Adding SSL for a domain name using a CNAME record

Question

I'm hosting a website with Fortrabbit. I point subdomain.mywebsite.com to mywebapp.eu1.frbit.net using a CNAME record, next I wanted to add ssl.

I created a certificate signing request and applied for a certificate using the following:

openssl req -new -nodes -keyout my-app.key -out my-app.csr -newkey rsa:2048

I've just received my certificate and have entered it in the admin panel, but visiting the subdomain.mywebsite.com gives me the following:

subdomain.mywebsite.com uses an invalid security certificate.

The certificate is only valid for *.eu1.frbit.net

(Error code: ssl_error_bad_cert_domain)

Have I entered details in the Certificate request incorrectly?

Country Name (2 letter code) [AU]:GB
State or Province Name (full name) [Some-State]:London
Locality Name (eg, city) []:London
Organization Name (eg, company) [Internet Widgits Pty Ltd]:My Website
Organizational Unit Name (eg, section) []:
Common Name (e.g. server FQDN or YOUR name) []:subdomain.mywebsite.com
Email Address []:admin@mywebsite.com

Please enter the following 'extra' attributes
to be sent with your certificate request
A challenge password []:
An optional company name []:My Website Ltd

Edit

Looking at this SSL and CNAMEs? I think I have it right, but the results say otherwise.

Answer 1

Normally, a SSL certificate only allows one Common Name (CN). You can add one or more Alternative Names (SANs) to a SSL certificate, but most CA's charge more to sign those certificates and your browser or application needs to understand SAN (although all modern web browsers do so).