Creating Wildcard self-signed certificates with openssl with subjectAltName (SAN - Subject Alternate Name)

For the past few hours I have been trying to create a self-signed certificate for all the sub-domains for my staging setup using wildcard subdomain.

There are a lot of guides and tutorials on the internet out there which explain the process of creating a self-signed certificate using openssl with a good amount details. Further there are also certain guides to create a self-signed cert for wildcard domain. It's fairly easy. You just specify that your Common Name (CN) a.k.a FQDN is *.yourdomain.com while creating the certificate signing request (CSR).

This will take care of all of your sub-domains under yourdomain.com (like www.yourdomain.com or mail.yourdomain.com), however your top level bare domain (yourdomain.com) itself is not covered under this certificate. When you use a certificate generated by specifying *.yourdomain.com the browsers will throw up an error when you hit your server with the top level domain name https://yourdomain.com/.

To address this X.509 certificate standard allows for a type of extension named subjectAltName (http://en.wikipedia.org/wiki/SubjectAltName). Using this you can specify that there are a few other domain names for which this certificate is valid.

This requires specifying the use of this extension while generating the certificate request AND while signing the certificate. To do this you will have to add a few things to your openssl configuration file (typically /etc/ssl/openssl.cnf on a Ubuntu like machine). Alternatively you can copy the config file to another location, add these extension stuff there and then specify the new config file for all your openssl commands. The commands shown below assume the default config file at /etc/ssl/openssl.cnf was updated with extension details.

Here is one blog post which details the updates needed for the openssl config file. http://grevi.ch/blog/ssl-certificate-request-with-subject-alternative-names-san. It also has commands for generating the private key, converting the key to a format which does not ask for password (a.k.a unencrypted key), generating the certificate request (CSR) and finally signing the certificate. The steps mentioned there until the generation of certificate request are correct. It is the last step of signing the certificate that is missing one small piece of information because of which the extension mentioned above (subjectAltName) doesn't get added to the final certificate, despite they being present in the certificate request.

After a lot of searching on the internet, copy pasting the commands exactly, trying my luck at IRC (irc.freenode.net#openssl) the answer finally appeared to me in the man pages (duh..! RTFM dude..!!).  The man page for x509 (man x509) command of openssl has this little entry :

-extfile filename
           file containing certificate extensions to use. If not specified then no extensions are added to the certificate.

So turns out that just specifying the extensions in the openssl config file is not sufficient, but you must also specify the same file as the file containing the extensions to be included in the command line using the above -extfile option.

With that added, you will get a self-signed certificate for your wildcard subdomain which is also valid for your top level bare domain.

Changes made to /etc/ssl/openssl.cnf

Uncomment the req_extensions = v3_req

req_extensions = v3_req # The extensions to add to a certificate request

Add subjectAltName to v3_req section

[ v3_req ]

# Extensions to add to a certificate request

basicConstraints = CA:FALSE
keyUsage = nonRepudiation, digitalSignature, keyEncipherment
subjectAltName = @alt_names

Finally add the alternate names for which you want this certificate to be valid. It would be your toplevel bare domain

[alt_names]
DNS.1 = yourdomain.com

This last section [alt_names] will not be present in the file. You can add it right after the [ v3_req ] section.

Once config update is done, create your certificate.
Here are the commands that I used to create such a certificate :

Create a private key :

openssl genrsa -des3 -out ssl/staging/yourdomain.com.key 2048

This will ask you for a password. Key in a simple one and remember it.

Convert the private key to an unencrypted format

openssl rsa -in ssl/staging/yourdomain.com.key -out ssl/staging/yourdomain.com.key.rsa

This will ask you for a password. Key in the same thing that you used in the previous step.

Create the certificate signing request

openssl req -new -key ssl/staging/yourdomain.com.key.rsa -out ssl/staging/yourdomain.com.csr

This will ask you for a bunch of fields. Enter *.yourdomain.com when it asks for Common Name (or FQDN). The fields after that can be left blank by just hitting return (Enter) key.

Sign the certificate with extensions

openssl x509 -req -extensions v3_req -days 365 -in ssl/staging/yourdomain.com.csr -signkey ssl/staging/yourdomain.com.key.rsa -out ssl/staging/yourdomain.com.crt -extfile /etc/ssl/openssl.cnf

Note that here we specify the openssl config file as the file file containing extensions as that is where we have defined it. Probably we can put the extensions in a separate file too, but I haven't tried that. (Homework?! :P)

Thats it. Now you have a self-signed wildcard subdomain certificate which is valid for your top level domain too.

Despite this the first time you hit your SSL enabled website with the above generated certificate your browser will show the standard "Invalid Certificate - Untrusted" page. It is because your certificate is self-signed. You can view the errors in the "Technical Details" section.