Does using SNI have any benefit whatsoever (whether security, scalability, or ot

Question

Does using SNI have any benefit whatsoever (whether security, scalability, or otherwise) if the TLS certificate validation is performed exclusively using the certificate fingerprint and without regard to the common name? (Examples of systems where this might plausibly be the case include DANE and Convergence.) Even weird benefits that only affect small niche use cases qualify as benefits.

I'm assuming that 100% of visitors will be using fingerprint validation of certificates, so backward-compatibility with common name validation is not a factor.

My motivation for eliminating SNI is that it leaks metadata to a passive eavesdropper. While this is not necessarily a big deal in most cases, it bothers me enough that if it has no benefit in the system I'm describing, I would remove it.

Explanation / Answer

With SNI the client sends the expected hostname within the ClientHello. This is necessary if you have multiple certificates behind the same IP address. It does not matter if you validate the certificates the usual way or if you verify it by fingerprint - the only question is if you have a single certificate on the IP (no SNI needed) or multiple certificates (SNI needed).

If the server is under your control and your application should only connect to your server, then a single certificate is enough, so no SNI is needed. But, if you use a CDN then you often have multiple certificates behind the same IP and the relation between the actual server/IP and the hosted certificates might change over time. In this case it is not enough to identify the server as owned by the CDN, but you have to make sure that your certificate is located on the server and you have to tell the server to use the configuration behind the certificate. In this case you need SNI.

And, while the Host header inside an HTTP request also specifies the requested hostname, this is only a property of HTTP (and it is not even required with HTTP/1.0). With protocols like SMTP you don't have such information but SNI is still used with these protocols on the client side (postfix, exim) and also the server side (exim).

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