server: a directory under /s/ answers 404, not a listing (closes #286)

http.FileServer answered /s/ and /s/css/ with an HTML listing of the
embedded files, and /s/css with a redirect to that listing. The static
handler now looks the name up in the embedded files first and answers
404 when it names a directory; files are served as before.

Model: opus-5-5

TODO.md

@@ -19,6 +19,8 @@ milestone: https://git.eeqj.de/sneak/dnswatcher/milestone/7
 
 # Completed Steps
 
+- 2026-10-15: a request under `/s/` that names a directory, such as `/s/` or
+  `/s/css`, answers 404 instead of a listing of its files (closes #286).
 - 2026-10-13: a hostname or a domain whose zone's nameservers all stop answering
   gets an NS Failure for each, then an NS Recovery (closes #284).
 - 2026-10-13: an IPv6 address is written in brackets before its port, as in

internal/server/routes.go

@@ -1,7 +1,9 @@
 package server
 
 import (
+	"io/fs"
 	"net/http"
+	"strings"
 	"time"
 
 	sentryhttp "github.com/getsentry/sentry-go/http"
@@ -47,13 +49,7 @@ func (s *Server) SetupRoutes() {
 	public.Get("/", s.handlers.HandleDashboard())
 
 	// Static assets (embedded CSS/JS)
-	public.Mount(
-		"/s",
-		http.StripPrefix(
-			"/s",
-			http.FileServer(http.FS(static.Static)),
-		),
-	)
+	public.Mount("/s", http.StripPrefix("/s", staticAssets()))
 
 	// Health check (standard well-known path)
 	public.Get(
@@ -86,3 +82,27 @@ func (s *Server) SetupRoutes() {
 		s.router.Mount("/metrics", metrics)
 	}
 }
+
+// staticAssets serves the embedded static assets, with the /s prefix
+// already stripped from the path. A request that names a directory
+// answers 404: http.FileServer would answer it with a listing of the
+// directory, or with a redirect to that listing when the trailing
+// slash is missing.
+func staticAssets() http.Handler {
+	files := http.FileServer(http.FS(static.Static))
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// fs.Stat refuses a name that is empty or ends in a slash,
+		// so /s/ and /s/css/ answer 404 here along with /s/css.
+		name := strings.TrimPrefix(r.URL.Path, "/")
+
+		info, err := fs.Stat(static.Static, name)
+		if err != nil || info.IsDir() {
+			http.NotFound(w, r)
+
+			return
+		}
+
+		files.ServeHTTP(w, r)
+	})
+}

internal/server/routes_test.go

@@ -1,6 +1,7 @@
 package server_test
 
 import (
+	"mime"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -288,3 +289,42 @@ func TestMetricsRateLimitComesBeforeAuth(t *testing.T) {
 		t.Errorf("another address: status = %d, want 200", rec.Code)
 	}
 }
+
+// TestStaticDirectoriesAnswerNotFound checks that a request naming a
+// directory of the static assets, with or without the trailing slash,
+// answers 404 rather than a listing of it or a redirect to one, and
+// that the stylesheet is still served as CSS.
+func TestStaticDirectoriesAnswerNotFound(t *testing.T) {
+	viper.Reset()
+	t.Setenv("DNSWATCHER_TARGETS", "example.com")
+
+	srv := routedServer(t)
+
+	for _, path := range []string{"/s", "/s/", "/s/css", "/s/css/"} {
+		rec := serve(srv, httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, path, nil,
+		))
+
+		if rec.Code != http.StatusNotFound {
+			t.Errorf("GET %s: status = %d, want 404", path, rec.Code)
+		}
+	}
+
+	rec := serve(srv, httptest.NewRequestWithContext(
+		t.Context(), http.MethodGet, "/s/css/tailwind.min.css", nil,
+	))
+
+	if rec.Code != http.StatusOK {
+		t.Errorf("GET stylesheet: status = %d, want 200", rec.Code)
+	}
+
+	contentType := rec.Header().Get("Content-Type")
+
+	mediaType, _, err := mime.ParseMediaType(contentType)
+	if err != nil || mediaType != "text/css" {
+		t.Errorf(
+			"GET stylesheet: Content-Type = %q, want text/css",
+			contentType,
+		)
+	}
+}