cmd/tailscale: add temporary debug command for getting DNS-01 LetsEncrypt cert

Not even close to usable or well integrated yet, but submitting this before
it bitrots or I lose it.

Updates #1235

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

cmd/tailscale/cli/cert.go

@@ -0,0 +1,212 @@
+// Copyright (c) 2021 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 cli
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/json"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"golang.org/x/crypto/acme"
+	"tailscale.com/client/tailscale"
+)
+
+func jout(v interface{}) {
+	j, err := json.MarshalIndent(v, "", "\t")
+	if err != nil {
+		panic(err)
+	}
+	fmt.Printf("%T: %s\n", v, j)
+}
+
+func debugGetCert(ctx context.Context, cert string) error {
+	key, err := acmeKey()
+	if err != nil {
+		return err
+	}
+	ac := &acme.Client{
+		Key: key,
+	}
+	d, err := ac.Discover(ctx)
+	if err != nil {
+		return err
+	}
+	jout(d)
+
+	if reg, _ := strconv.ParseBool(os.Getenv("TS_DEBUG_ACME_REGISTER")); reg {
+		acct, err := ac.Register(ctx, new(acme.Account), acme.AcceptTOS)
+		if err != nil {
+			return fmt.Errorf("Register: %v", err)
+		}
+		jout(acct)
+	}
+
+	order, err := ac.AuthorizeOrder(ctx, []acme.AuthzID{{Type: "dns", Value: cert}})
+	if err != nil {
+		return err
+	}
+	jout(order)
+
+	for _, aurl := range order.AuthzURLs {
+		az, err := ac.GetAuthorization(ctx, aurl)
+		if err != nil {
+			return err
+		}
+		jout(az)
+		for _, ch := range az.Challenges {
+			if ch.Type == "dns-01" {
+				rec, err := ac.DNS01ChallengeRecord(ch.Token)
+				if err != nil {
+					return err
+				}
+				err = tailscale.SetDNS(ctx, "_acme-challenge."+cert, rec)
+				log.Printf("SetDNS of %q = %v", rec, err)
+
+				chal, err := ac.Accept(ctx, ch)
+				if err != nil {
+					return fmt.Errorf("Accept: %v", err)
+				}
+				jout(chal)
+				break
+			}
+		}
+	}
+
+	order, err = ac.WaitOrder(ctx, order.URI)
+	if err != nil {
+		return fmt.Errorf("WaitOrder: %v", err)
+	}
+	jout(order)
+
+	certPrivKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		return err
+	}
+	var pemBuf bytes.Buffer
+	if err := encodeECDSAKey(&pemBuf, certPrivKey); err != nil {
+		return err
+	}
+	if err := ioutil.WriteFile("acme-debug.key", pemBuf.Bytes(), 0600); err != nil {
+		return err
+	}
+
+	csr, err := certRequest(certPrivKey, cert, nil)
+	if err != nil {
+		return err
+	}
+
+	der, _, err := ac.CreateOrderCert(ctx, order.FinalizeURL, csr, true)
+	if err != nil {
+		return fmt.Errorf("CreateOrder: %v", err)
+	}
+	pemBuf.Reset()
+	for _, b := range der {
+		pb := &pem.Block{Type: "CERTIFICATE", Bytes: b}
+		if err := pem.Encode(&pemBuf, pb); err != nil {
+			return err
+		}
+	}
+	if err := ioutil.WriteFile("acme-debug.crt", pemBuf.Bytes(), 0600); err != nil {
+		return err
+	}
+	os.Stdout.Write(pemBuf.Bytes())
+	return nil
+}
+
+// certRequest generates a CSR for the given common name cn and optional SANs.
+func certRequest(key crypto.Signer, cn string, ext []pkix.Extension, san ...string) ([]byte, error) {
+	req := &x509.CertificateRequest{
+		Subject:         pkix.Name{CommonName: cn},
+		DNSNames:        san,
+		ExtraExtensions: ext,
+	}
+	return x509.CreateCertificateRequest(rand.Reader, req, key)
+}
+
+func acmeKey() (crypto.Signer, error) {
+	cacheDir, err := os.UserCacheDir()
+	if err != nil {
+		return nil, err
+	}
+	file := filepath.Join(cacheDir, "tailscale-acme")
+	if err := os.MkdirAll(file, 0700); err != nil {
+		return nil, err
+	}
+	cacheFile := filepath.Join(file, "acme-account.key.pem")
+	if v, err := ioutil.ReadFile(cacheFile); err == nil {
+		priv, _ := pem.Decode(v)
+		if priv == nil || !strings.Contains(priv.Type, "PRIVATE") {
+			return nil, errors.New("acme/autocert: invalid account key found in cache")
+		}
+		return parsePrivateKey(priv.Bytes)
+	}
+
+	privKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		return nil, err
+	}
+	var pemBuf bytes.Buffer
+	if err := encodeECDSAKey(&pemBuf, privKey); err != nil {
+		return nil, err
+	}
+	if err := ioutil.WriteFile(cacheFile, pemBuf.Bytes(), 0600); err != nil {
+		return nil, err
+	}
+	return privKey, nil
+}
+
+func encodeECDSAKey(w io.Writer, key *ecdsa.PrivateKey) error {
+	b, err := x509.MarshalECPrivateKey(key)
+	if err != nil {
+		return err
+	}
+	pb := &pem.Block{Type: "EC PRIVATE KEY", Bytes: b}
+	return pem.Encode(w, pb)
+}
+
+// parsePrivateKey is a copy of x/crypto/acme's parsePrivateKey.
+//
+// Attempt to parse the given private key DER block. OpenSSL 0.9.8 generates
+// PKCS#1 private keys by default, while OpenSSL 1.0.0 generates PKCS#8 keys.
+// OpenSSL ecparam generates SEC1 EC private keys for ECDSA. We try all three.
+//
+// Inspired by parsePrivateKey in crypto/tls/tls.go.
+func parsePrivateKey(der []byte) (crypto.Signer, error) {
+	if key, err := x509.ParsePKCS1PrivateKey(der); err == nil {
+		return key, nil
+	}
+	if key, err := x509.ParsePKCS8PrivateKey(der); err == nil {
+		switch key := key.(type) {
+		case *rsa.PrivateKey:
+			return key, nil
+		case *ecdsa.PrivateKey:
+			return key, nil
+		default:
+			return nil, errors.New("acme/autocert: unknown private key type in PKCS#8 wrapping")
+		}
+	}
+	if key, err := x509.ParseECPrivateKey(der); err == nil {
+		return key, nil
+	}
+
+	return nil, errors.New("acme/autocert: failed to parse private key")
+}

cmd/tailscale/cli/debug.go

@@ -36,6 +36,7 @@ var debugCmd = &ffcli.Command{
 		fs.BoolVar(&debugArgs.netMap, "netmap", true, "whether to include netmap in --ipn mode")
 		fs.BoolVar(&debugArgs.localCreds, "local-creds", false, "print how to connect to local tailscaled")
 		fs.StringVar(&debugArgs.file, "file", "", "get, delete:NAME, or NAME")
+		fs.StringVar(&debugArgs.getCert, "get-acme-cert", "", "hostname to start ACME flow for (debug)")
 		return fs
 	})(),
 }
@@ -49,12 +50,16 @@ var debugArgs struct {
 	file       string
 	prefs      bool
 	pretty     bool
+	getCert    string
 }
 
 func runDebug(ctx context.Context, args []string) error {
 	if len(args) > 0 {
 		return errors.New("unknown arguments")
 	}
+	if debugArgs.getCert != "" {
+		return debugGetCert(ctx, debugArgs.getCert)
+	}
 	if debugArgs.localCreds {
 		port, token, err := safesocket.LocalTCPPortAndToken()
 		if err == nil {