Simple definition
SNI allows you to indicate the targeted domain name at the beginning of a TLS connection.
Technical definition
Server Name specification is a TLS extension sent by the client so that a server hosting multiple sites chooses the right certificate.
What’s the point?.
host multiple HTTPS sites on the same IP address.
Concrete example
a reverse proxy selects the certificate of example.fr with the SNI.
Common problems
- former client without SNI
- default certificate returned
- different SNI name from HTTP Host
To be noted: Always adapt the diagnosis to the version of the product and to the architecture actually deployed.