How to get response header from reverse proxy - go

Question

How can I detect and react on a response header from within a reverse proxy?
I'm writing a middleware method which is injected into a reverse proxy. I confirmed the middleware is being called.
The server sets a response header X-SERVER-VAR with a value of Foo. I need to detect and run some code based on the value.
I thought I'd read the ResponseWriter, thinking the server response should include the value, but nothing is logging.
func SessionHandler(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
log.Printf(w.Header().Get("X-SERVER-VAR"))
next.ServeHTTP(w, r)
})
}
I'm using github.com/gorilla/mux to attach this middleware handler.
I'm setting everything up with code similar to below (simplified for this question).
func newProxy(remote string) http.Handler {
proxyurl, err := url.Parse(remote)
if err != nil {
panic(err)
}
proxy := httputil.NewSingleHostReverseProxy(proxyurl)
return proxy
}
func main() {
r := mux.NewRouter()
r.Use(SessionHandler)
proxy := newProxy("https://www.example.com/")
r.PathPrefix("/").Handler(proxy)
log.Fatal(http.ListenAndServe(":9001", r))
}
If it isn't possible with middleware, is it possible to detect a response header from the server another way?

Answer 1

You can't expect to read the response headers before they're set.
Change this:
func SessionHandler(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
log.Printf(w.Header().Get("X-SERVER-VAR"))
next.ServeHTTP(w, r)
})
}
To this:
func SessionHandler(next http.Handler) http.Handler {
return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
next.ServeHTTP(w, r)
log.Printf(w.Header().Get("X-SERVER-VAR"))
})
}