12.5.1. Securing Communication
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.
12.5.1.1. 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 the 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.
12.5.1.2. 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 in two ways: by applying to a Certificate Authority (CA) for an SSL certificate or by creating a self-signed certificate.
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 or POP, change to the /etc/pki/dovecot/ directory, edit the certificate parameters in the /etc/pki/dovecot/dovecot-openssl.conf configuration file as you prefer, and type the following commands, as root:
dovecot]# rm -f certs/dovecot.pem private/dovecot.pem
dovecot]# /usr/libexec/dovecot/mkcert.sh
Once finished, make sure you have the following configurations in your /etc/dovecot/conf.d/10-ssl.conf file:
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_key = </etc/pki/dovecot/private/dovecot.pem
Execute the service dovecot restart command to restart the dovecot daemon.
Alternatively, the stunnel command can be used as an SSL encryption wrapper around the standard, non-secure connections to IMAP or POP services.
The stunnel utility uses external OpenSSL libraries included with Red Hat Enterprise Linux to provide strong cryptography and to protect the network connections. It is recommended to apply to a CA to obtain an SSL certificate, but it is also possible to create a self-signed certificate.
In order to use stunnel, first ensure the stunnel package is installed on your system by running, as root:
~]# yum install stunnel
To create a self-signed SSL certificate, change to the /etc/pki/tls/certs/ directory, and type the following command:
certs]# make stunnel.pem
Answer all of the questions to complete the process.
Once the certificate is generated, create an stunnel configuration file, for example /etc/stunnel/mail.conf, with the following content:
cert = /etc/pki/tls/certs/stunnel.pem
[pop3s]
accept = 995
connect = 110
[imaps]
accept = 993
connect = 143
Once you start stunnel with the created configuration file using the /usr/bin/stunnel /etc/stunnel/mail.conf command, it will be possible to use an IMAP or a POP email client and connect to the email server using SSL encryption.
For more information on stunnel, refer to the stunnel man page or the documents in the /usr/share/doc/stunnel-<version-number> / directory, where <version-number> is the version number of stunnel.
