main.go

package main

import (
	"fmt"
	"io"
	"io/ioutil"
	"log"
	"net/http"
)

func main() {
	http.HandleFunc("/", checkSite)
	http.ListenAndServe(":8080", nil)
}

func checkSite(w http.ResponseWriter, r *http.Request) {
	q := r.RequestURI[1:]
	req, err := http.NewRequest(http.MethodGet, "http://"+q, nil)
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	resp, err := http.Get("http://" + q)
	if err != nil {
		log.Println(err)
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	io.Copy(ioutil.Discard, resp.Body)
	resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		fmt.Fprintf(w, "Site \"%s\" is down.", q)
		return
	}
	fmt.Fprintf(w, "Site \"%s\" is up.", q)
}