Golang reverse proxy with custom authentication - authentication

Question

I am trying to authenticate a user by calling a REST api, before I proxy the request to a remote server.
However, I find that if I make the api call before proxy to remote server, the request will fail with the following error:
http: proxy error: http: ContentLength=139 with Body length 0.
If I remove the api call before proxy to remote server, the request can get through and return correct response.
My middleware is as following:
func AuthMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// the api call to external auth server
user_id, err := authenticate(r.FormValue("username"), r.FormValue("password"))
if err != nil {
http.Error(w, err.Error(), 401)
return
}
next.ServeHTTP(w, r)
})
}
My reverse proxy is as following:
func NewReverseProxy(target *url.URL) *httputil.ReverseProxy {
director := func(req *http.Request) {
req.URL.Scheme = target.Scheme
req.URL.Host = target.Host
req.URL.Path = target.Path
targetQuery := target.RawQuery
if targetQuery == "" || req.URL.RawQuery == "" {
req.URL.RawQuery = targetQuery + req.URL.RawQuery
} else {
req.URL.RawQuery = targetQuery + "&" + req.URL.RawQuery
}
if _, ok := req.Header["User-Agent"]; !ok {
// explicitly disable User-Agent so it's not set to default value
req.Header.Set("User-Agent", "")
}
}
return &httputil.ReverseProxy{Director: director}
}
And I am using Chi for routing
r.Use(AuthMiddleware)
r.Post("/", NewReverseProxy(targets).ServeHTTP)
What is the issue with this implementation?

Answer 1

If you do not care for the body anymore you can set the contentlength of the request to 0, reflecting the current state of the body:
func AuthMiddleware(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
// the api call to external auth server
user_id, err := authenticate(r.FormValue("username"), r.FormValue("password"))
if err != nil {
http.Error(w, err.Error(), 401)
return
}
r.ContentLength = 0
next.ServeHTTP(w, r)
})
}