feat: controlled error responses for HTTP honeypot

Add `withCustomError` middleware that intercepts HTTP error responses and replaces them with a custom error response.

This is used when the HTTP honeypot is configured to serve content from a directory. It ensures that all error responses from http.FileServerFS are controlled and predicatable.

internal/httpserver/httpserver.go

@@ -65,7 +65,7 @@ func determineConfig(cfg *config.Server) *responseConfig {
 		return &responseConfig{
 			mode:      modeDirectory,
 			fsRoot:    root,
-			fsHandler: http.FileServerFS(noDirectoryFS{root.FS()}),
+			fsHandler: withCustomError(http.FileServerFS(noDirectoryFS{root.FS()}), cfg.ErrorPagePath),
 		}
 	}
 

internal/httpserver/middleware.go

@@ -0,0 +1,48 @@
+package httpserver
+
+import (
+	"net/http"
+)
+
+// withCustomError is a middleware that intercepts 4xx/5xx HTTP error responses
+// and replaces them with a custom error response.
+func withCustomError(next http.Handler, errorPath string) http.HandlerFunc {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		e := &errorInterceptor{origWriter: w, origRequest: r, errorPath: errorPath}
+		next.ServeHTTP(e, r)
+	})
+}
+
+// errorInterceptor intercepts HTTP responses to override error status codes
+// and to serve a custom error response.
+type errorInterceptor struct {
+	origWriter  http.ResponseWriter
+	origRequest *http.Request
+	overridden  bool
+	errorPath   string
+}
+
+// WriteHeader intercepts error response codes (4xx or 5xx) to serve a custom
+// error response.
+func (e *errorInterceptor) WriteHeader(statusCode int) {
+	if statusCode >= 400 && statusCode <= 599 {
+		e.overridden = true
+		serveErrorPage(e.origWriter, e.origRequest, e.errorPath)
+		return
+	}
+	e.origWriter.WriteHeader(statusCode)
+}
+
+// Write writes the response body only if the response code was not overridden.
+// Otherwise, the body is discarded.
+func (e *errorInterceptor) Write(b []byte) (int, error) {
+	if !e.overridden {
+		return e.origWriter.Write(b)
+	}
+	return 0, nil
+}
+
+// Header returns the response headers from the original ResponseWriter.
+func (e *errorInterceptor) Header() http.Header {
+	return e.origWriter.Header()
+}