cmd/derper: accept more LetsEncrypt hostnames without explicit config

cmd/derper/derper.go

@@ -6,7 +6,9 @@
 package main // import "tailscale.com/cmd/derper"
 
 import (
+	"context"
 	"encoding/json"
+	"errors"
 	"expvar"
 	"flag"
 	"fmt"
@@ -17,6 +19,7 @@
 	"net/http"
 	"os"
 	"path/filepath"
+	"regexp"
 	"time"
 
 	"github.com/tailscale/wireguard-go/wgcfg"
@@ -159,6 +162,9 @@ func main() {
 			HostPolicy: autocert.HostWhitelist(*hostname),
 			Cache:      autocert.DirCache(*certDir),
 		}
+		if *hostname == "derp.tailscale.com" {
+			certManager.HostPolicy = prodAutocertHostPolicy
+		}
 		httpsrv.TLSConfig = certManager.TLSConfig()
 		go func() {
 			err := http.ListenAndServe(":80", certManager.HTTPHandler(tsweb.Port80Handler{mux}))
@@ -253,3 +259,12 @@ func serveSTUN() {
 		}
 	}
 }
+
+var validProdHostname = regexp.MustCompile(`^derp(\d+|\-\w+)?\.tailscale\.com\.?$`)
+
+func prodAutocertHostPolicy(_ context.Context, host string) error {
+	if validProdHostname.MatchString(host) {
+		return nil
+	}
+	return errors.New("invalid hostname")
+}

cmd/derper/derper_test.go

@@ -0,0 +1,31 @@
+// 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.
+
+package main
+
+import "testing"
+
+func TestProdAutocertHostPolicy(t *testing.T) {
+	tests := []struct {
+		in     string
+		wantOK bool
+	}{
+		{"derp.tailscale.com", true},
+		{"derp.tailscale.com.", true},
+		{"derp1.tailscale.com", true},
+		{"derp2.tailscale.com", true},
+		{"derp02.tailscale.com", true},
+		{"derp-nyc.tailscale.com", true},
+		{"derpfoo.tailscale.com", false},
+		{"derp02.bar.tailscale.com", false},
+		{"example.net", false},
+	}
+	for _, tt := range tests {
+		got := prodAutocertHostPolicy(nil, tt.in) == nil
+		if got != tt.wantOK {
+			t.Errorf("f(%q) = %v; want %v", tt.in, got, tt.wantOK)
+		}
+	}
+
+}