TLS Validator

Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>

src/config/load.rs

@@ -1267,6 +1267,10 @@ mod tests {
             cfg.server.proxy_protocol_trusted_cidrs,
             default_proxy_protocol_trusted_cidrs()
         );
+        assert_eq!(
+            cfg.censorship.unknown_sni_action,
+            UnknownSniAction::Drop
+        );
         assert_eq!(cfg.server.api.listen, default_api_listen());
         assert_eq!(cfg.server.api.whitelist, default_api_whitelist());
         assert_eq!(
@@ -1403,6 +1407,10 @@ mod tests {
             server.proxy_protocol_trusted_cidrs,
             default_proxy_protocol_trusted_cidrs()
         );
+        assert_eq!(
+            AntiCensorshipConfig::default().unknown_sni_action,
+            UnknownSniAction::Drop
+        );
         assert_eq!(server.api.listen, default_api_listen());
         assert_eq!(server.api.whitelist, default_api_whitelist());
         assert_eq!(
@@ -1473,6 +1481,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn unknown_sni_action_parses_and_defaults_to_drop() {
+        let cfg_default: ProxyConfig = toml::from_str(
+            r#"
+            [server]
+            [general]
+            [network]
+            [access]
+            [censorship]
+            "#,
+        )
+        .unwrap();
+        assert_eq!(cfg_default.censorship.unknown_sni_action, UnknownSniAction::Drop);
+
+        let cfg_mask: ProxyConfig = toml::from_str(
+            r#"
+            [server]
+            [general]
+            [network]
+            [access]
+            [censorship]
+            unknown_sni_action = "mask"
+            "#,
+        )
+        .unwrap();
+        assert_eq!(cfg_mask.censorship.unknown_sni_action, UnknownSniAction::Mask);
+    }
+
     #[test]
     fn dc_overrides_allow_string_and_array() {
         let toml = r#"

src/config/types.rs

@@ -1359,6 +1359,14 @@ impl Default for TimeoutsConfig {
     }
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+#[serde(rename_all = "lowercase")]
+pub enum UnknownSniAction {
+    #[default]
+    Drop,
+    Mask,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct AntiCensorshipConfig {
     #[serde(default = "default_tls_domain")]
@@ -1368,6 +1376,10 @@ pub struct AntiCensorshipConfig {
     #[serde(default)]
     pub tls_domains: Vec<String>,
 
+    /// Policy for TLS ClientHello with unknown (non-configured) SNI.
+    #[serde(default)]
+    pub unknown_sni_action: UnknownSniAction,
+
     /// Upstream scope used for TLS front metadata fetches.
     /// Empty value keeps default upstream routing behavior.
     #[serde(default = "default_tls_fetch_scope")]
@@ -1478,6 +1490,7 @@ impl Default for AntiCensorshipConfig {
         Self {
             tls_domain: default_tls_domain(),
             tls_domains: Vec::new(),
+            unknown_sni_action: UnknownSniAction::Drop,
             tls_fetch_scope: default_tls_fetch_scope(),
             mask: default_true(),
             mask_host: None,

src/error.rs

@@ -216,6 +216,9 @@ pub enum ProxyError {
     #[error("Invalid proxy protocol header")]
     InvalidProxyProtocol,
 
+    #[error("Unknown TLS SNI")]
+    UnknownTlsSni,
+
     #[error("Proxy error: {0}")]
     Proxy(String),
 

src/proxy/client.rs

@@ -317,6 +317,13 @@ fn record_handshake_failure_class(
     record_beobachten_class(beobachten, config, peer_ip, class);
 }
 
+#[inline]
+fn increment_bad_on_unknown_tls_sni(stats: &Stats, error: &ProxyError) {
+    if matches!(error, ProxyError::UnknownTlsSni) {
+        stats.increment_connects_bad();
+    }
+}
+
 fn is_trusted_proxy_source(peer_ip: IpAddr, trusted: &[IpNetwork]) -> bool {
     if trusted.is_empty() {
         static EMPTY_PROXY_TRUST_WARNED: OnceLock<AtomicBool> = OnceLock::new();
@@ -508,7 +515,10 @@ where
                         beobachten.clone(),
                     ));
                 }
-                HandshakeResult::Error(e) => return Err(e),
+                HandshakeResult::Error(e) => {
+                    increment_bad_on_unknown_tls_sni(stats.as_ref(), &e);
+                    return Err(e);
+                }
             };
 
             debug!(peer = %peer, "Reading MTProto handshake through TLS");
@@ -959,7 +969,10 @@ impl RunningClientHandler {
                     self.beobachten.clone(),
                 ));
             }
-            HandshakeResult::Error(e) => return Err(e),
+            HandshakeResult::Error(e) => {
+                increment_bad_on_unknown_tls_sni(stats.as_ref(), &e);
+                return Err(e);
+            }
         };
 
         debug!(peer = %peer, "Reading MTProto handshake through TLS");

src/proxy/handshake.rs

@@ -16,7 +16,7 @@ use tokio::io::{AsyncRead, AsyncWrite, AsyncWriteExt};
 use tracing::{debug, trace, warn};
 use zeroize::{Zeroize, Zeroizing};
 
-use crate::config::ProxyConfig;
+use crate::config::{ProxyConfig, UnknownSniAction};
 use crate::crypto::{AesCtr, SecureRandom, sha256};
 use crate::error::{HandshakeResult, ProxyError};
 use crate::protocol::constants::*;
@@ -510,6 +510,21 @@ fn decode_user_secrets(
     secrets
 }
 
+#[inline]
+fn find_matching_tls_domain<'a>(config: &'a ProxyConfig, sni: &str) -> Option<&'a str> {
+    if config.censorship.tls_domain.eq_ignore_ascii_case(sni) {
+        return Some(config.censorship.tls_domain.as_str());
+    }
+
+    for domain in &config.censorship.tls_domains {
+        if domain.eq_ignore_ascii_case(sni) {
+            return Some(domain.as_str());
+        }
+    }
+
+    None
+}
+
 async fn maybe_apply_server_hello_delay(config: &ProxyConfig) {
     if config.censorship.server_hello_delay_max_ms == 0 {
         return;
@@ -593,6 +608,25 @@ where
     }
 
     let client_sni = tls::extract_sni_from_client_hello(handshake);
+    let matched_tls_domain = client_sni
+        .as_deref()
+        .and_then(|sni| find_matching_tls_domain(config, sni));
+
+    if client_sni.is_some() && matched_tls_domain.is_none() {
+        auth_probe_record_failure(peer.ip(), Instant::now());
+        maybe_apply_server_hello_delay(config).await;
+        debug!(
+            peer = %peer,
+            sni = ?client_sni,
+            action = ?config.censorship.unknown_sni_action,
+            "TLS handshake rejected by unknown SNI policy"
+        );
+        return match config.censorship.unknown_sni_action {
+            UnknownSniAction::Drop => HandshakeResult::Error(ProxyError::UnknownTlsSni),
+            UnknownSniAction::Mask => HandshakeResult::BadClient { reader, writer },
+        };
+    }
+
     let secrets = decode_user_secrets(config, client_sni.as_deref());
 
     let validation = match tls::validate_tls_handshake_with_replay_window(
@@ -633,16 +667,8 @@ where
 
     let cached = if config.censorship.tls_emulation {
         if let Some(cache) = tls_cache.as_ref() {
-            let selected_domain = if let Some(sni) = client_sni.as_ref() {
+            let selected_domain = matched_tls_domain.unwrap_or(config.censorship.tls_domain.as_str());
-                if cache.contains_domain(sni).await {
+            let cached_entry = cache.get(selected_domain).await;
-                    sni.clone()
-                } else {
-                    config.censorship.tls_domain.clone()
-                }
-            } else {
-                config.censorship.tls_domain.clone()
-            };
-            let cached_entry = cache.get(&selected_domain).await;
             let use_full_cert_payload = cache
                 .take_full_cert_budget_for_ip(
                     peer.ip(),

src/proxy/tests/handshake_security_tests.rs

@@ -956,6 +956,89 @@ async fn stress_tls_sni_preferred_user_hint_scales_to_large_user_set() {
     }
 }
 
+#[tokio::test]
+async fn tls_unknown_sni_drop_policy_returns_hard_error() {
+    let secret = [0x48u8; 16];
+    let mut config = test_config_with_secret_hex("48484848484848484848484848484848");
+    config.censorship.unknown_sni_action = UnknownSniAction::Drop;
+
+    let replay_checker = ReplayChecker::new(128, Duration::from_secs(60));
+    let rng = SecureRandom::new();
+    let peer: SocketAddr = "198.51.100.190:44326".parse().unwrap();
+    let handshake =
+        make_valid_tls_client_hello_with_sni_and_alpn(&secret, 0, "unknown.example", &[b"h2"]);
+
+    let result = handle_tls_handshake(
+        &handshake,
+        tokio::io::empty(),
+        tokio::io::sink(),
+        peer,
+        &config,
+        &replay_checker,
+        &rng,
+        None,
+    )
+    .await;
+
+    assert!(matches!(
+        result,
+        HandshakeResult::Error(ProxyError::UnknownTlsSni)
+    ));
+}
+
+#[tokio::test]
+async fn tls_unknown_sni_mask_policy_falls_back_to_bad_client() {
+    let secret = [0x49u8; 16];
+    let mut config = test_config_with_secret_hex("49494949494949494949494949494949");
+    config.censorship.unknown_sni_action = UnknownSniAction::Mask;
+
+    let replay_checker = ReplayChecker::new(128, Duration::from_secs(60));
+    let rng = SecureRandom::new();
+    let peer: SocketAddr = "198.51.100.191:44326".parse().unwrap();
+    let handshake =
+        make_valid_tls_client_hello_with_sni_and_alpn(&secret, 0, "unknown.example", &[b"h2"]);
+
+    let result = handle_tls_handshake(
+        &handshake,
+        tokio::io::empty(),
+        tokio::io::sink(),
+        peer,
+        &config,
+        &replay_checker,
+        &rng,
+        None,
+    )
+    .await;
+
+    assert!(matches!(result, HandshakeResult::BadClient { .. }));
+}
+
+#[tokio::test]
+async fn tls_missing_sni_keeps_legacy_auth_path() {
+    let secret = [0x4Au8; 16];
+    let mut config = test_config_with_secret_hex("4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a4a");
+    config.censorship.unknown_sni_action = UnknownSniAction::Drop;
+
+    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_handshake(&secret, 0);
+
+    let result = handle_tls_handshake(
+        &handshake,
+        tokio::io::empty(),
+        tokio::io::sink(),
+        peer,
+        &config,
+        &replay_checker,
+        &rng,
+        None,
+    )
+    .await;
+
+    assert!(matches!(result, HandshakeResult::Success(_)));
+}
+
 #[tokio::test]
 async fn alpn_enforce_rejects_unsupported_client_alpn() {
     let secret = [0x33u8; 16];