SMTP

Question 1

Configure a mail transfer agent (MTA) to accept inbound email from other systems

Answer 1

Configure a mail transfer agent (MTA) to accept inbound email from other systems.

In order to test a mail server, you will need to set up a DNS server.

1. Remove the sendmail package (sendmail is more complicated to configure):
# yum erase sendmail

2. Install the postfix package:
# yum install -y postfix

3. Add a new rule to the firewall:
# iptables -I INPUT -m state --state NEW -m tcp -p tcp --dport 25 -j ACCEPT

4. Save the firewall configuration:
# service iptables save

5. Activate the postfix service at boot:
# chkconfig postfix on

6. Start the postfix service:
# service postfix start

7. Let’s assume that your server is called mail.example.com on the 192.168.1.0/24 network.
   Edit the /etc/postfix/main.cf file and change the following directives:
   myhostname = mail.example.com
   mydomain = example.com
   myorigin = $mydomain
   inet_interfaces = all
   mydestination = $mydomain, $myhostname, localhost.$mydomain, localhost
   mynetworks = 192.168.1.0/24, 127.0.0.0/8

8. Check the syntax:
# postfix check

9. Check the non-default configuration:
# postconf -n

10. Reload the postfix configuration:
# service postfix reload

11. Set the ‘allow_postfix_local_write_mail_spool‘ SElinux boolean to ‘on’:
# setsebool -P allow_postfix_local_write_mail_spool on

12. Test from a client (it should display: 25/tcp open smtp):
# yum install -y nmap
# nmap mail.example.com

Alternatively, test from a client:
# yum install -y telnet
# telnet mail.example.com 25

Question 2

Configure an MTA to forward (relay) email through a smart host

Answer 2

Configure an MTA to forward (relay) email through a smart host.

In order to test this configuration, you will need to set up a DNS server and configure a central mail server (here at 192.168.1.1) to receive mails.

1. Remove the sendmail package (sendmail is more complicated to configure):
# yum erase sendmail

2. Install the postfix package:
# yum install -y postfix

3. Activate the postfix service at boot:
# chkconfig postfix on

4. Start the postfix service:
# service postfix start

5. Let’s assume that your local server is called server.example.com on the 192.168.1.0/24 network and your smart host (outgoing mail gateway) is at 192.168.1.1.
   Edit the /etc/postfix/main.cf file and change the following directives:
   myhostname = server.example.com
   mydomain = example.com
   myorigin = $mydomain
   inet_interfaces = all
   mydestination = $myhostname, localhost.$mydomain, localhost
   mynetworks = 192.168.1.0/24, 127.0.0.0/8
   relayhost = 192.168.1.1

6. Check the syntax:
# postfix check

7. Check the non-default configuration:
# postconf -n

8. Reload the postfix configuration:
# service postfix reload