File Server - index.html quirk

Getting Help
1

Hi, I’m currently learning Go’s File Server, and I found a little quirk when working on index.html that I can’t get my head around. So here’s the situation …

File structure

|— cmd/
|    |— main.go
|— resources/
     |— hello/
     |    |— hello.html
     |    |— hello.js
     |— index/
          |— index.html
          |— index.js

main.go

func newRouter() *mux.Router {
	r := mux.NewRouter()
	r.HandleFunc("/", indexHandler).Methods("GET")
	r.HandleFunc("/hello", helloHandler).Methods("GET")
	r.PathPrefix("/static/").Handler(staticFileHandler()).Methods("GET")
	return r
}

func indexHandler(w http.ResponseWriter, r *http.Request) {
	b, err := os.ReadFile("../resources/index/index.html")
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
	fmt.Fprintf(w, string(b))
}

func helloHandler(w http.ResponseWriter, r *http.Request) {
	b, err := os.ReadFile("../resources/hello/hello.html")
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
	}
	fmt.Fprintf(w, string(b))
}

func staticFileHandler() http.Handler {
	dir := http.Dir("../")
	return http.StripPrefix("/static/", http.FileServer(dir))
}

func main() {
	r := newRouter()
	http.ListenAndServe(":8080", r)
}

So if we run main.go, and send a GET request of
http://localhost:8080/static/resources/hello
This is what we will get from the response …

<pre>
<a href="hello.html">hello.html</a>
<a href="hello.js">hello.js</a>
</pre>

… which is the files in “hello” folder, as expected.

So if we send another GET request of
http://localhost:8080/static/resources/index
Guess what? we will get the content of index.html directly. Say,

<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="UTF-8">
		<title>index</title>
	</head>
	<body>
		<h1>index</h1>
		<script src="/static/resources/index/index.js"></script>
	</body>
</html>

To further prove that, I changed the name of index.html to, maybe, default.html
then only I got this …

<pre>
<a href="default.html">default.html</a>
<a href="index.js">index.js</a>
</pre>

So my question is, what so special about index.html? I thought index.html was just a naming convention thing. Why Go http.FileServer() treats it differently? Is there any rationale behind it?

Thanks.

2

Hi @naixyeur,

Indeed, this is a behavior of ServeFile() that FileServer calls internally.

This is historic Web server behavior. If the URL maps to a directory, the standard action is to see if an index.html file is inside, and serve that instead of the directory contents.

See e.g., Webserver directory index - Wikipedia:

When an HTTP client (generally a web browser) requests a URL that points to a directory structure instead of an actual web page within the directory, the web server will generally serve a default page, which is often referred to as a main or “index” page.

1 Like
3

It seems like I’ve asked a silly question due to lack of understanding of fundamental knowledge about web service.
Thanks for your reply, Christoph.

4

Not your fault at all! No one can have all the historically grown peculiarities of IT tools and protocols in the back of their head. IMHO the net/http docs should mention that behavior.

1 Like
5

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