TIL Common Name (CN) is legacy and subjectAltName must always be used.

Seems I’ve been living under a rock for far too log. From RFC2818:

Although the use of the Common Name is existing practice, it is deprecated and Certification Authorities are encouraged to use the dNSName instead.

So in today’s world CN is only evaluated when subjectAltName is not present and if it’s set all host names, IPs, emails, etc. must be specified in subjectAltName.

As a bonus, below is a one-liner to generate CSR with subjectAltName:

openssl req -new -newkey rsa:2048 -keyout example.com.key -sha256 -nodes -days 36500 -out example.com.csr -subj "/C=US/ST=IL/L=Chicago/O=Fortune500/OU=IT/CN=example.com" -reqexts v3_req -config <(cat /etc/pki/tls/openssl.cnf <(printf "[ v3_req ]\nsubjectAltName = DNS:example.com,DNS:www.example.com"))
Posted on July 6, 2016 at 2:52 pm by sergeyt · Permalink

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