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[[Category:OPS335]][[Category:OPS335 Labs]]
 
[[Category:OPS335]][[Category:OPS335 Labs]]
  
==MAIL SERVER RESOURCES==
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==OVERVIEW & PREPARATION==
  
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{{Admon/important|Warning|Your lab 3 must be complete with a functioning DNS server for your domain before this lab will work.}}
  
Online References:
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You may not be aware of it as an user, but email is a very <u>complex</u> system to administer. In fact, the more modern e-mail systems (eg. web-based mail applications, etc) are more technically involved than the other archaic, hard-to-configure, and sometimes inter-operable mail systems.
  
* [https://prezi.com/iuk-advzak_o/mail-servers-postfix/ OPS335 Mail Server Notes ] Course Note providing Concepts and Overview of mail server setup
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We are going to spread the remaining email labs over a few weeks, so that by the end of this topic, you will have a sufficient understanding of what services are involved in sending, filtering, and reading email. You will also have the skills to configure a basic mail setup using the default services provided for your Centos7 Linux distribution.
* [http://www.fredshack.com/docs/postfix.html Postfix for Dummies] Good Basic Guide to setup Postfix MTA
 
  
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Believe it or not, this is a simple diagram of you sending an email to someone else:
  
==OVERVIEW &amp; PREPARATION==
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[[Image:Email-servers.png]]
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 '''VM2'''machine 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.
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This lab will show you how to set up a Mail User Agent ('''MUA'''), using the '''mailx''' package on your '''vm2''' machine to send and receive e-mails on your local VM. In this case, the '''Postfix''' package which represents your '''MTA''' is most likely already installed and running on your local VM. In addition to sending and receiving emails on your Local VM, you will also be able to send a text-based e-mail from your '''vm2 machine''' to your '''Seneca mail account'''. You will also learn how to make multiple MTAs in the same network collaborate in sending emails. In addition, you will learn where the message store (MS) is located that stores mail messages until they are viewed and either deleted or transferred to other folders.
  
The '''diagram''' below shows the layout of the what this lab should be able to accomplish:
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Although, you will not be able to receive mail messages from outside sources (such as your Seneca email account), this lab acts as a starting point in order to run a basic email server. You are NOT required to go into tremendous depth (just the minimum requirements). For example, we will not go over every aspect of the Postfix MTA service, but you should know what it represents and what is its main purpose, as opposed to the following: [https://en.wikipedia.org/wiki/Postfix_%28software%29#Architecture complex diagram 1] , [https://www.credativ.de/blog/postfix-architecture-overview complex diagram 2].
  
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===Online References:===
  
[[Image:email-setup-simple.png]]
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* [http://www.simplehelp.net/2008/12/01/how-to-send-email-from-the-linux-command-line/ Mail Send Command] (examples how to send e-mail using mail command)
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* [http://www.johnkerl.org/doc/mail-how-to.html#prompt_commands View and Manage Received e-mail Mesages] (Common commands to view and manage received email messages)
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* [https://support.google.com/mail/answer/29436?hl=en Reading Full Email Headers] (Explanation of message header information)
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* [http://wiki.dovecot.org/MailServerOverview Here's an overview] (common mail server terms)
  
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==INVESTIGATION 1: INSTALL, SET-UP, AND USE THE MAIL USER AGENT ('''MUA''')==
  
==INVESTIGATION 1: INSTALL &amp; SETUP THE MAIL TRANSFER AGENT ('''MTA''')==
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We will be using a simple text-based '''Mail User Agent (MUA)''' called '''mailx''' in this lab to '''both send and receive''' mail messages within your '''vm2''' machine and to '''only send''' mail messages from your '''vm2''' machine to your Seneca e-mail account.
  
{{Admon/important|Prerequistites|<ol><li>Ensure <u>all</u> machines have been updated and that the '''clocks are set to the correct date and time'''.<br></li><li>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.</li></ol>}}
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'''NOTE:''' Because you're using private IP addresses and no external DNS servers are pointing to your network, you '''<u>cannot</u>''' send e-mail messages from outside your environment to your '''vm2''' machine.
  
=== Confirm Network Connections ===
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=== Installing the Mail User Agent (MUA)===
  
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.
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'''Perform the following Steps:'''
  
'''Perform the following steps:'''
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#Make certain you are in your '''vm2''' machine.
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#Install the '''mailx''' application (MUA) using yum
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:'''NOTE:''' You can refer to the link below to acquaint yourself on how to send e-mail messages using '''mailx''' application:<br> [http://www.simplehelp.net/2008/12/01/how-to-send-email-from-the-linux-command-line/ Mail Send Command Examples]
  
#Make certain that you can connect to the Internet on your '''Host Machine''' via web-browser.<br> 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.
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===Sending a Mail Message from your vm2 Machine to your Seneca Email Account===
#Start a web-browser on your '''Host Machine''', and access your Seneca e-mail account (https://myseneca.ca).
 
#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===
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{{Admon/important|Note|These instructions no longer work reliably. You can still send email to your own email server, and look at the server logs to see that it did really get sent. But it probably won't be accepted for one of a multitude of good reasons. If it doesn't work for you: don't worry about it for lab submission purposes.}}
  
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.<br>In our case, between our servers '''vm2''' and an outside server which is your '''Seneca College''' email account (https://myseneca.ca).  
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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:'''
 
'''Perform the following steps:'''
  
#Switch to your '''VM2''' machine.
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#Make certain you are still in your '''vm2''' machine.
#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.
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#Test email from your machine by sending an email to your '''Seneca email account''' using the following command:<br>'''mail -s "Lab4a - test1" <Your Seneca email address>'''<br><br>'''NOTE:''' after you type in the body of the mail message, move to an empty line, and then press the key combination '''&lt;ctrl&gt;&lt;d&gt;''' to send the message.<br><br>
#Issue the following command to install the '''postfix''' application (MTA):<br>'''yum install postfix'''<br><br>
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#Check your Seneca email account (Inbox / Junk Email Folder) to see if you got the email (note that it may take a <u>few minutes to arrive</u>, 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.
#What is the purpose of installing the postfix application in terms of using e-mail? Record your answer in your lab logbook.
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#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.
#We need to <u>configure</u> our MTA (postfix application) to recognize our recently-created domain name that was created in lab3.<br>Edit the Postfix configuration file called '''/etc/postfix/main.cf''' replacing the existing contents with only the contents displayed below:<br><br>'''mydomain = senecaID.org'''<br>'''myorigin = $mydomain'''<br>'''mynetworks = 127.0.0.0/8, 192.168.X.0/24'''<br>'''relay_domains ='''<br>'''inet_interfaces=all'''<br><br>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.<br><br>
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#Once you have succeeded in sending the first email, send a second email to the same destination using the following command:<br>'''mail -r "someone@hacker.com (Canadian Revenue Agency)" -s "Lab4a - test2" <Your Seneca email address>
#Edit the '''/etc/postfix/aliases''' file and add your first name as an alias to be resolved to your VM2's account name:<br><br>'''your-first-name''' &nbsp; &nbsp; &nbsp; '''your-vm2-regular-username'''<br><br>
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#Check your email to see if you got the email. If you did, make a note of the return address. How would you think that including the '''-r''' option could be used by penetration hackers to gain access to a computer system? What sort of steps do you think should be taken to help prevent this type of attack from happening?
#In order to update the MTA's database for the newly-create alias, issue the following command:<br>'''postalias hash:/etc/aliases'''<br><br>
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#Finally, to start and enable your MTA, issue the following commands:<br>'''systemctl start postfix.service'''<br>'''systemctl enable postfix.service'''<br><br>
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===Sending a Mail Message within your vm2 Machine===
#Check the '''/var/log/messages''' file to see that your MTA server started without error. If there are any errors, correct them before continuing.
 
  
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We will now test both your MUA (mailx) and MTA (postfix) by sending and receiving e-mail messages on the local vm2 machine only.
  
'''Record steps, commands, and your observations in INVESTIGATION 1 in your OPS335 lab log-book'''
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'''Perform the following Steps:'''
  
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#Send an email message locally (i.e. only within your vm2 machine) by issuing the command:<br>'''mail -s "Lab4a - Local - Test1" <yourSenecaID>'''
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#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.
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#Login with your '''regular user''' and issue the following command to read the mail message you send to yourself:<br>'''mail'''<br><br>'''NOTE:''' You can 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:<br>[http://www.johnkerl.org/doc/mail-how-to.html#prompt_commands Commands to View and Manage Received e-mail Mesages]<br><br>
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#Issue the following command: '''cat /var/spool/mail/<yourSenecaID>'''<br>What do you see? What does this show you in terms of where mail is stored on your e-mail server?
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#If you received an e-mail message, the message and subject line should appear as a listing in your mail command.<br><br>'''NOTE:''' If you did not receive a mail message, check your mail server settings, check to see if your mail server is running and also check '''/var/log/maillog''' and '''/var/log/messages''' (this step requires '''root''' privilege).<br><br>
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#Once you have received the message, 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.
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#Exit the mail program by typing the letter '''q''' and press ENTER.
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#Re-issue the '''mail''' command. What happened? Issue the command: '''cat /var/spool/mail/<yourSenecaID>'''. What do you notice?
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#Exit the mail command.
  
==INVESTIGATION 2: INSTALL &amp; SETUP THE MAIL USER AGENT ('''MUA''')==
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'''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'''.
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==INVESTIGATION 2: SETUP MTA TO SEND MAIL MESSAGES (NO ENCRYPTION)==
  
'''NOTE:''' Due to the simplicity of this mail server setup, and the setup of Seneca College's mail server, you '''<u>cannot</u>''' send Seneca e-mail messages to your '''VM2''' machine.
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We will be using the '''Postfix''' application as the '''MTA''', and we will be setting it up on your '''vm2''' and '''vm3''' machines. They will act as the "sending" email servers for your internal network. You will be able to send email out of your network, and receive email from within your network, but you will '''<u>not</u>''' receive email from outside of your network due to the following reasons:
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* Individuals outside of your domain will never find the MX records because there are no other DNS servers pointing to your DNS server (i.e. you haven't paid for it).
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* Even if the individuals could read your MX records, your local network is using IP addresses on a '''private subnet''', which is not routeable on the Internet, so it cannot be reached from outside of your system.
  
=== Installing the Mail User Agent (MUA)===
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=== Verify the Postfix Service Status ===
  
'''Perform the following Steps:'''
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'''Perform the following steps:'''
  
#Make certain you are in your '''vm2''' machine.
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#The '''postfix''' application should be installed by default. If it isn't, install it.
#Issue the following command to install the '''mailx''' application (MUA):<br>'''yum install mailx'''<br><br>
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#Postfix is capable of sending email with the default configuration, so start and enable this service, and verify that the postfix service is running.
'''NOTE:''' You can refer to the link below to acquaint yourself on how to send e-mail messages using '''mailx''' application:<br> [http://www.simplehelp.net/2008/12/01/how-to-send-email-from-the-linux-command-line/ Mail Send Command Examples]
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#Look for the running postfix service in the list of listening ports by issuing the following command:<br><source>ss -atnp</source>
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#Which service is postfix running? Locate the port used by SMTP, and look for connections with the state LISTEN (i.e. currently listening).
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#Write your observations in your lab logbook.
  
===Testing mail with the internet===
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=== Testing the connection to the Postfix Service ===
  
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.
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We will be demonstrating the use of the '''nc''' application to test that the postfix service is running and listening.
  
 
'''Perform the following steps:'''
 
'''Perform the following steps:'''
  
#Make certain you are still in your '''vm2''' machine.
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# If the '''nc''' command is not installed on your vm2 machine, install it (install '''nc''' command for your '''vm3''' as well).
#Test email from your machine by sending an email to your '''Seneca email account''' using the following command:<br>'''mail -s "Lab4a - test1" <Your Seneca email address>'''<br><br>'''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.<br><br>
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# Connect from your '''vm2''' to itself using the '''nc''' command by issuing the following command:<br><source >nc localhost 25</source>
#Check your Seneca email account to see if you got the email (note that it may take a <u>few minutes to arrive</u>, 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.
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# You should see a response: <br><source >220 vm2.yourdomain.ops ESMTP Postfix</source>
#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.
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# You could theoretically use SMTP commands to send an email here, but this would be a very unusual use of your mail server. You have an '''MUA''' for a reason.
#Test email from your '''Host Machine''' by sending an email to your Seneca account using the following command:<br>'''mail -s "Lab4a - test2" -r hacker@evil.com <Your Seneca email address>
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#Enter the command '''QUIT''' to close the connection to the server, then '''<ctrl>-c''' to terminate the nc command.
#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)===
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::'''NOTE:''' If it worked, this indicates that the postfix service is running, listening, and responding to connections.
  
We will now test both your MUA (mailx) and MTA (postfix) by sending and receiving e-mail messages on the local VM2 machine only.
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<ol><li value="4">Let's see if it works from other machines. Use '''nc''' to connect to '''vm2''' from '''vm3''' and see if it works. If your firewall is set up properly, the nc command should not permit a connection (i.e. ''no route to host'').</li>
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<li>Create an iptables rule to allow incoming connections to your '''SMTP''' server on your '''vm2'''.</li>
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<li>Once you open the port in the firewall, retry the '''nc''' command. You should get a different error this time (e.g. ''connection refused''). This time the problem is that your service isn't listening on the outside interface, it's currently configured to listen only on the loopback (lo) interface.</li>
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<li>Make sure the new iptables rule gets saved so that it will be loaded automatically from startup.</li>
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</ol>
  
'''Perform the following Steps:'''
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=== Listening on all interfaces ===
  
#Send an email message locally (i.e. only within) your VM2 machine by issuing the command:<br>'''mail -s "Lab4a - Local - Test1" <yourSenecaID>'''
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We need to configure the MTA not only to listen to connections from other (separate) MTAs, but to set the domain name and server name in order to allow the user to issue emails in the "standard way", and allow mail messages to provide a correct email address for replies.
#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.
 
#Issue the following command to read the mail message you send to yourself:<br>'''mail'''<br><br>'''NOTE:''' You can 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:<br>[http://www.johnkerl.org/doc/mail-how-to.html#prompt_commands Commands to View and Manage Received e-mail Mesages]<br><br>
 
#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'''.
 
#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.
 
#Exit the mail program by typing the letter '''q''' and press ENTER.
 
#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?<br>Record your answer in your OPS335 lab log-book.
 
  
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'''Perform the following steps:'''
  
===Test email using host names===
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# In your '''vm2''' machine, launch in editing session for the postfix configuration file called: '''/etc/postfix/main.cf'''
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# Our first editing change to the Postfix configuration will be to make the service "listen" for incoming connections on the external interface (i.e '''eth0''' from the VMs point of view).<br>Change the value of the following parameter to what is displayed below:<br><source>inet_interfaces = all</source>
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# We should also set the string that will end up in the '''From:''' header in messages sent by this server.<br>Change the '''mydomain'''  option to YOUR domain name (shown below):<source>mydomain = yoursenecaid.ops</source>
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# Also you must set the '''hostname''' for this server so that will correctly specify the hostname in the '''From:''' header in a sent mail message.<br>Make certain the following parameter only appears once (shown below):<source>myorigin = $myhostname</source>
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#Ensure that your '''hostname''' and '''DOMAIN''' name is properly set on your machine, otherwise you will need to set the '''myhostname''' parameter.
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<br>
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{{Admon/important|Warning|Make sure there are no other un-commented copies of those above-mentioned parameters in the Postfix configuration file.}}
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<br><ol><li value="6">Restart the postfix service, then  use the '''ss''' command to confirm that the your MTA is now listening on <u>all</u> interfaces (not just loopback)</li>
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<li>Test by connecting to it (using the '''nc''' command) from your '''vm3''' machine.</li></ol>
  
Let's complete this lab by sending email messages using our recently-created domain name:
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'''Record steps, commands, and your observations in INVESTIGATION 2 in your OPS335 lab log-book'''
  
'''Perform the following Steps:'''
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==INVESTIGATION 3: SENDING EMAIL BETWEEN MTAs for vm2 and vm3 (NO ENCRYPTION)==
  
#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.
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Your '''vm2''' server should now be capable of '''sending''' and '''receiving''' email, but we can't be certain until we test it. This also would not help the users on the other machines in the network, which are still not capable of receiving email.
#Restart your MTA (postfix) server if you changed the hostname on either mail server.
 
#Try sending mail to yourself on both machines. For example, as the username <senecaID> on VM 3, use the command:<br>'''mail <senecaID>@vm2.<yourdomain>.org'''<br><br>'''NOTE:''' the machine will prompt for a subject: enter "testing" without the quotes.<br><br>
 
#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 <u>NOT</u> USE CTRL-Z).
 
#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?
 
#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'''
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'''Perform the following steps:'''
  
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# Repeat the configuration from investigation 2 on '''vm3''' (swap vm2 and vm3 when issuing command so that you are configuring vm3, and using your vm2 server to test the connections).
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# Once that is complete, send an email from '''root on vm2''' to '''root on vm3''', and then reply '''from vm3 to vm2'''.
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# If both messages arrive, both MTAs are working.  If not, use the troubleshooting tools and techniques you have already learned to diagnose and fix the problem.<br><br>
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{{Admon/important |Backup your VMs!|You MUST perform a '''full backup''' of ALL of your VMs whenever you complete your '''OPS335 labs''' or when working on your '''OPS335 assignments'''. You should be using the dump or rsync command, and you should use  the Bash shell script that you were adviced to create in order to backup all of your VMs.}}
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<br>
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'''Record steps, commands, and your observations in INVESTIGATION 3 in your OPS335 lab log-book'''
  
 
==COMPLETING THE LAB==
 
==COMPLETING THE LAB==
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).
 
  
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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 vm2 to vm3, and from vm3 to vm2), and to your seneca email (or other external mail server).
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===Online Submission===
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Follow the instructions for lab 4a on blackboard.
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<!--
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===Andrew's sections===
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You may choose to:
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* Submit screenshots of your work on Blackboard, in which case you don't need to come to the lab.
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* Or come to the lab, show me your work, and talk to me about it. I want to hear what you've learned and answer any questions you have.
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 +
You'll get the same grade regardless of how you choose to submit your work.
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::<span style="color:green;font-size:1.5em;">&#x2713;</span>Arrange proof that you can send e-mail from your '''vm2''' machine to your '''Seneca College e-mail account''', and than you can '''send and receive e-mail messages between on your vm2 and vm3 machines'''.
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::<span style="color:green;font-size:1.5em;">&#x2713;</span>Download and run '''https://ict.senecacollege.ca/~andrew.smith/ops335/labcheck4a.bash''' on your '''host''' machine.
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::<span style="color:green;font-size:1.5em;">&#x2713;</span>Completed Lab4a log-book notes.
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-->
  
 
==EXPLORATION QUESTIONS==
 
==EXPLORATION QUESTIONS==
  
#What did changing the relay_domains parameter do?
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#Briefly list the steps to install the MUA on your server for text-based messaging.
#What firewall rule or rules, if any, did you have to enter so that email between your VMs would work?
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#Briefly list the steps to trouble-shoot your server if you could not send e-mail messages from your vm2 machine to an external e-mail server.
#What is the meaning of the square brackets surrounding the IP address in the examples?
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#Write the command to send an e-mail message from your vm2 to your Seneca College e-mail account.
#What were the results of sending email between the VMs? Show log segments to verify your answers.
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#What are the commands to issue in the mail prompt to:<ul><li>Read the first e-mail message displayed</li><li>Save the 4th e-mail message to the file pathname: ~/maildir/3.msg.txt</li><li>Delete the 3rd e-mail message displayed</li><li>Exit the mail command prompt and return to the shell</li></ul>
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#What were the results of sending emails locally on your vm2 machine? Show log segments to verify your answers.
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# What is the purpose of an MTA?
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# What is the purpose of an MUA?
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# Draw a simple diagram showing how an MUA and an MTA are used to send e-mail messages between different servers.
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# List the steps to test a running postfix service using the nc application.

Latest revision as of 20:17, 4 January 2021


OVERVIEW & PREPARATION

Important.png
Warning
Your lab 3 must be complete with a functioning DNS server for your domain before this lab will work.

You may not be aware of it as an user, but email is a very complex system to administer. In fact, the more modern e-mail systems (eg. web-based mail applications, etc) are more technically involved than the other archaic, hard-to-configure, and sometimes inter-operable mail systems.

We are going to spread the remaining email labs over a few weeks, so that by the end of this topic, you will have a sufficient understanding of what services are involved in sending, filtering, and reading email. You will also have the skills to configure a basic mail setup using the default services provided for your Centos7 Linux distribution.

Believe it or not, this is a simple diagram of you sending an email to someone else:

Email-servers.png

This lab will show you how to set up a Mail User Agent (MUA), using the mailx package on your vm2 machine to send and receive e-mails on your local VM. In this case, the Postfix package which represents your MTA is most likely already installed and running on your local VM. In addition to sending and receiving emails on your Local VM, you will also be able to send a text-based e-mail from your vm2 machine to your Seneca mail account. You will also learn how to make multiple MTAs in the same network collaborate in sending emails. In addition, you will learn where the message store (MS) is located that stores mail messages until they are viewed and either deleted or transferred to other folders.

Although, you will not be able to receive mail messages from outside sources (such as your Seneca email account), this lab acts as a starting point in order to run a basic email server. You are NOT required to go into tremendous depth (just the minimum requirements). For example, we will not go over every aspect of the Postfix MTA service, but you should know what it represents and what is its main purpose, as opposed to the following: complex diagram 1 , complex diagram 2.

Online References:

INVESTIGATION 1: INSTALL, SET-UP, AND USE THE MAIL USER AGENT (MUA)

We will be using a simple text-based Mail User Agent (MUA) called mailx in this lab to both send and receive mail messages within your vm2 machine and to only send mail messages from your vm2 machine to your Seneca e-mail account.

NOTE: Because you're using private IP addresses and no external DNS servers are pointing to your network, you cannot send e-mail messages from outside your environment to your vm2 machine.

Installing the Mail User Agent (MUA)

Perform the following Steps:

  1. Make certain you are in your vm2 machine.
  2. Install the mailx application (MUA) using yum
NOTE: You can refer to the link below to acquaint yourself on how to send e-mail messages using mailx application:
Mail Send Command Examples

Sending a Mail Message from your vm2 Machine to your Seneca Email Account

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Note
These instructions no longer work reliably. You can still send email to your own email server, and look at the server logs to see that it did really get sent. But it probably won't be accepted for one of a multitude of good reasons. If it doesn't work for you: don't worry about it for lab submission purposes.

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, and then press the key combination <ctrl><d> to send the message.

  3. Check your Seneca email account (Inbox / Junk Email Folder) 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. Once you have succeeded in sending the first email, send a second email to the same destination using the following command:
    mail -r "someone@hacker.com (Canadian Revenue Agency)" -s "Lab4a - test2" <Your Seneca email address>
  6. Check your email to see if you got the email. If you did, make a note of the return address. How would you think that including the -r option could be used by penetration hackers to gain access to a computer system? What sort of steps do you think should be taken to help prevent this type of attack from happening?

Sending a Mail Message within your vm2 Machine

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. Login with your regular user and issue the following command to read the mail message you send to yourself:
    mail

    NOTE: You can 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. Issue the following command: cat /var/spool/mail/<yourSenecaID>
    What do you see? What does this show you in terms of where mail is stored on your e-mail server?
  5. If you received an e-mail message, the message and subject line should appear as a listing in your mail command.

    NOTE: If you did not receive a mail message, check your mail server settings, check to see if your mail server is running and also check /var/log/maillog and /var/log/messages (this step requires root privilege).

  6. Once you have received the message, 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.
  7. Exit the mail program by typing the letter q and press ENTER.
  8. Re-issue the mail command. What happened? Issue the command: cat /var/spool/mail/<yourSenecaID>. What do you notice?
  9. Exit the mail command.

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

INVESTIGATION 2: SETUP MTA TO SEND MAIL MESSAGES (NO ENCRYPTION)

We will be using the Postfix application as the MTA, and we will be setting it up on your vm2 and vm3 machines. They will act as the "sending" email servers for your internal network. You will be able to send email out of your network, and receive email from within your network, but you will not receive email from outside of your network due to the following reasons:

  • Individuals outside of your domain will never find the MX records because there are no other DNS servers pointing to your DNS server (i.e. you haven't paid for it).
  • Even if the individuals could read your MX records, your local network is using IP addresses on a private subnet, which is not routeable on the Internet, so it cannot be reached from outside of your system.

Verify the Postfix Service Status

Perform the following steps:

  1. The postfix application should be installed by default. If it isn't, install it.
  2. Postfix is capable of sending email with the default configuration, so start and enable this service, and verify that the postfix service is running.
  3. Look for the running postfix service in the list of listening ports by issuing the following command:
    ss -atnp
  4. Which service is postfix running? Locate the port used by SMTP, and look for connections with the state LISTEN (i.e. currently listening).
  5. Write your observations in your lab logbook.

Testing the connection to the Postfix Service

We will be demonstrating the use of the nc application to test that the postfix service is running and listening.

Perform the following steps:

  1. If the nc command is not installed on your vm2 machine, install it (install nc command for your vm3 as well).
  2. Connect from your vm2 to itself using the nc command by issuing the following command:
    nc localhost 25
  3. You should see a response:
    220 vm2.yourdomain.ops ESMTP Postfix
  4. You could theoretically use SMTP commands to send an email here, but this would be a very unusual use of your mail server. You have an MUA for a reason.
  5. Enter the command QUIT to close the connection to the server, then <ctrl>-c to terminate the nc command.
NOTE: If it worked, this indicates that the postfix service is running, listening, and responding to connections.
  1. Let's see if it works from other machines. Use nc to connect to vm2 from vm3 and see if it works. If your firewall is set up properly, the nc command should not permit a connection (i.e. no route to host).
  2. Create an iptables rule to allow incoming connections to your SMTP server on your vm2.
  3. Once you open the port in the firewall, retry the nc command. You should get a different error this time (e.g. connection refused). This time the problem is that your service isn't listening on the outside interface, it's currently configured to listen only on the loopback (lo) interface.
  4. Make sure the new iptables rule gets saved so that it will be loaded automatically from startup.

Listening on all interfaces

We need to configure the MTA not only to listen to connections from other (separate) MTAs, but to set the domain name and server name in order to allow the user to issue emails in the "standard way", and allow mail messages to provide a correct email address for replies.

Perform the following steps:

  1. In your vm2 machine, launch in editing session for the postfix configuration file called: /etc/postfix/main.cf
  2. Our first editing change to the Postfix configuration will be to make the service "listen" for incoming connections on the external interface (i.e eth0 from the VMs point of view).
    Change the value of the following parameter to what is displayed below:
    inet_interfaces = all
  3. We should also set the string that will end up in the From: header in messages sent by this server.
    Change the mydomain option to YOUR domain name (shown below):
    mydomain = yoursenecaid.ops
  4. Also you must set the hostname for this server so that will correctly specify the hostname in the From: header in a sent mail message.
    Make certain the following parameter only appears once (shown below):
    myorigin = $myhostname
  5. Ensure that your hostname and DOMAIN name is properly set on your machine, otherwise you will need to set the myhostname parameter.


Important.png
Warning
Make sure there are no other un-commented copies of those above-mentioned parameters in the Postfix configuration file.

  1. Restart the postfix service, then use the ss command to confirm that the your MTA is now listening on all interfaces (not just loopback)
  2. Test by connecting to it (using the nc command) from your vm3 machine.

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

INVESTIGATION 3: SENDING EMAIL BETWEEN MTAs for vm2 and vm3 (NO ENCRYPTION)

Your vm2 server should now be capable of sending and receiving email, but we can't be certain until we test it. This also would not help the users on the other machines in the network, which are still not capable of receiving email.

Perform the following steps:

  1. Repeat the configuration from investigation 2 on vm3 (swap vm2 and vm3 when issuing command so that you are configuring vm3, and using your vm2 server to test the connections).
  2. Once that is complete, send an email from root on vm2 to root on vm3, and then reply from vm3 to vm2.
  3. If both messages arrive, both MTAs are working. If not, use the troubleshooting tools and techniques you have already learned to diagnose and fix the problem.

Important.png
Backup your VMs!
You MUST perform a full backup of ALL of your VMs whenever you complete your OPS335 labs or when working on your OPS335 assignments. You should be using the dump or rsync command, and you should use the Bash shell script that you were adviced to create in order to backup all of your VMs.


Record steps, commands, and your observations in INVESTIGATION 3 in your OPS335 lab log-book

COMPLETING THE LAB

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 vm2 to vm3, and from vm3 to vm2), and to your seneca email (or other external mail server).

Online Submission

Follow the instructions for lab 4a on blackboard.

EXPLORATION QUESTIONS

  1. Briefly list the steps to install the MUA on your server for text-based messaging.
  2. Briefly list the steps to trouble-shoot your server if you could not send e-mail messages from your vm2 machine to an external e-mail server.
  3. Write the command to send an e-mail message from your vm2 to your Seneca College e-mail account.
  4. What are the commands to issue in the mail prompt to:
    • Read the first e-mail message displayed
    • Save the 4th e-mail message to the file pathname: ~/maildir/3.msg.txt
    • Delete the 3rd e-mail message displayed
    • Exit the mail command prompt and return to the shell
  5. What were the results of sending emails locally on your vm2 machine? Show log segments to verify your answers.
  6. What is the purpose of an MTA?
  7. What is the purpose of an MUA?
  8. Draw a simple diagram showing how an MUA and an MTA are used to send e-mail messages between different servers.
  9. List the steps to test a running postfix service using the nc application.