HTTP fileserver which allows domain fronting

dfdemoserver.go

// Program dfdemoserver is an HTTP fileserver which allows domain fronting,
// meant for demos.
package main

/*
 * dfdemoserver.go
 * HTTP fileserver which allows domain fronting
 * By J. Stuart McMurray
 * Created 20190922
 * Last Modified 20190922
 */

import (
	"flag"
	"fmt"
	"log"
	"net/http"
	"os"
	"path/filepath"
	"strings"
)

const (
	/* dirPerm is the permissions used when we make a served files
	directory */
	dirPerm = 0700
)

func main() {
	var (
		cert = flag.String(
			"cert",
			"cert.pem",
			"TLS certificate `file`",
		)
		key = flag.String(
			"key",
			"key.pem",
			"TLS key `file`",
		)
		dir = flag.String(
			"files",
			"files",
			"Served files `directory`",
		)
		laddr = flag.String(
			"listen",
			"0.0.0.0:443",
			"Listen `address`",
		)
	)
	flag.Usage = func() {
		fmt.Fprintf(
			os.Stderr,
			`Usage: %v [options]

A simple little fileserver which serves files from subdirectories named the
same as the host header (with any slashes changed to underscores).  The parent
directory of the files directories will be created if it does not exist.

Options:
`,
			os.Args[0],
		)
		flag.PrintDefaults()
	}
	flag.Parse()

	/* Be in files directory */
	if err := os.MkdirAll(*dir, dirPerm); nil != err {
		log.Fatalf(
			"Unable to make files directory %q: %v",
			*dir,
			err,
		)
	}
	if err := os.Chdir(*dir); nil != err {
		log.Fatalf(
			"Unable to chdir to files directory %q: %v",
			*dir,
			err,
		)
	}

	/* Serve HTTP */
	http.HandleFunc("/", handle)
	log.Fatalf(
		"Error serving HTTPS: %v",
		http.ListenAndServeTLS(*laddr, *cert, *key, nil),
	)
}

/* handle serves files from the directory named by the Host: header. */
func handle(w http.ResponseWriter, r *http.Request) {
	/* Get the sanitized directory name */
	dir := strings.Replace(filepath.Clean(string(r.Host)), "/", "_", -1)
	if ".." == dir {
		http.Error(w, "Hacker Detected!", http.StatusPaymentRequired)
		return
	}

	log.Printf("[%v] %v %v %v", r.RemoteAddr, r.Method, r.Host, r.URL)

	/* Let the library do the hard work */
	http.FileServer(http.Dir(dir)).ServeHTTP(w, r)
}