OPS335 Lab 4

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This lab will show you how to set up a Postfix email server (MTA) on your VM2 machine to send an e-mail message to an external e-mail server (i.e. myseneca.ca). You will also be setting up on your VM2machine the mailx package (MUA) to allow users of that VM to send a text-based e-mail to your Seneca mail account (only send, but not receive), and to send and receive e-mails locally on your VM2 machine.

In order to send e-mail messages between your different VMs, a more complex set-up is required, and will be addressed in the second part of lab4.

The diagram below shows the layout of the what this lab should be able to accomplish:



  1. Ensure all machines have been updated and that the clocks are set to the correct date and time.
  2. Due to the changes made in this lab3, you will now need your vm1 running (as the DNS server) in order for any of your virtual machines to be able to use the internet.

Confirm Network Connections

Before proceeding with this lab, we need to confirm our machines have connectivity to the Internet before we can install and start sending e-mail messages.

Perform the following steps:

  1. Make certain that you can connect to the Internet on your Host Machine via web-browser.
    Since your host should be using VM1 as its DNS server, you will need to make certain that the VM1 machine is running as well.
  2. Start a web-browser on your Host Machine, and access your Seneca e-mail account (https://myseneca.ca).
  3. Confirm that you can access the web from your VM2 machine. You can install the wget or lynx applications in order to test this for your text-based server.

Install and Configure Postfix Package (MTA) on a VM

We will now be installing the postfix application which will act as the Mail Transfer Agent (MTA) that will send e-mail messages to other e-mail servers.
In our case, between our servers vm2 and an outside server which is your Seneca College email account (https://myseneca.ca).

Perform the following steps:

  1. Switch to your VM2 machine.
  2. Check the status of the sendmail service by using the systemctl command. If the sendmail service is running, use the systemctl command to stop and disable that service.
  3. Issue the following command to install the postfix application (MTA):
    yum install postfix

  4. What is the purpose of installing the postfix application in terms of using e-mail? Record your answer in your lab logbook.
  5. We need to configure our MTA (postfix application) to recognize our recently-created domain name that was created in lab3.
    Edit the Postfix configuration file called /etc/postfix/main.cf replacing the existing contents with only the contents displayed below:

    mydomain = senecaID.org
    myorigin = $mydomain
    mynetworks =, 192.168.X.0/24
    relay_domains =

    Your MTA has the ability to resolve "fake" public address to an actual user account name. For example, if you have a user account called "msaul", you can have the MTA resolve the fake name "murray.saul" to the account name "msaul". In order to do this, you will need to create an alias which is contained in the MTA's alias file.

  6. Edit the /etc/postfix/aliases file and add your first name as an alias to be resolved to your VM2's account name:

    your-first-name       your-vm2-regular-username

  7. In order to update the MTA's database for the newly-create alias, issue the following command:
    postalias hash:/etc/aliases

  8. Finally, to start and enable your MTA, issue the following commands:
    systemctl start postfix.service
    systemctl enable postfix.service

  9. Check the /var/log/messages file to see that your MTA server started without error. If there are any errors, correct them before continuing.

Record steps, commands, and your observations in INVESTIGATION 1 in your OPS335 lab log-book


We will be using a simple text-based Mail User Agent (MUA) called mailx in this lab to send and receive mail messages within your VM1 machine and to send mail messages to your Seneca e-mail account.

NOTE: Due to the simplicity of this mail server setup, and the setup of Seneca College's mail server, you cannot send Seneca e-mail messages to your VM2 machine.

Installing the Mail User Agent (MUA)

Perform the following Steps:

  1. Make certain you are in your vm2 machine.
  2. Issue the following command to install the mailx application (MUA):
    yum install mailx

  3. Refer to the link below to acquaint yourself on how to send e-mail messages using mailx application:
    Mail Send Command Examples

Testing mail with the internet

We will now test to see if your MTA for your VM2 machine is correctly running by sending email messages from your VM2 machine to your Seneca e-mail account.

Perform the following steps:

  1. Make certain you are still in your vm2 machine.
  2. Test email from your machine by sending an email to your Seneca email account using the following command:
    mail -s "Lab4a - test1" <Your Seneca email address>

    NOTE: after you type in the body of the mail message, move to an empty line, type period "." and press the ENTER key to send the message.

  3. Check your Seneca email account to see if you got the email (note that it may take a few minutes to arrive, so you may also wish to try an alternate email account if you have one like gmail, etc). When you do receive that email, make a note of the return address.
  4. If you did not receive the mail, check the mail logs on your vm2 machine to determine any errors messages that would indicate a mail server setup problem.
  5. Test email from your Host Machine by sending an email to your Seneca account using the following command:
    mail -s "Lab4a - test2" -r hacker@evil.com <Your Seneca email address>
  6. Check your Seneca email to see if you got the email. If you did make a note of the return address. Why would you think including the -r option could be used by penetration hackers to gain access to a computer system?

Testing email on Local VM Machine (vm2)

We will now test both your MUA (mailx) and MTA (postfix) by sending and receiving e-mail messages on the local VM2 machine only.

Perform the following Steps:

  1. Send an email message locally (i.e. only within) your VM2 machine by issuing the command:
    mail -s "Lab4a - Local - Test1" <yourSenecaID>
  2. After you type in the body of the mail message, move to an empty line, type period "." and press the ENTER key to send the message.
  3. Issue the following command to read the mail message you send to yourself:

    NOTE: Refer to the link below to view a reference chart on how to read and delete received e-mail messages at the mail command prompt:
    Commands to View and Manage Received e-mail Mesages

  4. If you received an e-mail message, the message and subject line should appear as a listing in your mail command. If you did not receive a mail message, check your mail server settings, check to see if you mail server is running and also check /var/log/messages.
  5. Type the mail message number that is displayed in your e-mail message list in the prompt and press ENTER. You should be able to confirm the message body that you sent.
  6. Exit the mail program by typing the letter q and press ENTER.
  7. Re-issue the mail command. What happened? If the mail message that you read no longer shows is displayed, what would be the reason for this? Record your answer in your OPS335 lab log-book.

Test email using host names

Let's complete this lab by sending email messages using our recently-created domain name:

Perform the following Steps:

  1. Use the host and hostname commands to confirm that each of your machines has the same hostname and that the DNS service verifies this to be true.
  2. Restart your MTA (postfix) server if you changed the hostname on either mail server.
  3. Try sending mail to yourself on both machines. For example, as the username <senecaID> on VM 3, use the command:
    mail <senecaID>@vm2.<yourdomain>.org

    NOTE: the machine will prompt for a subject: enter "testing" without the quotes.

  4. Enter the body of your e-mail message. When you have completed entering your email message, enter a period (.) in column 1 on the last line of your letter. This will send the signal: end-of-file and your letter will be mailed. You should end up back at the shell prompt. If you make a mistake, use CTRL-C to cancel the email (DO NOT USE CTRL-Z).
  5. Use the mailq command on both machines (you will need to be root) to view the mail queue. Why would this command be useful for an administrator?
  6. Use the mail command to check to see if you have received any mail.

Record steps, commands, and your observations in INVESTIGATION 2 in your OPS335 lab log-book


Upon completion of this lab you should have postfix mail servers running on two machines, and starting automatically when they do. These servers must have sent email both ways between each other (from VM 2 to VM 3, and from VM 3 to VM 2), and to your seneca email (or other external mail server).


  1. What did changing the relay_domains parameter do?
  2. What firewall rule or rules, if any, did you have to enter so that email between your VMs would work?
  3. What is the meaning of the square brackets surrounding the IP address in the examples?
  4. What were the results of sending email between the VMs? Show log segments to verify your answers.