Creating Multiple Domain (UCC) CSR

Multiple Names on One Certificate

While it is not possible without TLS extensions to serve different certificates for a single IP (See here on how to setup apache on Debian for TLS extensions.) it is possible to have a single certificate that works with any number of hostnames. I’m not talking about a wildcard certificate but a certificate that allows completely different hostnames to be valid for a single certificate. For example www.foo.com and www.foo.org can share a certificate. This approach would not be appropriate except in certain circumstances. You wouldn’t want to have two different customers using the same certificate but a single customer may wish to use one certificate for all of their domains. Both Internet Explorer and Firefox honor certificates of this type. From what I read some Java SSL libraries do not handle this type of certificate properly but Java was the only exception.

x509 certificates, those that are served in SSL communications, offer a feature known as Subject Altnerative Names. A subject Alternative Name is an attribute that lists an alternate name for the subject of the certificate (that’s oddly fitting isn’t it?). In a web context that subject is the hostname. However it’s not just hostnames that can be an alternative subject. Email is an option as is IP addresses.

The first step is to create a CSR (certificate signing request) that contains the subject alternative names that you desire for your certificate. I will show how to do that using openssl. You will likely need to modify the default openssl.cnf file. In Debian this is located in /etc/ssl/openssl.cnf. Note that you may prefer to make modifications to a local copy and tell openssl to use your locally modified copy using the -config option. For simplicity I will omit -config localopenssl.cnf from my examples.

Config File Settings

You need to tell openssl to create a CSR that includes x509 V3 extensions and you also need to tell openssl to include a list of subject alternative names in your CSR. In my openssl.cnf I have the following:

In the [req] section

[req] req_extensions = v3_req

In the v3_req section:

[ v3_req ]  

# Extensions to add to a certificate request 

basicConstraints = CA:FALSE 

keyUsage = nonRepudiation, digitalSignature, keyEncipherment  

# Some CAs do not yet support subjectAltName in CSRs. 

# Instead the additional names are form entries on web 

# pages where one requests the certificate... 

subjectAltName          = @alt_names  [alt_names] DNS.1   = www.foo.com DNS.2   = www.foo.org

Generating the CSR

Then the CSR is generated using:

$ openssl req -new -out $CSR_FILENAME -key $KEY_FILE

To check to see if you got everything correct use:

$ openssl req -text -noout -in $CSR_FILENAME

You should see something similar to this:

Attributes:

Requested Extensions:             X509v3 

Basic Constraints:                 CA:FALSE             

X509v3 

Key Usage:                 Digital Signature, Non Repudiation, Key Encipherment             X509v3 

Subject Alternative Name:                 DNS:www.foo.com, DNS:www.foo.org