Gate non-loopback remote binds

docs/NATIVE_V1_SPEC.md

@@ -506,6 +506,7 @@ native_auth_rate_limit_per_minute = 30
 attach_ticket_ttl_secs = 86400
 allow_tcp_forwarding = true
 allow_remote_forwarding = false
+allow_remote_non_loopback_bind = false
 allow_agent_forwarding = false
 accept_env = ["LANG", "LC_*", "TERM", "COLORTERM"]
 ```

src/bin/dosh-server.rs

@@ -1443,10 +1443,7 @@ async fn start_remote_forwards(
             .bind_host
             .clone()
             .unwrap_or_else(|| "127.0.0.1".to_string());
-        anyhow::ensure!(
+        remote_bind_allowed(&config, &bind_host)?;
-            is_loopback_bind(&bind_host),
-            "remote forwarding may only bind loopback in this server build"
-        );
         let listen_port = forward.listen_port;
         let target_host = forward
             .target_host
@@ -1547,6 +1544,16 @@ fn is_loopback_bind(host: &str) -> bool {
     matches!(host, "127.0.0.1" | "localhost" | "::1")
 }
 
+fn remote_bind_allowed(config: &ServerConfig, host: &str) -> Result<()> {
+    if !is_loopback_bind(host) {
+        anyhow::ensure!(
+            config.allow_remote_non_loopback_bind,
+            "remote forwarding non-loopback bind disabled"
+        );
+    }
+    Ok(())
+}
+
 fn stream_open_allowed(
     config: &ServerConfig,
     client: &ClientState,
@@ -2151,4 +2158,17 @@ mod tests {
         assert_eq!(data.stream_id, 42);
         assert_eq!(data.bytes, b"hello");
     }
+
+    #[test]
+    fn remote_non_loopback_bind_requires_explicit_config() {
+        let config = ServerConfig::default();
+        remote_bind_allowed(&config, "127.0.0.1").unwrap();
+        assert!(remote_bind_allowed(&config, "0.0.0.0").is_err());
+
+        let config = ServerConfig {
+            allow_remote_non_loopback_bind: true,
+            ..ServerConfig::default()
+        };
+        remote_bind_allowed(&config, "0.0.0.0").unwrap();
+    }
 }

src/config.rs

@@ -33,6 +33,8 @@ pub struct ServerConfig {
     #[serde(default)]
     pub allow_remote_forwarding: bool,
     #[serde(default)]
+    pub allow_remote_non_loopback_bind: bool,
+    #[serde(default)]
     pub allow_agent_forwarding: bool,
     #[serde(default = "default_accept_env")]
     pub accept_env: Vec<String>,
@@ -61,6 +63,7 @@ impl Default for ServerConfig {
             native_auth_rate_limit_per_minute: default_native_auth_rate_limit_per_minute(),
             allow_tcp_forwarding: true,
             allow_remote_forwarding: false,
+            allow_remote_non_loopback_bind: false,
             allow_agent_forwarding: false,
             accept_env: default_accept_env(),
         }