Add dynamic SNI mask target mode

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

src/config/hot_reload.rs

@@ -620,6 +620,7 @@ fn warn_non_hot_changes(old: &ProxyConfig, new: &ProxyConfig, non_hot_changed: b
         || old.censorship.tls_domains != new.censorship.tls_domains
         || old.censorship.tls_fetch_scope != new.censorship.tls_fetch_scope
         || old.censorship.mask != new.censorship.mask
+        || old.censorship.mask_dynamic != new.censorship.mask_dynamic
         || old.censorship.mask_host != new.censorship.mask_host
         || old.censorship.mask_port != new.censorship.mask_port
         || old.censorship.exclusive_mask != new.censorship.exclusive_mask

src/config/load.rs

@@ -372,6 +372,7 @@ const CENSORSHIP_CONFIG_KEYS: &[&str] = &[
     "tls_fetch_scope",
     "tls_fetch",
     "mask",
+    "mask_dynamic",
     "mask_host",
     "mask_port",
     "exclusive_mask",
@@ -2047,11 +2048,6 @@ impl ProxyConfig {
             *mask_host = normalize_mask_host_to_ascii(mask_host, "censorship.mask_host")?;
         }
 
-        // Default mask_host to tls_domain if not set and no unix socket configured.
-        if config.censorship.mask_host.is_none() && config.censorship.mask_unix_sock.is_none() {
-            config.censorship.mask_host = Some(config.censorship.tls_domain.clone());
-        }
-
         for (domain, target) in &config.censorship.exclusive_mask {
             if !is_valid_tls_domain_name(domain) {
                 return Err(ProxyError::Config(format!(

src/config/types.rs

@@ -1726,6 +1726,10 @@ pub struct AntiCensorshipConfig {
     #[serde(default = "default_true")]
     pub mask: bool,
 
+    /// Use the ClientHello SNI as the mask TCP target for configured TLS domains.
+    #[serde(default = "default_true")]
+    pub mask_dynamic: bool,
+
     #[serde(default)]
     pub mask_host: Option<String>,
 
@@ -1861,6 +1865,7 @@ impl Default for AntiCensorshipConfig {
             tls_fetch_scope: default_tls_fetch_scope(),
             tls_fetch: TlsFetchConfig::default(),
             mask: default_true(),
+            mask_dynamic: default_true(),
             mask_host: None,
             mask_port: default_mask_port(),
             exclusive_mask: HashMap::new(),

src/proxy/masking.rs

@@ -385,7 +385,7 @@ mod tls_domain_mask_host_tests {
         let mut config = ProxyConfig::default();
         config.censorship.tls_domain = "a.com".to_string();
         config.censorship.tls_domains = vec!["b.com".to_string(), "c.com".to_string()];
-        config.censorship.mask_host = Some("a.com".to_string());
+        config.censorship.mask_host = None;
         config
     }
 
@@ -419,6 +419,15 @@ mod tls_domain_mask_host_tests {
         assert_eq!(mask_host_for_initial_data(&config, &initial_data), "b.com");
     }
 
+    #[test]
+    fn mask_host_uses_primary_domain_when_dynamic_masking_is_disabled() {
+        let mut config = config_with_tls_domains();
+        config.censorship.mask_dynamic = false;
+        let initial_data = client_hello_with_sni("b.com");
+
+        assert_eq!(mask_host_for_initial_data(&config, &initial_data), "a.com");
+    }
+
     #[test]
     fn exclusive_mask_target_overrides_only_matching_sni() {
         let mut config = config_with_tls_domains();
@@ -577,24 +586,32 @@ fn default_mask_tcp_target_for_initial_data<'a>(
         .as_deref()
         .unwrap_or(&config.censorship.tls_domain);
 
-    if !configured_mask_host.eq_ignore_ascii_case(&config.censorship.tls_domain) {
+    if config.censorship.mask_host.is_none() && config.censorship.mask_dynamic {
+        let extracted_sni = if sni.is_none() {
+            tls::extract_sni_from_client_hello(initial_data)
+        } else {
+            None
+        };
+        if let Some(host) = sni
+            .or(extracted_sni.as_deref())
+            .and_then(|sni| matching_tls_domain_for_sni(config, sni))
+        {
+            return MaskTcpTarget {
+                host,
+                port: config.censorship.mask_port,
+            };
+        }
+    }
+
+    if let Some(mask_host) = config.censorship.mask_host.as_deref() {
         return MaskTcpTarget {
-            host: configured_mask_host,
+            host: mask_host,
             port: config.censorship.mask_port,
         };
     }
 
-    let extracted_sni = if sni.is_none() {
-        tls::extract_sni_from_client_hello(initial_data)
-    } else {
-        None
-    };
-    let host = sni
-        .or(extracted_sni.as_deref())
-        .and_then(|sni| matching_tls_domain_for_sni(config, sni))
-        .unwrap_or(configured_mask_host);
     MaskTcpTarget {
-        host,
+        host: configured_mask_host,
         port: config.censorship.mask_port,
     }
 }