Hack The Sec-Leading Resource Of Linux Tutorial

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>