Merge pull request #721 from lie-must-die/feat/unknown-sni-reject-handshake

Feat/unknown sni reject handshake

docs/Config_params/CONFIG_PARAMS.en.md

@@ -2297,7 +2297,7 @@ Note: This section also accepts the legacy alias `[server.admin_api]` (same sche
 | --- | ---- | ------- |
 | [`tls_domain`](#tls_domain) | `String` | `"petrovich.ru"` |
 | [`tls_domains`](#tls_domains) | `String[]` | `[]` |
-| [`unknown_sni_action`](#unknown_sni_action) | `"drop"`, `"mask"`, `"accept"` | `"drop"` |
+| [`unknown_sni_action`](#unknown_sni_action) | `"drop"`, `"mask"`, `"accept"`, `"reject_handshake"` | `"drop"` |
 | [`tls_fetch_scope`](#tls_fetch_scope) | `String` | `""` |
 | [`tls_fetch`](#tls_fetch) | `Table` | built-in defaults |
 | [`mask`](#mask) | `bool` | `true` |
@@ -2348,13 +2348,17 @@ Note: This section also accepts the legacy alias `[server.admin_api]` (same sche
     tls_domains = ["example.net", "example.org"]
     ```
 ## unknown_sni_action
-  - **Constraints / validation**: `"drop"`, `"mask"` or `"accept"`.
+  - **Constraints / validation**: `"drop"`, `"mask"`, `"accept"` or `"reject_handshake"`.
   - **Description**: Action for TLS ClientHello with unknown / non-configured SNI.
+    - `drop` — close the connection without any response (silent FIN after `server_hello_delay` is applied). Timing-indistinguishable from the Success branch, but wire-quieter than what a real web server would do.
+    - `mask` — transparently proxy the connection to `mask_host:mask_port` (TLS fronting). The client receives a real ServerHello from the backend with its real certificate. Maximum camouflage, but opens an outbound connection for every misdirected request.
+    - `accept` — pretend the SNI is valid and continue on the auth path. Weakens active-probing resistance; only meaningful in narrow scenarios.
+    - `reject_handshake` — emit a fatal TLS `unrecognized_name` alert (RFC 6066, AlertDescription = 112) and close the connection. Identical on the wire to a modern nginx with `ssl_reject_handshake on;` on its default vhost: looks like an ordinary HTTPS server that simply does not host the requested name. Recommended when the goal is maximal parity with a stock web server rather than TLS fronting. `server_hello_delay` is intentionally **not** applied to this branch, so the alert is emitted "instantly" the way a reference nginx would.
   - **Example**:
 
     ```toml
     [censorship]
-    unknown_sni_action = "drop"
+    unknown_sni_action = "reject_handshake"
     ```
 ## tls_fetch_scope
   - **Constraints / validation**: `String`. Value is trimmed during load; whitespace-only becomes empty.
@@ -3110,5 +3114,3 @@ If your backend or network is very bandwidth-constrained, reduce cap first. If p
     username = "alice"
     password = "secret"
     ```
-
-

docs/Config_params/CONFIG_PARAMS.ru.md

@@ -2224,7 +2224,7 @@
     ```
 ## relay_client_idle_soft_secs
   - **Ограничения / валидация**: Должно быть `> 0`; Должно быть меньше или равно `relay_client_idle_hard_secs`.
-  - **Описание**: Мягкий порог простоя (в секундах) для неактивности uplink клиента в промежуточном узле. При достижении этого порога сессия помечается как кандидат на простой и может быть удалена в зависимости от политики.
+  - **Описание**: Мягкий порог простоя (в секундах) для неактивности uplink клиента в промежуточном узле. При достижении этого порога сессия помечается как кандидат на простой и может быть удалена в зависимости от политики.
   - **Пример**:
 
     ```toml
@@ -2303,7 +2303,7 @@
 | --- | ---- | ------- |
 | [`tls_domain`](#tls_domain) | `String` | `"petrovich.ru"` |
 | [`tls_domains`](#tls_domains) | `String[]` | `[]` |
-| [`unknown_sni_action`](#unknown_sni_action) | `"drop"`, `"mask"`, `"accept"` | `"drop"` |
+| [`unknown_sni_action`](#unknown_sni_action) | `"drop"`, `"mask"`, `"accept"`, `"reject_handshake"` | `"drop"` |
 | [`tls_fetch_scope`](#tls_fetch_scope) | `String` | `""` |
 | [`tls_fetch`](#tls_fetch) | `Table` | built-in defaults |
 | [`mask`](#mask) | `bool` | `true` |
@@ -2353,13 +2353,17 @@
     tls_domains = ["example.net", "example.org"]
     ```
 ## unknown_sni_action
-  - **Ограничения / валидация**: `"drop"`, `"mask"` или `"accept"`.
+  - **Ограничения / валидация**: `"drop"`, `"mask"`, `"accept"` или `"reject_handshake"`.
   - **Описание**: Действие для TLS ClientHello с неизвестным/ненастроенным SNI.
+    - `drop` — закрыть соединение без ответа (молчаливый FIN после применения `server_hello_delay`). Поведение, неотличимое по таймингу от Success-ветки, но более «тихое», чем у обычного веб-сервера.
+    - `mask` — прозрачно проксировать соединение на `mask_host:mask_port` (TLS-fronting). Клиент получает настоящий ServerHello от реального бэкенда с его сертификатом. Максимальный камуфляж, но порождает исходящее соединение на каждый чужой запрос.
+    - `accept` — притвориться, что SNI валиден, и продолжить auth-путь. Снижает защиту от активного пробинга; осмысленно только в узких сценариях.
+    - `reject_handshake` — отправить фатальный TLS-alert `unrecognized_name` (RFC 6066, AlertDescription = 112) и закрыть соединение. Поведение, идентичное современному nginx с `ssl_reject_handshake on;` на дефолтном vhost'е: на wire-уровне выглядит как обычный HTTPS-сервер, у которого просто нет такого домена. Рекомендуется, если цель — максимальная похожесть на стоковый веб-сервер, а не tls-fronting. `server_hello_delay` на эту ветку не применяется, чтобы alert улетал «мгновенно», как у эталонного nginx.
   - **Пример**:
 
     ```toml
     [censorship]
-    unknown_sni_action = "drop"
+    unknown_sni_action = "reject_handshake"
     ```
 ## tls_fetch_scope
   - **Ограничения / валидация**: `String`. Значение обрезается во время загрузки; значение, состоящее только из пробелов, становится пустым.
@@ -3117,5 +3121,3 @@
     username = "alice"
     password = "secret"
     ```
-
-

docs/FAQ.en.md

@@ -210,6 +210,13 @@ If you need to allow connections with any domains (ignoring SNI mismatches), add
 unknown_sni_action = "mask"
 ```
 
+Alternatively, if you want telemt to behave like a vanilla nginx with `ssl_reject_handshake on;` on unknown SNI (emit a TLS `unrecognized_name` alert and close the connection), use:
+```toml
+[censorship]
+unknown_sni_action = "reject_handshake"
+```
+This does not recover stale clients, but it makes port 443 wire-indistinguishable from a stock web server that simply does not host the requested vhost.
+
 ### How to view metrics
 
 1. Open the configuration file: `nano /etc/telemt/telemt.toml`.

docs/FAQ.ru.md

@@ -227,6 +227,13 @@ curl -s http://127.0.0.1:9091/v1/users | jq
 unknown_sni_action = "mask"
 ```
 
+Альтернатива: если вы хотите, чтобы telemt на неизвестный SNI вёл себя как обычный nginx с `ssl_reject_handshake on;` (отдавал TLS-alert `unrecognized_name` и закрывал соединение), используйте:
+```toml
+[censorship]
+unknown_sni_action = "reject_handshake"
+```
+Это не пропускает старых клиентов, но делает поведение на 443-м порту неотличимым от стокового веб-сервера, у которого просто нет такого виртуального хоста.
+
 ## Как посмотреть метрики
 
 1. Откройте файл конфигурации: `nano /etc/telemt/telemt.toml`.

src/config/load.rs

@@ -1977,6 +1977,22 @@ mod tests {
             cfg_accept.censorship.unknown_sni_action,
             UnknownSniAction::Accept
         );
+
+        let cfg_reject: ProxyConfig = toml::from_str(
+            r#"
+            [server]
+            [general]
+            [network]
+            [access]
+            [censorship]
+            unknown_sni_action = "reject_handshake"
+            "#,
+        )
+        .unwrap();
+        assert_eq!(
+            cfg_reject.censorship.unknown_sni_action,
+            UnknownSniAction::RejectHandshake
+        );
     }
 
     #[test]

src/config/types.rs

@@ -1571,6 +1571,13 @@ pub enum UnknownSniAction {
     Drop,
     Mask,
     Accept,
+    /// Reject the TLS handshake by sending a fatal `unrecognized_name` alert
+    /// (RFC 6066, AlertDescription = 112) before closing the connection.
+    /// Mimics nginx `ssl_reject_handshake on;` behavior on the default vhost —
+    /// the wire response indistinguishable from a stock modern web server
+    /// that simply does not host the requested name.
+    #[serde(rename = "reject_handshake")]
+    RejectHandshake,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]

src/proxy/handshake.rs

@@ -1132,9 +1132,20 @@ where
                     "TLS handshake accepted by unknown SNI policy"
                 );
             }
-            action @ (UnknownSniAction::Drop | UnknownSniAction::Mask) => {
+            action @ (UnknownSniAction::Drop
+            | UnknownSniAction::Mask
+            | UnknownSniAction::RejectHandshake) => {
                 auth_probe_record_failure_in(shared, peer.ip(), Instant::now());
-                maybe_apply_server_hello_delay(config).await;
+                // For Drop/Mask we apply the synthetic ServerHello delay so
+                // the fail-closed path is timing-indistinguishable from the
+                // success path. For RejectHandshake we deliberately skip the
+                // delay: a stock modern nginx with `ssl_reject_handshake on;`
+                // responds with the alert essentially immediately, so
+                // injecting 8-24ms here would itself become a distinguisher
+                // against the public baseline we are trying to blend into.
+                if !matches!(action, UnknownSniAction::RejectHandshake) {
+                    maybe_apply_server_hello_delay(config).await;
+                }
                 let log_now = Instant::now();
                 if should_emit_unknown_sni_warn_in(shared, log_now) {
                     warn!(
@@ -1153,8 +1164,33 @@ where
                         "TLS handshake rejected by unknown SNI policy"
                     );
                 }
+                if matches!(action, UnknownSniAction::RejectHandshake) {
+                    // TLS alert record layer:
+                    //   0x15            ContentType.alert
+                    //   0x03 0x03       legacy_record_version = TLS 1.2
+                    //                   (matches what modern nginx emits in
+                    //                   the first server -> client record,
+                    //                   per RFC 8446 5.1 guidance)
+                    //   0x00 0x02       length = 2
+                    // Alert payload:
+                    //   0x02            AlertLevel.fatal
+                    //   0x70            AlertDescription.unrecognized_name (112, RFC 6066)
+                    const TLS_ALERT_UNRECOGNIZED_NAME: [u8; 7] =
+                        [0x15, 0x03, 0x03, 0x00, 0x02, 0x02, 0x70];
+                    if let Err(e) = writer.write_all(&TLS_ALERT_UNRECOGNIZED_NAME).await {
+                        debug!(
+                            peer = %peer,
+                            error = %e,
+                            "Failed to write unrecognized_name TLS alert"
+                        );
+                    } else {
+                        let _ = writer.flush().await;
+                    }
+                }
                 return match action {
-                    UnknownSniAction::Drop => HandshakeResult::Error(ProxyError::UnknownTlsSni),
+                    UnknownSniAction::Drop | UnknownSniAction::RejectHandshake => {
+                        HandshakeResult::Error(ProxyError::UnknownTlsSni)
+                    }
                     UnknownSniAction::Mask => HandshakeResult::BadClient { reader, writer },
                     UnknownSniAction::Accept => unreachable!(),
                 };

src/proxy/tests/handshake_security_tests.rs

@@ -1007,6 +1007,55 @@ async fn tls_unknown_sni_mask_policy_falls_back_to_bad_client() {
     assert!(matches!(result, HandshakeResult::BadClient { .. }));
 }
 
+#[tokio::test]
+async fn tls_unknown_sni_reject_handshake_policy_emits_unrecognized_name_alert() {
+    use tokio::io::{AsyncReadExt, duplex};
+
+    let secret = [0x4Au8; 16];
+    let mut config = test_config_with_secret_hex("4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a");
+    config.censorship.unknown_sni_action = UnknownSniAction::RejectHandshake;
+
+    let replay_checker = ReplayChecker::new(128, Duration::from_secs(60));
+    let rng = SecureRandom::new();
+    let peer: SocketAddr = "198.51.100.192:44326".parse().unwrap();
+    let handshake =
+        make_valid_tls_client_hello_with_sni_and_alpn(&secret, 0, "unknown.example", &[b"h2"]);
+
+    // Wire up a duplex so we can inspect what the server writes towards the
+    // client. We own the "peer side" half to read from it.
+    let (server_side, mut peer_side) = duplex(1024);
+    let (server_read, server_write) = tokio::io::split(server_side);
+
+    let result = handle_tls_handshake(
+        &handshake,
+        server_read,
+        server_write,
+        peer,
+        &config,
+        &replay_checker,
+        &rng,
+        None,
+    )
+    .await;
+
+    assert!(matches!(
+        result,
+        HandshakeResult::Error(ProxyError::UnknownTlsSni)
+    ));
+
+    // Drain what the server wrote. We expect exactly one TLS alert record:
+    //   0x15 0x03 0x03 0x00 0x02 0x02 0x70
+    // (ContentType.alert, TLS 1.2, length=2, fatal, unrecognized_name)
+    drop(result); // drops the server-side writer so peer_side sees EOF
+    let mut buf = Vec::new();
+    peer_side.read_to_end(&mut buf).await.unwrap();
+    assert_eq!(
+        buf,
+        [0x15, 0x03, 0x03, 0x00, 0x02, 0x02, 0x70],
+        "reject_handshake must emit a fatal unrecognized_name TLS alert"
+    );
+}
+
 #[tokio::test]
 async fn tls_unknown_sni_accept_policy_continues_auth_path() {
     let secret = [0x4Bu8; 16];