4.8. Using stunnel
The stunnel program is an encryption wrapper between a client and a server. It listens on the port specified in its configuration file, encrypts the communitation with the client, and forwards the data to the original daemon listening on its usual port. This way, you can secure any service that itself does not support any type of encryption, or improve the security of a service that uses a type of encryption that you want to avoid for security reasons, such as SSL versions 2 and 3, affected by the POODLE SSL vulnerability (CVE-2014-3566). See https://access.redhat.com/solutions/1234773 for details. CUPS is an example of a component that does not provide a way to disable SSL in its own configuration.
4.8.1. Installing stunnel
Install the stunnel package by entering the following command as
yum install stunnel
4.8.2. Configuring stunnel as a TLS Wrapper
To configure stunnel, follow these steps:
- You need a valid certificate for stunnel regardless of what service you use it with. If you do not have a suitable certificate, you can apply to a Certificate Authority to obtain one, or you can create a self-signed certificate.
WarningAlways use certificates signed by a Certificate Authority for servers running in a production environment. Self-signed certificates are only appropriate for testing purposes or private networks.See Section 184.108.40.206, “Creating a Certificate Signing Request” for more information about certificates granted by a Certificate Authority. On the other hand, to create a self-signed certificate for stunnel, enter the
/etc/pki/tls/certs/directory and type the following command as
make stunnel.pemAnswer all of the questions to complete the process.
- When you have a certificate, create a configuration file for stunnel. It is a text file in which every line specifies an option or the beginning of a service definition. You can also keep comments and empty lines in the file to improve its legibility, where comments start with a semicolon.The stunnel RPM package contains the
/etc/stunnel/directory, in which you can store the configuration file. Although stunnel does not require any special format of the file name or its extension, use
/etc/stunnel/stunnel.conf. The following content configures stunnel as a TLS wrapper:
cert = /etc/pki/tls/certs/stunnel.pem ; Allow only TLS, thus avoiding SSL sslVersion = TLSv1 chroot = /var/run/stunnel setuid = nobody setgid = nobody pid = /stunnel.pid socket = l:TCP_NODELAY=1 socket = r:TCP_NODELAY=1 [service_name] accept = port connect = port TIMEOUTclose = 0Alternatively, you can avoid SSL by replacing the line containing
sslVersion = TLSv1with the following lines:
options = NO_SSLv2 options = NO_SSLv3The purpose of the options is as follows:
cert— the path to your certificate
sslVersion— the version of SSL; note that you can use
TLShere even though SSL and TLS are two independent cryptographic protocols
chroot— the changed root directory in which the stunnel process runs, for greater security
setgid— the user and group that the stunnel process runs as;
nobodyis a restricted system account
pid— the file in which stunnel saves its process ID, relative to
socket— local and remote socket options; in this case, disable Nagle's algorithm to improve network latency
[service_name]— the beginning of the service definition; the options used below this line apply to the given service only, whereas the options above affect stunnel globally
accept— the port to listen on
connect— the port to connect to; this must be the port that the service you are securing uses
TIMEOUTclose— how many seconds to wait for the close_notify alert from the client;
0instructs stunnel not to wait at all
options— OpenSSL library options
Example 4.3. Securing CUPSTo configure stunnel as a TLS wrapper for CUPS, use the following values:
[cups] accept = 632 connect = 631Instead of
632, you can use any free port that you prefer.
631is the port that CUPS normally uses.
- Create the
chrootdirectory and give the user specified by the
setuidoption write access to it. To do so, enter the following commands as
chown nobody:nobody /var/run/stunnelThis allows stunnel to create the PID file.
- If your system is using firewall settings that disallow access to the new port, change them accordingly. See Section 5.6.7, “Opening Ports using GUI” for details.
- When you have created the configuration file and the
chrootdirectory, and when you are sure that the specified port is accessible, you are ready to start using stunnel.
4.8.3. Starting, Stopping, and Restarting stunnel
To start stunnel, enter the following command as
By default, stunnel uses
/var/log/secureto log its output.
To terminate stunnel, kill the process by running the following command as
kill `cat /var/run/stunnel/stunnel.pid`
If you edit the configuration file while stunnel is running, terminate stunnel and start it again for your changes to take effect.