Hack The Sec-Leading Resource Of Linux Tutorial: SMTP

Configure Postfix SMTP Relay (Smart Host) on CentOS 8 Postfix is a Mail Transport Agent (MTA), which can be easily configured as a private relay host, passing mail to other mail servers. Unlike Sendmail, Postfix is considered a very secure MTA, offering a high level of flexibility and ease of administration. In this article, I am configuring Postfix on CentOS 8 as SMTP smart host (relay host) using SASL authentication to send out mail. 1. Install Postfix<blockquote style="border: none; margin: 0 0 0 40px; padding: 0px;"><a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgExCIgNV6dSQHUBuJSk7cnAf8X8jYG1wuC6oSMjlXn1enx_vc-wDCcb_coF_D3hCLkPHYGoXdQXiGHgFztfmQ7QuWBhMi26j_bg6vXcEFldqhLIv6e_TuiCOQi-eTeZq5VDeJsZ74-GVyJ_A18TzThvzbTHtBzIqLHcyomSTExNZtzcOzq7FxywZS3SQ/s1200/hackthesec.co.in_postfix.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" data-original-height="840" data-original-width="1200" height="224" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgExCIgNV6dSQHUBuJSk7cnAf8X8jYG1wuC6oSMjlXn1enx_vc-wDCcb_coF_D3hCLkPHYGoXdQXiGHgFztfmQ7QuWBhMi26j_bg6vXcEFldqhLIv6e_TuiCOQi-eTeZq5VDeJsZ74-GVyJ_A18TzThvzbTHtBzIqLHcyomSTExNZtzcOzq7FxywZS3SQ/s320/hackthesec.co.in_postfix.png" width="320" /></a></blockquote><div class="separator" style="clear: both; text-align: left;">Postfix should be preinstalled on CentOS 8 by default. If for some reason you don’t have the service installed, use the below command to install the relevant package:</div><div class="separator" style="clear: both; text-align: left;"> </div><pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# dnf install postfix</code></pre>2. Install SASL pluginPackage cyrus-sasl-plain contains the Cyrus SASL plugins which support PLAIN and LOGIN authentication.<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# dnf install cyrus-sasl-plain</code></pre>3. Edit the configuration fileI am configuring the service to act as SMTP smart host, sending the mail to hackthesec mail server, which is my ISP mail server, using SASL authentication (login and password).Edit /etc/postfix/main.cf configuration file and update the below relevant lines:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">...
meta_directory = /etc/postfix
myhostname = chronos
mydomain = hackthesec.co.in
local_transport = error: this is a null client
myorigin = $myhostname.$mydomain
# list of trusted network addresses, that can relay through this MTA
mynetworks = 127.0.0.0/8 [::1]/128
relayhost = [hackthesec.co.in]
disable_dns_lookups = yes

# SASL authentication 
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:</code>admin@hackthesec.co.in:mypassword</pre><pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">smtp_sasl_security_options = noanonymous
smtp_tls_security_level = encrypt
...</code></pre>4. Test the configurationTest the configuration for any obvious errors. If the configuration is correct, the below command should not give any output:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# postfix check</code></pre>5. Start and enable Postfix serviceLaunch and enable Postfix daemon:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# systemctl start postfix
[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# systemctl enable postfix</code></pre>6. Test the e-mail deliveryVerify the configuration by sending the test e-mail from the command line. Prepare test e-mail body:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# echo "Subject: Mail Delivery Test" > /tmp/mail.txt
[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# echo "This is the example e-mail body" >> /tmp/mail.txt</code></pre>Send the test e-mail using sendmail script with increased verbosity:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# sendmail -v admin@hackthesec.co.in < /tmp/mail.txt</code></pre>Monitor the system journal to check if the e-mail was successfully relayed to the mail server:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# journalctl -u postfix </code></pre>Last but not least – check the recipient’s Inbox (or SPAM box), the test e-mail should be delivered successfully.If for some reason the e-mail has not arrived yet, you can display mail queue on your relay host:<pre class=" language-markup" style="background: rgb(39, 40, 34); border-radius: 0.3em; box-sizing: border-box; color: #f8f8f2; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 15.36px; hyphens: none; line-height: 1.2; margin-bottom: 20px; margin-top: 0px; max-width: 100%; overflow: auto; padding: 10px; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal;"><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;">[<a class="token email-link" href="root@hackthesec" style="background-color: transparent; box-sizing: border-box; text-decoration-line: none;">root@hackthesec</a> ~]# mailq</code></pre><div><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;"> </code></div><div><code class=" language-markup" style="box-sizing: border-box; direction: ltr; font-family: Consolas, Monaco, "Andale Mono", monospace; font-size: 14.7456px; hyphens: none; line-height: 1.2; tab-size: 4; text-shadow: rgba(0, 0, 0, 0.3) 0px 1px; word-break: normal; word-spacing: normal;"> </code></div>

Configure Postfix SMTP Relay (Smart Host) on CentOS 8

Configure Postfix SMTP Relay (Smart Host) on CentOS 8 Postfix is a Mail Transport Agent (MTA), which can be easily configured as a private ...

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Install Postfix to configure SMTP server. SMTP uses 25/TCP</h5>
<h5 style="background-color: white; border: 0px; font-stretch: inherit; margin: 0px 0px 14px; padding: 0px; vertical-align: baseline; word-wrap: break-word;">
Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail, intended as an alternative to the widely used Sendmail MTA.</h5>
<h5 style="background-color: white; border: 0px; font-stretch: inherit; margin: 0px 0px 14px; padding: 0px; vertical-align: baseline; word-wrap: break-word;">
<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6erBvDea_BmtV8j6nHRCfkyP1HK7xWFm-jBQpRByg78AFExVdUV0TEm-_ECHCjSH1HpwNtZ-tuVkrFZaLhZYGHabCL5H-90tCexaLTrAat0bCFYNwzFN6SJI_CgC__uDysSUzEnaOdyGM/s1600/postfix-mail-server_www.hackthesec.co.in.png" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" height="150" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi6erBvDea_BmtV8j6nHRCfkyP1HK7xWFm-jBQpRByg78AFExVdUV0TEm-_ECHCjSH1HpwNtZ-tuVkrFZaLhZYGHabCL5H-90tCexaLTrAat0bCFYNwzFN6SJI_CgC__uDysSUzEnaOdyGM/s200/postfix-mail-server_www.hackthesec.co.in.png" width="200" /></a><ul style="text-align: left;">
<li><a href="http://www.hackthesec.co.in/2016/02/how-to-install-dovecot-to-configure.html" target="_blank">SMTP-Auth to use Dovecot's SASL function</a></li>
<li><a href="http://www.hackthesec.co.in/2016/02/how-to-install-dovecot-to-configure.html" target="_blank">How to Install Dovecot to configure POP/IMAP server</a></li>
</ul>
Postfix is installed even if <a href="http://www.hackthesec.co.in/search/label/Centos" target="_blank">CentOS </a>system was installed with [Minimal Install], but if Postfix is not, Install it first like follows.</h5>
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<pre style="background: rgb(54, 54, 54); border-radius: 7px; border: 1px solid rgb(221, 221, 221); font-stretch: inherit; margin-bottom: 15px; margin-top: 15px; overflow-x: auto; overflow-y: hidden; padding: 18px 20px; vertical-align: baseline;">[root@mail ~]# yum -y install postfix</pre>
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<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjtUFqbeQIjF1sMQ24NxFVepEZtUi97GyhdDw5rkJdnjhF2GdWUUl5ZA18_KoVGGvv_b4dN8X3dSOJGnIlPXERtkXRkSjXeiMlc17UuWMGn2CfqyNvQNVgHOiYxC1CJg5BA_ddr1QZ6Nr32/s1600/Postfix_architecture_www.hackthesec.co.in.png" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" height="292" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjtUFqbeQIjF1sMQ24NxFVepEZtUi97GyhdDw5rkJdnjhF2GdWUUl5ZA18_KoVGGvv_b4dN8X3dSOJGnIlPXERtkXRkSjXeiMlc17UuWMGn2CfqyNvQNVgHOiYxC1CJg5BA_ddr1QZ6Nr32/s640/Postfix_architecture_www.hackthesec.co.in.png" width="640" /></a></div>
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Postfix Configuration. </div>
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<pre style="background-attachment: initial; background-clip: initial; background-color: #363636; background-image: initial; background-origin: initial; background-position: initial; background-repeat: initial; background-size: initial; border-image-outset: initial; border-image-repeat: initial; border-image-slice: initial; border-image-source: initial; border-image-width: initial; border-radius: 7px; border: 1px solid rgb(221, 221, 221); font-stretch: inherit; margin-bottom: 15px; margin-top: 15px; overflow-x: auto; overflow-y: hidden; padding: 18px 20px; vertical-align: baseline;">[root@mail ~]# vi /etc/postfix/main.cf

# line 75: uncomment and specify hostname
myhostname = mail.hackthesec.co.in

# line 83: uncomment and specify domain name
mydomain = hackthesec.co.in

# line 99: uncomment
myorigin = $mydomain

# line 116: change
inet_interfaces = all

# line 164: add
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

# line 264: uncomment and specify your local network
mynetworks = 127.0.0.0/8, 10.0.0.0/24

# line 419: uncomment (use Maildir)
home_mailbox = Maildir/

# line 574: add
smtpd_banner = $myhostname ESMTP

# add follows to the end
# limit an email size for 10M
message_size_limit = 10485760
# limit a mailbox for 1G
mailbox_size_limit = 1073741824

# for SMTP-Auth
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
smtpd_sasl_local_domain = $myhostname
smtpd_recipient_restrictions = permit_mynetworks,permit_auth_destination,permit_sasl_authenticated,reject</pre>
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[root@mail ~]# systemctl restart postfix 
[root@mail ~]# systemctl enable postfix </pre>
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Install and Configure Postfix to configure SMTP server

Install Postfix to configure SMTP server. SMTP uses 25/TCP Postfix is a free and open-source mail transfer agent (MTA) that routes and d...