Allow SSL_TLSEXT_ERR_NOACK from servername callback

Author: ctz

src/callbacks.rs

@@ -160,6 +160,9 @@ impl ServerNameCallbackConfig {
 
         match result {
             SSL_TLSEXT_ERR_OK => Ok(()),
+            // in practice no client does anything if SNI is not acked, and rustls
+            // acks any syntactically valid extension (and ignores invalid ones, because OpenSSL)
+            SSL_TLSEXT_ERR_NOACK => Ok(()),
             _ => Err(Error::not_supported(
                 "SSL_CTX_servername_callback_func return error",
             )),