11.5. Mail User Agents
There are scores of mail programs available under Red Hat Enterprise Linux. There are full-featured, graphical email client programs, such as Mozilla Mail or Ximian Evolution, as well as text-based email programs such as
mutt
.
The remainder of this section focuses on securing communication between the client and server.
11.5.1. Securing Communication
Popular MUAs included with Red Hat Enterprise Linux, such as Mozilla Mail, Ximian Evolution, and
mutt
offer SSL-encrypted email sessions.
Like any other service that flows over a network unencrypted, important email information, such as usernames, passwords, and entire messages, may be intercepted and viewed by users on the network. Additionally, since the standard POP and IMAP protocols pass authentication information unencrypted, it is possible for an attacker to gain access to user accounts by collecting usernames and passwords as they are passed over the network.
11.5.1.1. Secure Email Clients
Most Linux MUAs designed to check email on remote servers support SSL encryption. To use SSL when retrieving email, it must be enabled on both the email client and server.
SSL is easy to enable on the client-side, often done with the click of a button in the MUA's configuration window or via an option in the MUA's configuration file. Secure IMAP and POP have known port numbers (993 and 995, respectively) that the MUA uses to authenticate and download messages.
11.5.1.2. Securing Email Client Communications
Offering SSL encryption to IMAP and POP users on the email server is a simple matter.
First, create an SSL certificate. This can be done two ways: by applying to a Certificate Authority (CA) for an SSL certificate or by creating a self-signed certificate.
Warning
Self-signed certificates should be used for testing purposes only. Any server used in a production environment should use an SSL certificate granted by a CA.
To create a self-signed SSL certificate for IMAP, change to the
/usr/share/ssl/certs/
directory and type the following commands as root:
rm -f imapd.pem
make imapd.pem
Answer all of the questions to complete the process.
To create a self-signed SSL certificate for POP, change to the
/usr/share/ssl/certs/
directory, and type the following commands as root:
rm -f ipop3d.pem
make ipop3d.pem
Again, answer all of the questions to complete the process.
Important
Please be sure to remove the default
imapd.pem
and ipop3d.pem
files before issuing each make
command.
Once finished, execute the
/sbin/service xinetd restart
command to restart the xinetd
daemon which controls imapd
and ipop3d
.
Alternatively, the
stunnel
command can be used as an SSL encryption wrapper around the standard, non-secure daemons, imapd
or pop3d
.
The
stunnel
program uses external OpenSSL libraries included with Red Hat Enterprise Linux to provide strong cryptography and protect the connections. It is best to apply to a CA to obtain an SSL certificate, but it is also possible to create a self-signed certificate.
To create a self-signed SSL certificate, change to the
/usr/share/ssl/certs/
directory, and type the following command:
make stunnel.pem
Again, answer all of the questions to complete the process.
Once the certificate is generated, it is possible to use the
stunnel
command to start the imapd
mail daemon using the following command:
/usr/sbin/stunnel -d 993 -l /usr/sbin/imapd imapd
Once this command is issued, it is possible to open an IMAP email client and connect to the email server using SSL encryption.
To start the
pop3d
using the stunnel
command, type the following command:
/usr/sbin/stunnel -d 995 -l /usr/sbin/pop3d pop3d
For more information about how to use
stunnel
, read the stunnel
man page or refer to the documents in the /usr/share/doc/stunnel-<version-number>
/ directory, where <version-number> is the version number for stunnel
.