cmd/derper: add a basic DERP server

Mostly Crawshaw's earlier code, refactored a bit.

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/derper/derper.go

@@ -0,0 +1,141 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// The derper binary is a simple DERP server.
+package main // import "tailscale.com/cmd/derper"
+
+import (
+	"encoding/json"
+	"flag"
+	"io"
+	"io/ioutil"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+
+	"github.com/tailscale/wireguard-go/wgcfg"
+	"golang.org/x/crypto/acme/autocert"
+	"tailscale.com/atomicfile"
+	"tailscale.com/derp"
+	"tailscale.com/derp/derphttp"
+	"tailscale.com/types/key"
+)
+
+var (
+	addr       = flag.String("a", ":443", "server address")
+	configPath = flag.String("c", "", "config file path")
+	certDir    = flag.String("certdir", defaultCertDir(), "directory to store LetsEncrypt certs, if addr's port is :443")
+	hostname   = flag.String("hostname", "derp.tailscale.com", "LetsEncrypt host name, if addr's port is :443")
+)
+
+func defaultCertDir() string {
+	cacheDir, err := os.UserCacheDir()
+	if err == nil {
+		return filepath.Join(cacheDir, "tailscale", "derper-certs")
+	}
+	return ""
+}
+
+type config struct {
+	PrivateKey wgcfg.PrivateKey
+}
+
+func loadConfig() config {
+	if *configPath == "" {
+		log.Fatalf("derper: -c <config path> not specified")
+	}
+	b, err := ioutil.ReadFile(*configPath)
+	switch {
+	case os.IsNotExist(err):
+		return writeNewConfig()
+	case err != nil:
+		log.Fatal(err)
+		panic("unreachable")
+	default:
+		var cfg config
+		if err := json.Unmarshal(b, &cfg); err != nil {
+			log.Fatalf("derper: config: %v", err)
+		}
+		return cfg
+	}
+}
+
+func writeNewConfig() config {
+	key, err := wgcfg.NewPrivateKey()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if err := os.MkdirAll(filepath.Dir(*configPath), 0777); err != nil {
+		log.Fatal(err)
+	}
+	cfg := config{
+		PrivateKey: key,
+	}
+	b, err := json.MarshalIndent(cfg, "", "\t")
+	if err != nil {
+		log.Fatal(err)
+	}
+	if err := atomicfile.WriteFile(*configPath, b, 0666); err != nil {
+		log.Fatal(err)
+	}
+	return cfg
+}
+
+func main() {
+	flag.Parse()
+
+	cfg := loadConfig()
+
+	letsEncrypt := false
+	if _, port, _ := net.SplitHostPort(*addr); port == "443" {
+		letsEncrypt = true
+	}
+
+	s := derp.NewServer(key.Private(cfg.PrivateKey), log.Printf)
+
+	mux := http.NewServeMux()
+	mux.Handle("/derp", derphttp.Handler(s))
+	mux.Handle("/", http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "text/plain")
+		w.WriteHeader(200)
+		io.WriteString(w, "Tailscale DERP server.")
+	}))
+
+	httpsrv := &http.Server{
+		Addr:    *addr,
+		Handler: mux,
+	}
+
+	var err error
+	if letsEncrypt {
+		if *certDir == "" {
+			log.Fatalf("missing required --certdir flag")
+		}
+		log.Printf("derper: serving on %s with TLS", *addr)
+		certManager := &autocert.Manager{
+			Prompt:     autocert.AcceptTOS,
+			HostPolicy: autocert.HostWhitelist(*hostname),
+			Cache:      autocert.DirCache(*certDir),
+		}
+		httpsrv.TLSConfig = certManager.TLSConfig()
+		go func() {
+			err := http.ListenAndServe(":80", certManager.HTTPHandler(nil))
+			if err != nil {
+				if err != http.ErrServerClosed {
+					log.Fatal(err)
+				}
+			}
+		}()
+		err = httpsrv.ListenAndServeTLS("", "")
+	} else {
+		log.Printf("derper: serving on %s", *addr)
+		err = httpsrv.ListenAndServe()
+	}
+	if err != nil && err != http.ErrServerClosed {
+		log.Fatalf("derper: %v", err)
+	}
+}