feat: Disable SNI for STARTTLS (#7499)

Many clients don't send it currently, so it is unlikely that servers depend on it:
https://mastodon.social/@cks/114690055923939576.

For "implicit TLS", do not turn it off yet, it will serve as a fallback in case of rare server that
needs it. If the server only supports STARTTLS and requires SNI then it is really weird, likely
should not happen.

src/imap/client.rs

@@ -207,6 +207,7 @@ impl Client {
         hostname: &str,
         strict_tls: bool,
     ) -> Result<Self> {
+        let use_sni = true;
         let tcp_stream = connect_tcp_inner(addr).await?;
         let account_id = context.get_id();
         let events = context.events.clone();
@@ -215,6 +216,7 @@ impl Client {
             strict_tls,
             hostname,
             addr.port(),
+            use_sni,
             alpn(addr.port()),
             logging_stream,
             &context.tls_session_store,
@@ -251,6 +253,7 @@ impl Client {
         host: &str,
         strict_tls: bool,
     ) -> Result<Self> {
+        let use_sni = false;
         let tcp_stream = connect_tcp_inner(addr).await?;
 
         let account_id = context.get_id();
@@ -275,6 +278,7 @@ impl Client {
             strict_tls,
             host,
             addr.port(),
+            use_sni,
             "",
             tcp_stream,
             &context.tls_session_store,
@@ -294,6 +298,7 @@ impl Client {
         strict_tls: bool,
         proxy_config: ProxyConfig,
     ) -> Result<Self> {
+        let use_sni = true;
         let proxy_stream = proxy_config
             .connect(context, domain, port, strict_tls)
             .await?;
@@ -301,6 +306,7 @@ impl Client {
             strict_tls,
             domain,
             port,
+            use_sni,
             alpn(port),
             proxy_stream,
             &context.tls_session_store,
@@ -340,6 +346,7 @@ impl Client {
         proxy_config: ProxyConfig,
         strict_tls: bool,
     ) -> Result<Self> {
+        let use_sni = false;
         let proxy_stream = proxy_config
             .connect(context, hostname, port, strict_tls)
             .await?;
@@ -362,6 +369,7 @@ impl Client {
             strict_tls,
             hostname,
             port,
+            use_sni,
             "",
             proxy_stream,
             &context.tls_session_store,

src/net.rs

@@ -131,11 +131,13 @@ pub(crate) async fn connect_tls_inner(
     alpn: &str,
     tls_session_store: &TlsSessionStore,
 ) -> Result<impl SessionStream + 'static> {
+    let use_sni = true;
     let tcp_stream = connect_tcp_inner(addr).await?;
     let tls_stream = wrap_tls(
         strict_tls,
         host,
         addr.port(),
+        use_sni,
         alpn,
         tcp_stream,
         tls_session_store,

src/net/http.rs

@@ -74,19 +74,33 @@ where
         }
         "https" => {
             let port = parsed_url.port_u16().unwrap_or(443);
-            let load_cache = true;
+            let (use_sni, load_cache) = (true, true);
 
             if let Some(proxy_config) = proxy_config_opt {
                 let proxy_stream = proxy_config
                     .connect(context, host, port, load_cache)
                     .await?;
-                let tls_stream =
+                let tls_stream = wrap_rustls(
-                    wrap_rustls(host, port, "", proxy_stream, &context.tls_session_store).await?;
+                    host,
+                    port,
+                    use_sni,
+                    "",
+                    proxy_stream,
+                    &context.tls_session_store,
+                )
+                .await?;
                 Box::new(tls_stream)
             } else {
                 let tcp_stream = crate::net::connect_tcp(context, host, port, load_cache).await?;
-                let tls_stream =
+                let tls_stream = wrap_rustls(
-                    wrap_rustls(host, port, "", tcp_stream, &context.tls_session_store).await?;
+                    host,
+                    port,
+                    use_sni,
+                    "",
+                    tcp_stream,
+                    &context.tls_session_store,
+                )
+                .await?;
                 Box::new(tls_stream)
             }
         }

src/net/proxy.rs

@@ -429,9 +429,11 @@ impl ProxyConfig {
                     load_cache,
                 )
                 .await?;
+                let use_sni = true;
                 let tls_stream = wrap_rustls(
                     &https_config.host,
                     https_config.port,
+                    use_sni,
                     "",
                     tcp_stream,
                     &context.tls_session_store,

src/net/tls.rs

@@ -13,12 +13,14 @@ pub async fn wrap_tls<'a>(
     strict_tls: bool,
     hostname: &str,
     port: u16,
+    use_sni: bool,
     alpn: &str,
     stream: impl SessionStream + 'static,
     tls_session_store: &TlsSessionStore,
 ) -> Result<impl SessionStream + 'a> {
     if strict_tls {
-        let tls_stream = wrap_rustls(hostname, port, alpn, stream, tls_session_store).await?;
+        let tls_stream =
+            wrap_rustls(hostname, port, use_sni, alpn, stream, tls_session_store).await?;
         let boxed_stream: Box<dyn SessionStream> = Box::new(tls_stream);
         Ok(boxed_stream)
     } else {
@@ -32,6 +34,7 @@ pub async fn wrap_tls<'a>(
         };
         let tls = async_native_tls::TlsConnector::new()
             .min_protocol_version(Some(async_native_tls::Protocol::Tlsv12))
+            .use_sni(use_sni)
             .request_alpns(&alpns)
             .danger_accept_invalid_hostnames(true)
             .danger_accept_invalid_certs(true);
@@ -90,6 +93,7 @@ impl TlsSessionStore {
 pub async fn wrap_rustls<'a>(
     hostname: &str,
     port: u16,
+    use_sni: bool,
     alpn: &str,
     stream: impl SessionStream + 'a,
     tls_session_store: &TlsSessionStore,
@@ -117,6 +121,7 @@ pub async fn wrap_rustls<'a>(
     let resumption = tokio_rustls::rustls::client::Resumption::store(resumption_store)
         .tls12_resumption(tokio_rustls::rustls::client::Tls12Resumption::Disabled);
     config.resumption = resumption;
+    config.enable_sni = use_sni;
 
     let tls = tokio_rustls::TlsConnector::from(Arc::new(config));
     let name = rustls_pki_types::ServerName::try_from(hostname)?.to_owned();

src/smtp/connect.rs

@@ -228,6 +228,7 @@ async fn connect_secure_proxy(
     strict_tls: bool,
     proxy_config: ProxyConfig,
 ) -> Result<Box<dyn SessionBufStream>> {
+    let use_sni = true;
     let proxy_stream = proxy_config
         .connect(context, hostname, port, strict_tls)
         .await?;
@@ -235,6 +236,7 @@ async fn connect_secure_proxy(
         strict_tls,
         hostname,
         port,
+        use_sni,
         alpn(port),
         proxy_stream,
         &context.tls_session_store,
@@ -253,6 +255,7 @@ async fn connect_starttls_proxy(
     strict_tls: bool,
     proxy_config: ProxyConfig,
 ) -> Result<Box<dyn SessionBufStream>> {
+    let use_sni = false;
     let proxy_stream = proxy_config
         .connect(context, hostname, port, strict_tls)
         .await?;
@@ -266,6 +269,7 @@ async fn connect_starttls_proxy(
         strict_tls,
         hostname,
         port,
+        use_sni,
         "",
         tcp_stream,
         &context.tls_session_store,
@@ -316,6 +320,7 @@ async fn connect_starttls(
     strict_tls: bool,
     tls_session_store: &TlsSessionStore,
 ) -> Result<Box<dyn SessionBufStream>> {
+    let use_sni = false;
     let tcp_stream = connect_tcp_inner(addr).await?;
 
     // Run STARTTLS command and convert the client back into a stream.
@@ -327,6 +332,7 @@ async fn connect_starttls(
         strict_tls,
         host,
         addr.port(),
+        use_sni,
         "",
         tcp_stream,
         tls_session_store,