Simple definition
Extension of certificate listing names DNS or addresses covered by the certificate.
Technical definition
Extension of certificate listing names DNS In a professional environment, this mechanism usually integrates with other services to validate multiple host names with the same certificate.
What is it used for?
validate multiple host names with the same certificate.
Practical example
a certificate covers server.exemple.fr and autodiscover.exemple.fr The SAN.
Common issues
- expired certificate or name not covered
- incomplete confidence chain
- wrong key, revocation or automation of renewal
Key takeaway: Always adapt the diagnosis to the product version and the architecture actually deployed.