Important: This page was written in 2002. I was contacted a couple of years ago by the author of stunnel, indicating that stunnel has evolved so much since then that as far as stunnel information goes, this page is essentially worse than useless. I don't use stunnel anymore so am unable to update it. I should probably remove this page entirely, but I can't quite bring myself to do it. So please don't rely on the information on this page.
stunnel provides "drop-in" SSL-secured connectivity between two hosts. It is most often used to retroactively secure an existing application when encryption and/or authentication were not part of the original design.
When neither the client nor the server support SSL encapsulation of a connection, stunnel can be used on both sides of the connection.
In its default configuration, stunnel does not perform authentication; it merely encrypts the channel without verifying the endpoints.
This document describes how to set up stunnel to use signed certificates to perform bilateral authentication. Under this scheme, both the client and the server provide sufficient credentials (in the form of SSL certificates signed by a mutually acceptable authority) to establish their identity.
Black Text represents output from the software or prompts for information that will appear.
Green Text represents commands or information that (at least the first time) can be entered exactly as shown.
Red Text represents commands or information that must be customized to the particular situation before being entered. For example, WWWI should be changed to the name of your own organization.
stunnel has two significant limitations that affect its ability to perform bilateral authentication. The first is that there is no provision for encrypted keys. The second is that stunnel does not perform browser-style checking of the CN name field against the hostname of the connection. In combination, these limitations mean that if someone can access an stunnel key, they will not need a passphrase to decode it and that they can use that key from any connected host without regard for its hostname or IP address.
It is therefore vital to ensure that systems with stunnel keys are carefully secured and that the keys are stored with the most restrictive possible permissions.
This is not a strictly speaking a limitation of stunnel, but rather a difficulty HTTP-over-SSL has traditionally worked around by allowing the web server to prompt for a passphrase on startup, and by using browsers that compare the common name (CN) of a server's certificate to the hostname. Perhaps a future version of stunnel will offer an option to enable similar functionality.
YourPrompt&ht; openssl genrsa -out client.key 1024
Generating RSA private key, 1024 bit long modulus
e is 65537 (0x10001)
YourPrompt> openssl req -new -key client.key -out
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
Country Name (2 letter code) [AU]:US
State or Province Name (full name) [Some-State]:Texas
Locality Name (eg, city) :Montgomery
Organization Name (eg, company) [Internet Widgits Pty Ltd]:WWWI
Organizational Unit Name (eg, section) :WWWI Stunnel Services
Common Name (eg, YOUR name) :WWWI Client
Email Address :.
Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password :.
An optional company name :.
CA signing: client.csr -> client.crt:
Using configuration from ca.config
Enter PEM pass phrase:CAKeyPassPhrase
Check that the request matches the signature
The Subjects Distinguished Name is as follows
organizationalUnitName:PRINTABLE:'WWWI Stunnel Services'
commonName :PRINTABLE:'WWWI Client'
Certificate is to be certified until Feb 22 22:22:22 2003 GMT (365 days)
Sign the certificate? [y/n]:y
1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated
CA verifying: client.crt <-> CA cert
-----BEGIN RSA PRIVATE KEY-----
-----END RSA PRIVATE KEY-----
There are no extra blank lines or extraneous text needed in this file.
For this example, we will use mysql-3.23.X, which includes both client and server pieces and does not support native SSL connections.
YourPrompt> nohup mysqld --bind-address=127.0.0.1
Binding to the localhost IP address (127.0.0.1) ensures that other systems cannot gain unencrypted/unauthenticated access to MySQL.
YourPrompt> stunnel -A /usr/local/stunnel/stunnel-auth.pem -v 2 -p /usr/local/stunnel/stunnel-server.pem -d server.example.com:3307 -r localhost:3306
YourPrompt> stunnel -A /usr/local/stunnel/stunnel-auth.pem -v 2 -p /usr/local/stunnel/stunnel-client.pem -c -d localhost:3306 -r server.example.com:3307
YourPrompt> mysql -h 127.0.0.1
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 6615 to server version: 3.23.49
Type 'help;' or '\h' for help. Type '\c' to clear the buffer.
If problems occur with the mysql connection, check the stunnel log files or use -f to run stunnel in the foreground on each machine. It will provide information about any errors that occur in the certificate exchange process.
It is important to remember that from the MySQL server, all connections will appear to be originating from localhost. MySQL's internal user/host/password tables must be configured appropriately. Also, the SSL certificate exchange process can be relatively time consuming compared to an SQL query. Therefore, connection pooling becomes more important as a tool to improve performance when security is implemented in this fashion.
It is possible to further increase security by requiring stunnel to compare the certificate presented by its peer against a stored list of acceptable certificates. To take advantage of this feature:
Particular thanks go to the creators of stunnel, OpenSSL and MySQL for making such great products.
See also the general thanks and credits on the SSL page.
This document was written by Jeffrey D. Wheelhouse (firstname.lastname@example.org). This is version 1.0.3, created February 22, 2002 and last updated on February 2, 2013 to indicate it is now almost but not quite completely wrong. Comments, suggestions, and corrections are welcomed via email. This document and its contents are placed into the public domain for unlimited use and distribution.
Home | Software | SSL | Stunnel