golang simple http proxy @ https://medium.com/@mlowicki/http-s-proxy-in-golang-in-less-than-100-lines-of-code-6a51c2f2c38c

proxy.go

package main

import (
	"crypto/tls"
	"flag"
	"io"
	"log"
	"net"
	"net/http"
	"time"
)

func handleTunneling(w http.ResponseWriter, r *http.Request) {
	dest_conn, err := net.DialTimeout("tcp", r.Host, 10*time.Second)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	w.WriteHeader(http.StatusOK)
	hijacker, ok := w.(http.Hijacker)
	if !ok {
		http.Error(w, "Hijacking not supported", http.StatusInternalServerError)
		return
	}
	client_conn, _, err := hijacker.Hijack()
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
	}
	go transfer(dest_conn, client_conn)
	go transfer(client_conn, dest_conn)
}
func transfer(destination io.WriteCloser, source io.ReadCloser) {
	defer destination.Close()
	defer source.Close()
	io.Copy(destination, source)
}
func handleHTTP(w http.ResponseWriter, req *http.Request) {
	resp, err := http.DefaultTransport.RoundTrip(req)
	if err != nil {
		http.Error(w, err.Error(), http.StatusServiceUnavailable)
		return
	}
	defer resp.Body.Close()
	copyHeader(w.Header(), resp.Header)
	w.WriteHeader(resp.StatusCode)
	io.Copy(w, resp.Body)
}
func copyHeader(dst, src http.Header) {
	for k, vv := range src {
		for _, v := range vv {
			dst.Add(k, v)
		}
	}
}
func main() {
	var pemPath string
	flag.StringVar(&pemPath, "pem", "server.pem", "path to pem file")
	var keyPath string
	flag.StringVar(&keyPath, "key", "server.key", "path to key file")
	var proto string
	flag.StringVar(&proto, "proto", "https", "Proxy protocol (http or https)")
	flag.Parse()
	if proto != "http" && proto != "https" {
		log.Fatal("Protocol must be either http or https")
	}
	server := &http.Server{
		Addr: ":8888",
		Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
			log.Println(r)
			if r.Method == http.MethodConnect {
				handleTunneling(w, r)
			} else {
				handleHTTP(w, r)
			}
		}),
		// Disable HTTP/2.
		TLSNextProto: make(map[string]func(*http.Server, *tls.Conn, http.Handler)),
	}
	if proto == "http" {
		log.Println("Serving on port 8888 with http protocol")
		log.Fatal(server.ListenAndServe())
	} else {
		log.Println("Serving on port 8888 with https protocol")
		log.Fatal(server.ListenAndServeTLS(pemPath, keyPath))
	}
}

Also need to check if the file descriptors are enough to accept many requests:

$ ulimit -a
-t: cpu time (seconds)              unlimited
-f: file size (blocks)              unlimited
-d: data seg size (kbytes)          unlimited
-s: stack size (kbytes)             8192
-c: core file size (blocks)         unlimited
-m: resident set size (kbytes)      unlimited
-u: processes                       31157
-n: file descriptors                1024
-l: locked-in-memory size (kbytes)  64
-v: address space (kbytes)          unlimited
-x: file locks                      unlimited
-i: pending signals                 31157
-q: bytes in POSIX msg queues       819200
-e: max nice                        0
-r: max rt priority                 0
-N 15:                              unlimited