Mail

Install and send to local users

Installing and testing postfix

To send email, an email server is needed, so we are going to install postfix as a first step. We also add a user that we are going to use for testing email delivery.

apt-get install postfix
adduser mailuser # we use "123" as a password here

Now that we have an email server, let's check if it is up and running using telnet to connect to it:

telnet localhost 25

Note: To exit postfix, you have to enter the "escape character" it shows at the start. In the default case that is ^], which means CTRL + ]. After that type quit to exit telnet.

Connected to the postfix server we can send an email to our test user:

  1. mail from:<email@domain.com>
  2. rcpt to:mailuser
  3. data
  4. subject:Some email subject
  5. Text for email body (end the body with a . on an empty line)

If everything is configured and working correctly, the mail should show up for the user:

cat /var/mail/mailuser

Using an alias

We can also setup an alias for our mail user to which we can send mail as well:

nano /etc/aliases
# myalias:mailuser
newaliases

# -> rcpt to:myalias

Setting up MX records

We want to use the server as the mail server for our local machine, so first we have to specify an MX record in the DNS settings of the server (/etc/bind/zones/db.mi.hdm-stuttgart.de):

        IN      MX      ns5.mi.hdm-stuttgart.de.
sdi5a   IN      A       ns5.mi.hdm-stuttgart.de.
sdi5b   IN      A       141.62.75.112

This MX record has to point to a valid A record previously defined. In our case, we are using the same A record as for the nameserver settings. Also the "subdomain" we are using for emails has to be a correct A record as well.

Note: The bottom A record is not necessary for local testing since the record for the local hostname is already defined in the /etc/hosts, but we need it later.

We also have to configure postfix to accept to allow for remote connections from any host (/etc/postfix/main.cf):

# remove "mynetworks"
inet_interfaces = all
myhostname = sid5b.mi.hdm-stuttgart.de
mydestination # add "mi.hdm-stuttgart.de"

After restarting postfix (service postfix restart), we can connect from our local machine to the server using telnet:

telnet serverip 25

Note: When trying to send email from one server to another a relay access denied error pops up. This is because we are trying to send a mail from outside the network to a domain that our server is not authoritative for, thus the receive connector does not grant us the permission for sending/relaying.

Authentication Setup and Virtual Users

Add authentication to postfix

To authenticate users for sending emails, we need a sasl implementation, in our case, we chose dovecot:

apt-get install dovecot-imapd dovecot-pop3

Now, we have to configure dovecot to listen on the correct protocols (/etc/dovecot/dovecot.conf):

# uncomment "listen"
protocol = pop3 imap

However, remote login via telnet with password is disabled by default, so we enable plaintext authentication in the /etc/dovcot/conf.d/10-auth.conf:

disable_plaintext_auth = no

After restarting dovecot (service dovecot restart) we can connect authenticated:

telnet localhost 142
a login "mailuser" "123" # -> a OK

To connect dovecot with postfix, we setup the unix listener in /etc/dovecot/conf.d/10-master.conf:

unix_listener /var/spool/postfix/private/auth {
  mode = 0666
  user = postfix
}

We also connect postfix with dovecot using the configuration in /etc/postfix/main.cf:

smtpd_recipient_restrictions = permit_sasl_authenticated, permit_mynetworks, reject_unauth_destination, permit
smtpd_sasl_type = dovecot
smtpd_sasl_auth_enable = yes
smtpd_sasl_path = private/auth

Now we can use our server in an email client (we used Thunderbird) and as a proof that everything is configured correctly send and receive emails (also from one server to another)!

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