Certbot

https://unix.stackexchange.com/questions/301266/certbot-add-www-domain-to-existing-domain-certificate

DNS

certbot certonly --manual --preferred-challenges dns -d "paulelam.com"

certbot certonly --manual --preferred-challenges dns --expand -d "paulelam.com" -d "www.paulelam.com"

Examine certificates

openssl x509 -in certificate.pem -text

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