Specify whether you want Amazon Route 53 to send the value of FullyQualifiedDomainName to the endpoint in the client_hello message during TLS negotiation. This allows the endpoint to respond to HTTPS health check requests with the applicable SSL/TLS certificate.
Some endpoints require that HTTPS requests include the host name in the client_hello message. If you don't enable SNI, the status of the health check will be SSL alert handshake_failure. A health check can also have that status for other reasons. If SNI is enabled and you're still getting the error, check the SSL/TLS configuration on your endpoint and confirm that your certificate is valid.
The SSL/TLS certificate on your endpoint includes a domain name in the Common Name field and possibly several more in the Subject Alternative Names field. One of the domain names in the certificate should match the value that you specify for FullyQualifiedDomainName. If the endpoint responds to the client_hello message with a certificate that does not include the domain name that you specified in FullyQualifiedDomainName, a health checker will retry the handshake. In the second attempt, the health checker will omit FullyQualifiedDomainName from the client_hello message.