Golang : Listen and Serve on sub domain example

At this moment, Golang's ListenAndServer() is able to handle URL with such pattern


However, if you try to use ListenAndServe() function sub domain URL on ... let say for example, api.example.com such as

 package main

 import (

 func handler(w http.ResponseWriter, r *http.Request) {
 w.Write([]byte("Hello, World!"))

 func main() {
 http.HandleFunc("/", handler)
 fmt.Println(http.ListenAndServe("api.example.com:8080/", nil)) //<-- program will terminate with error 


the program will terminate with the error message listen tcp: unknown port tcp/8080/.

So, what's going on ? From the official documentation ServeMux :

Patterns may optionally begin with a host name, restricting matches to URLs on that host only. Host-specific patterns take precedence over general patterns, so

that a handler might register for the two patterns "/codesearch" and "codesearch.google.com/" without also taking over requests for "http://www.google.com/".

Aha! Because of the restriction to match URLs on that host only.... in order to make ListenAndServe() able to handle api.example.com, you will need to introduce a new ServeMux(a HTTP request multiplexer) to enable the program to handle and resolve the sub domain.

This code example below is taken from another tutorial on how to use multiplexer, should be able to handle api.example.com sub domain nicely.

 package main

 import "net/http"

 type home struct{}

 func (h home) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  w.Write([]byte("Hello, World!"))

 type page struct {
  body string

 func (p page) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  // echo back the page first URI

 // use map instead
 type multiplexer map[string]http.Handler

 func (m multiplexer) ServeHTTP(w http.ResponseWriter, r *http.Request) {
  if handler, ok := m[r.RequestURI]; ok {
 handler.ServeHTTP(w, r)
  } else {

 var mux = multiplexer{
  "/": home{},
  "/references/": page{"references"},
  "/tutorials/":  page{"tutorials"},

 func main() {
  http.ListenAndServe("api.example.com:8080", mux) // port 8080 is optional

Reference :


By Adam Ng

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