total 8
-rw-r----- 1 root opendkim 891 Nov 29 08:42 default.private
-rw-r--r-- 1 root opendkim 320 Nov 29 08:42 default.txt
[[email protected] keys]#

default.private is the private key for the domain and default.txt is public key that we will publish in DNS record (TXT) in the domain. A Selector ( default ) is created while generating keys, a selector can be unique keyword which is associated in keys and included in DKIM signature.

Step:4 Edit the Following Files :

  • /etc/opendkim.conf —- Config file of opendkim
  • /etc/opendkim/KeyTable —- As name suggest it defines the path of private key for the domain
  • /etc/opendkim/SigningTable — This file tells OpenDKIM how to apply the keys.
  • /etc/opendkim/TrustedHosts — This file defines which hosts are allowed to use keys.

Edit the file “/etc/opendkim.conf” & set the below parameters.


Edit the KeyTable file and replace the with your domain name.

[[email protected] ~]# cat /etc/opendkim/KeyTable
# To use this file, uncomment the #KeyTable option in /etc/opendkim.conf,
# then uncomment the following line and replace with your domain
# name, then restart OpenDKIM. Additional keys may be added on separate lines.
[[email protected] ~]#

Edit the SigningTable file and define who will sign the outgoing mails.

[[email protected] ~]# cat /etc/opendkim/SigningTable 
# Enables signing for any address on the listed domain(s), but will work only if
# "refile:/etc/opendkim/SigningTable" is included in /etc/opendkim.conf.
# Create additional lines for additional domains.


As i am using * in above parameter which means all the users on domain are allowed to sign the emails.

Edit the TrustedHosts file , add Server’s FQDN and domain name below localhost ip (

[[email protected] ~]# cat /etc/opendkim/TrustedHosts 
# To use this file, uncomment the #ExternalIgnoreList and/or the #InternalHosts
# option in /etc/opendkim.conf then restart OpenDKIM. Additional hosts
# may be added on separate lines (IP addresses, hostnames, or CIDR ranges).
# The localhost IP ( should always be the first entry in this file.
[[email protected] ~]#

Step:5 Edit Postfix Config File (/etc/postfix/

Add the below lines at end of /etc/postfix/ file.

[[email protected] ~]# vi /etc/postfix/
smtpd_milters = inet:
non_smtpd_milters = $smtpd_milters
milter_default_action = accept

Step:6 Start OpenDKIM & postfix Service

[[email protected] ~]# hash -r
[[email protected] ~]# systemctl start opendkim ; systemctl enable opendkim ; systemctl restart postfix
ln -s '/usr/lib/systemd/system/opendkim.service' '/etc/systemd/system/'
[[email protected] ~]#

Step:7 Update the TXT DNS record of your domain.

Use the output of default.txt and update the DNS Record (TXT) of the Domain.

Step:8 Send a Test email and view the logs.

Check whether email is signed or not.

Wow , Our email is signed and domainKeys configuration task is completed now.

B) cd /etc/opendkim/keys