ipn/localapi: move cert fetching code to localapi, cache, add cert subcommand

Updates #1235

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

client/tailscale/tailscale.go

@@ -8,6 +8,7 @@
 import (
 	"bytes"
 	"context"
+	"crypto/tls"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -18,7 +19,9 @@
 	"net/url"
 	"strconv"
 	"strings"
+	"time"
 
+	"go4.org/mem"
 	"tailscale.com/client/tailscale/apitype"
 	"tailscale.com/ipn"
 	"tailscale.com/ipn/ipnstate"
@@ -293,3 +296,48 @@ func CurrentDERPMap(ctx context.Context) (*tailcfg.DERPMap, error) {
 	}
 	return &derpMap, nil
 }
+
+// CertPair returns a cert and private key for the provided DNS domain.
+//
+// It returns a cached certificate from disk if it's still valid.
+func CertPair(ctx context.Context, domain string) (certPEM, keyPEM []byte, err error) {
+	res, err := send(ctx, "GET", "/localapi/v0/cert/"+domain+"?type=pair", 200, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+	// with ?type=pair, the response PEM is first the one private
+	// key PEM block, then the cert PEM blocks.
+	i := mem.Index(mem.B(res), mem.S("--\n--"))
+	if i == -1 {
+		return nil, nil, fmt.Errorf("unexpected output: no delimiter")
+	}
+	i += len("--\n")
+	keyPEM, certPEM = res[:i], res[i:]
+	if mem.Contains(mem.B(certPEM), mem.S(" PRIVATE KEY-----")) {
+		return nil, nil, fmt.Errorf("unexpected output: key in cert")
+	}
+	return certPEM, keyPEM, nil
+}
+
+// GetCertificate fetches a TLS certificate for the TLS ClientHello in hi.
+//
+// It returns a cached certificate from disk if it's still valid.
+//
+// It's the right signature to use as the value of
+// tls.Config.GetCertificate.
+func GetCertificate(hi *tls.ClientHelloInfo) (*tls.Certificate, error) {
+	if hi == nil || hi.ServerName == "" {
+		return nil, errors.New("no SNI ServerName")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), time.Minute)
+	defer cancel()
+	certPEM, keyPEM, err := CertPair(ctx, hi.ServerName)
+	if err != nil {
+		return nil, err
+	}
+	cert, err := tls.X509KeyPair(certPEM, keyPEM)
+	if err != nil {
+		return nil, err
+	}
+	return &cert, nil
+}

cmd/tailscale/cli/cert.go

@@ -5,278 +5,74 @@
 package cli
 
 import (
-	"bytes"
 	"context"
-	"crypto"
+	"crypto/tls"
-	"crypto/ecdsa"
+	"flag"
-	"crypto/elliptic"
-	"crypto/rand"
-	"crypto/rsa"
-	"crypto/x509"
-	"crypto/x509/pkix"
-	"encoding/json"
-	"encoding/pem"
-	"errors"
 	"fmt"
-	"io"
 	"io/ioutil"
 	"log"
-	"os"
+	"net/http"
-	"path/filepath"
-	"strings"
-	"time"
 
-	"golang.org/x/crypto/acme"
+	"github.com/peterbourgon/ff/v2/ffcli"
 	"tailscale.com/client/tailscale"
-	"tailscale.com/ipn/ipnstate"
 )
 
-func jout(v interface{}) {
+var certCmd = &ffcli.Command{
-	j, err := json.MarshalIndent(v, "", "\t")
+	Name:       "cert",
-	if err != nil {
+	Exec:       runCert,
-		panic(err)
+	ShortHelp:  "get TLS certs",
-	}
+	ShortUsage: "cert [flags] <domain>",
-	fmt.Printf("%T: %s\n", v, j)
+	FlagSet: (func() *flag.FlagSet {
+		fs := flag.NewFlagSet("cert", flag.ExitOnError)
+		fs.StringVar(&certArgs.certFile, "cert-file", "", "output cert file; defaults to DOMAIN.crt")
+		fs.StringVar(&certArgs.keyFile, "key-file", "", "output cert file; defaults to DOMAIN.key")
+		fs.BoolVar(&certArgs.serve, "serve-demo", false, "if true, serve on port :443 using the cert as a demo, instead of writing out the files to disk")
+		return fs
+	})(),
 }
 
-func checkCertDomain(st *ipnstate.Status, domain string) error {
+var certArgs struct {
-	if domain == "" {
+	certFile string
-		return errors.New("missing domain name")
+	keyFile  string
-	}
+	serve    bool
-	for _, d := range st.CertDomains {
-		if d == domain {
-			return nil
-		}
-	}
-	// Transitional way while server doesn't yet populate CertDomains: also permit the client
-	// attempting Self.DNSName.
-	okay := st.CertDomains[:len(st.CertDomains):len(st.CertDomains)]
-	if st.Self != nil {
-		if v := strings.Trim(st.Self.DNSName, "."); v != "" {
-			if v == domain {
-				return nil
-			}
-			okay = append(okay, v)
-		}
-	}
-	switch len(okay) {
-	case 0:
-		return errors.New("your Tailscale account does not support getting TLS certs")
-	case 1:
-		return fmt.Errorf("invalid domain %q; only %q is permitted", domain, okay[0])
-	default:
-		return fmt.Errorf("invalid domain %q; must be one of %q", domain, okay)
-	}
 }
 
-func debugGetCert(ctx context.Context, domain string) error {
+func runCert(ctx context.Context, args []string) error {
-	st, err := tailscale.Status(ctx)
+	if certArgs.serve {
-	if err != nil {
+		s := &http.Server{
-		return fmt.Errorf("getting tailscale status: %w", err)
+			TLSConfig: &tls.Config{
-	}
+				GetCertificate: tailscale.GetCertificate,
-	if err := checkCertDomain(st, domain); err != nil {
+			},
-		return err
+			Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				fmt.Fprintf(w, "<h1>Hello from Tailscale</h1>It works.")
+			}),
+		}
+		log.Printf("running TLS server on :443 ...")
+		return s.ListenAndServeTLS("", "")
 	}
 
-	key, err := acmeKey()
+	if len(args) != 1 {
+		return fmt.Errorf("Usage: tailscale cert [flags] <domain>")
+	}
+	domain := args[0]
+
+	if certArgs.certFile == "" {
+		certArgs.certFile = domain + ".crt"
+	}
+	if certArgs.keyFile == "" {
+		certArgs.keyFile = domain + ".key"
+	}
+	certPEM, keyPEM, err := tailscale.CertPair(ctx, domain)
 	if err != nil {
 		return err
 	}
-	ac := &acme.Client{
+	if err := ioutil.WriteFile(certArgs.certFile, certPEM, 0644); err != nil {
-		Key: key,
-	}
-
-	logf := log.Printf
-
-	a, err := ac.GetReg(ctx, "unused")
-	switch {
-	case err == nil:
-		// Great, already registered.
-		logf("Already had ACME account.")
-	case err == acme.ErrNoAccount:
-		a, err = ac.Register(ctx, new(acme.Account), acme.AcceptTOS)
-		if err == acme.ErrAccountAlreadyExists {
-			// Potential race. Double check.
-			a, err = ac.GetReg(ctx, "unused")
-		}
-		if err != nil {
-			return fmt.Errorf("acme.Register: %w", err)
-		}
-		logf("Registered ACME account.")
-		jout(a)
-	default:
-		return fmt.Errorf("acme.GetReg: %w", err)
-
-	}
-	if a.Status != acme.StatusValid {
-		return fmt.Errorf("unexpected ACME account status %q", a.Status)
-	}
-
-	order, err := ac.AuthorizeOrder(ctx, []acme.AuthzID{{Type: "dns", Value: domain}})
-	if err != nil {
 		return err
 	}
-	jout(order)
+	if err := ioutil.WriteFile(certArgs.keyFile, keyPEM, 0600); err != nil {
-
-	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."+domain, 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
-			}
-		}
-	}
-
-	t0 := time.Now()
-	orderURI := order.URI
-	for {
-		order, err = ac.WaitOrder(ctx, orderURI)
-		if err == nil {
-			break
-		}
-		if oe, ok := err.(*acme.OrderError); ok && oe.Status == acme.StatusInvalid {
-			if time.Since(t0) > 2*time.Minute {
-				return errors.New("timeout waiting for order to not be invalid")
-			}
-			log.Printf("order invalid; waiting...")
-			select {
-			case <-time.After(5 * time.Second):
-				continue
-			case <-ctx.Done():
-				return ctx.Err()
-			}
-		}
-		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
+	fmt.Printf("Wrote public cert to %v\n", certArgs.certFile)
-	if err := encodeECDSAKey(&pemBuf, certPrivKey); err != nil {
+	fmt.Printf("Wrote private key to %v\n", certArgs.keyFile)
-		return err
-	}
-	if err := ioutil.WriteFile(domain+".key", pemBuf.Bytes(), 0600); err != nil {
-		return err
-	}
-
-	csr, err := certRequest(certPrivKey, domain, 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(domain+".crt", pemBuf.Bytes(), 0644); err != nil {
-		return err
-	}
-	os.Stdout.Write(pemBuf.Bytes())
-	fmt.Printf("\nPublic cert and private key written to %s.crt and %s.key\n", domain, domain)
 	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/cli.go

@@ -107,6 +107,7 @@ func Run(args []string) error {
 			webCmd,
 			fileCmd,
 			bugReportCmd,
+			certCmd,
 		},
 		FlagSet:   rootfs,
 		Exec:      func(context.Context, []string) error { return flag.ErrHelp },

cmd/tailscale/cli/debug.go

@@ -36,7 +36,6 @@
 		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
 	})(),
 }
@@ -50,16 +49,12 @@
 	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 {

cmd/tailscale/depaware.txt

@@ -69,7 +69,6 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         tailscale.com/version                                        from tailscale.com/cmd/tailscale/cli+
         tailscale.com/version/distro                                 from tailscale.com/cmd/tailscale/cli+
         tailscale.com/wgengine/filter                                from tailscale.com/types/netmap
-        golang.org/x/crypto/acme                                     from tailscale.com/cmd/tailscale/cli
         golang.org/x/crypto/blake2b                                  from golang.org/x/crypto/nacl/box
         golang.org/x/crypto/chacha20                                 from golang.org/x/crypto/chacha20poly1305
         golang.org/x/crypto/chacha20poly1305                         from crypto/tls+
@@ -170,7 +169,7 @@ tailscale.com/cmd/tailscale dependencies: (generated by github.com/tailscale/dep
         reflect                                                      from crypto/x509+
         regexp                                                       from github.com/tailscale/goupnp/httpu+
         regexp/syntax                                                from regexp
-        runtime/debug                                                from golang.org/x/sync/singleflight+
+        runtime/debug                                                from golang.org/x/sync/singleflight
         sort                                                         from compress/flate+
         strconv                                                      from compress/flate+
         strings                                                      from bufio+

cmd/tailscaled/depaware.txt

@@ -177,6 +177,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/wgengine/wgcfg/nmcfg                           from tailscale.com/ipn/ipnlocal
         tailscale.com/wgengine/wglog                                 from tailscale.com/wgengine
    W 💣 tailscale.com/wgengine/winnet                                from tailscale.com/wgengine/router
+        golang.org/x/crypto/acme                                     from tailscale.com/ipn/localapi
         golang.org/x/crypto/blake2b                                  from golang.org/x/crypto/nacl/box
         golang.org/x/crypto/blake2s                                  from golang.zx2c4.com/wireguard/device+
         golang.org/x/crypto/chacha20                                 from golang.org/x/crypto/chacha20poly1305

ipn/localapi/cert.go

@@ -0,0 +1,400 @@
+// 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.
+
+//go:build !ios && !android
+// +build !ios,!android
+
+package localapi
+
+import (
+	"bytes"
+	"context"
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/tls"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"encoding/json"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"net"
+	"net/http"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+
+	"golang.org/x/crypto/acme"
+	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/paths"
+	"tailscale.com/types/logger"
+)
+
+func (h *Handler) certDir() (string, error) {
+	base := paths.DefaultTailscaledStateFile()
+	if base == "" {
+		return "", errors.New("no default DefaultTailscaledStateFile")
+	}
+	full := filepath.Join(filepath.Dir(base), "certs")
+	if err := os.MkdirAll(full, 0700); err != nil {
+		return "", err
+	}
+	return full, nil
+}
+
+var acmeDebug, _ = strconv.ParseBool(os.Getenv("TS_DEBUG_ACME"))
+
+func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
+	if !h.PermitWrite {
+		http.Error(w, "cert access denied", http.StatusForbidden)
+		return
+	}
+	dir, err := h.certDir()
+	if err != nil {
+		h.logf("certDir: %v", err)
+		http.Error(w, "failed to get cert dir", 500)
+		return
+	}
+
+	suff := strings.TrimPrefix(r.URL.Path, "/localapi/v0/cert/")
+	if suff == r.URL.Path {
+		http.Error(w, "internal handler config wired wrong", 500)
+		return
+	}
+	domain := suff
+
+	mu := &h.certMu
+	mu.Lock()
+	defer mu.Unlock()
+
+	logf := logger.WithPrefix(h.logf, fmt.Sprintf("cert(%q): ", domain))
+	traceACME := func(v interface{}) {
+		if !acmeDebug {
+			return
+		}
+		j, _ := json.MarshalIndent(v, "", "\t")
+		log.Printf("acme %T: %s", v, j)
+	}
+
+	pair, err := h.getCertPEM(r.Context(), logf, traceACME, dir, domain, time.Now())
+	if err != nil {
+		logf("getCertPEM: %v", err)
+		http.Error(w, fmt.Sprint(err), 500)
+		return
+	}
+
+	w.Header().Set("Content-Type", "text/plain")
+	switch r.URL.Query().Get("type") {
+	case "", "crt", "cert":
+		w.Write(pair.certPEM)
+	case "key":
+		w.Write(pair.keyPEM)
+	case "pair":
+		w.Write(pair.keyPEM)
+		w.Write(pair.certPEM)
+	default:
+		http.Error(w, `invalid type; want "cert" (default), "key", or "pair"`, 400)
+	}
+}
+
+type keyPair struct {
+	certPEM []byte
+	keyPEM  []byte
+	cached  bool
+}
+
+func (h *Handler) getCertPEM(ctx context.Context, logf logger.Logf, traceACME func(interface{}), dir, domain string, now time.Time) (*keyPair, error) {
+	keyFile := filepath.Join(dir, domain+".key")
+	certFile := filepath.Join(dir, domain+".crt")
+
+	if keyPEM, err := os.ReadFile(keyFile); err == nil {
+		certPEM, _ := os.ReadFile(certFile)
+		if validCertPEM(domain, keyPEM, certPEM, now) {
+			return &keyPair{certPEM: certPEM, keyPEM: keyPEM, cached: true}, nil
+		}
+	}
+
+	key, err := acmeKey(dir)
+	if err != nil {
+		return nil, fmt.Errorf("acmeKey: %w", err)
+	}
+	ac := &acme.Client{Key: key}
+
+	a, err := ac.GetReg(ctx, "" /* pre-RFC param */)
+	switch {
+	case err == nil:
+		// Great, already registered.
+		logf("already had ACME account.")
+	case err == acme.ErrNoAccount:
+		a, err = ac.Register(ctx, new(acme.Account), acme.AcceptTOS)
+		if err == acme.ErrAccountAlreadyExists {
+			// Potential race. Double check.
+			a, err = ac.GetReg(ctx, "" /* pre-RFC param */)
+		}
+		if err != nil {
+			return nil, fmt.Errorf("acme.Register: %w", err)
+		}
+		logf("registered ACME account.")
+		traceACME(a)
+	default:
+		return nil, fmt.Errorf("acme.GetReg: %w", err)
+
+	}
+	if a.Status != acme.StatusValid {
+		return nil, fmt.Errorf("unexpected ACME account status %q", a.Status)
+	}
+
+	// Before hitting LetsEncrypt, see if this is a domain that Tailscale will do DNS challenges for.
+	st := h.b.StatusWithoutPeers()
+	if err := checkCertDomain(st, domain); err != nil {
+		return nil, err
+	}
+
+	order, err := ac.AuthorizeOrder(ctx, []acme.AuthzID{{Type: "dns", Value: domain}})
+	if err != nil {
+		return nil, err
+	}
+	traceACME(order)
+
+	for _, aurl := range order.AuthzURLs {
+		az, err := ac.GetAuthorization(ctx, aurl)
+		if err != nil {
+			return nil, err
+		}
+		traceACME(az)
+		for _, ch := range az.Challenges {
+			if ch.Type == "dns-01" {
+				rec, err := ac.DNS01ChallengeRecord(ch.Token)
+				if err != nil {
+					return nil, err
+				}
+				key := "_acme-challenge." + domain
+
+				var resolver net.Resolver
+				var ok bool
+				txts, _ := resolver.LookupTXT(ctx, key)
+				for _, txt := range txts {
+					if txt == rec {
+						ok = true
+						logf("TXT record already existed")
+						break
+					}
+				}
+				if !ok {
+					err = h.b.SetDNS(ctx, key, rec)
+					if err != nil {
+						return nil, fmt.Errorf("SetDNS %q => %q: %w", key, rec, err)
+					}
+					logf("did SetDNS")
+				}
+
+				chal, err := ac.Accept(ctx, ch)
+				if err != nil {
+					return nil, fmt.Errorf("Accept: %v", err)
+				}
+				traceACME(chal)
+				break
+			}
+		}
+	}
+
+	wait0 := time.Now()
+	orderURI := order.URI
+	for {
+		order, err = ac.WaitOrder(ctx, orderURI)
+		if err == nil {
+			break
+		}
+		if oe, ok := err.(*acme.OrderError); ok && oe.Status == acme.StatusInvalid {
+			if time.Since(wait0) > 2*time.Minute {
+				return nil, errors.New("timeout waiting for order to not be invalid")
+			}
+			log.Printf("order invalid; waiting...")
+			select {
+			case <-time.After(5 * time.Second):
+				continue
+			case <-ctx.Done():
+				return nil, ctx.Err()
+			}
+		}
+		return nil, fmt.Errorf("WaitOrder: %v", err)
+	}
+	traceACME(order)
+
+	certPrivKey, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		return nil, err
+	}
+	var privPEM bytes.Buffer
+	if err := encodeECDSAKey(&privPEM, certPrivKey); err != nil {
+		return nil, err
+	}
+	if err := ioutil.WriteFile(keyFile, privPEM.Bytes(), 0600); err != nil {
+		return nil, err
+	}
+
+	csr, err := certRequest(certPrivKey, domain, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	der, _, err := ac.CreateOrderCert(ctx, order.FinalizeURL, csr, true)
+	if err != nil {
+		return nil, fmt.Errorf("CreateOrder: %v", err)
+	}
+
+	var certPEM bytes.Buffer
+	for _, b := range der {
+		pb := &pem.Block{Type: "CERTIFICATE", Bytes: b}
+		if err := pem.Encode(&certPEM, pb); err != nil {
+			return nil, err
+		}
+	}
+	if err := ioutil.WriteFile(certFile, certPEM.Bytes(), 0644); err != nil {
+		return nil, err
+	}
+
+	return &keyPair{certPEM: certPEM.Bytes(), keyPEM: privPEM.Bytes()}, 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 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")
+}
+
+func acmeKey(dir string) (crypto.Signer, error) {
+	pemName := filepath.Join(dir, "acme-account.key.pem")
+	if v, err := ioutil.ReadFile(pemName); 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(pemName, pemBuf.Bytes(), 0600); err != nil {
+		return nil, err
+	}
+	return privKey, nil
+}
+
+func validCertPEM(domain string, keyPEM, certPEM []byte, now time.Time) bool {
+	if len(keyPEM) == 0 || len(certPEM) == 0 {
+		return false
+	}
+	tlsCert, err := tls.X509KeyPair(certPEM, keyPEM)
+	if err != nil {
+		return false
+	}
+	var leaf *x509.Certificate
+	intermediates := x509.NewCertPool()
+	for i, certDER := range tlsCert.Certificate {
+		cert, err := x509.ParseCertificate(certDER)
+		if err != nil {
+			return false
+		}
+		if i == 0 {
+			leaf = cert
+		} else {
+			intermediates.AddCert(cert)
+		}
+	}
+	if leaf == nil {
+		return false
+	}
+	_, err = leaf.Verify(x509.VerifyOptions{
+		DNSName:       domain,
+		CurrentTime:   now,
+		Intermediates: intermediates,
+	})
+	return err == nil
+}
+
+func checkCertDomain(st *ipnstate.Status, domain string) error {
+	if domain == "" {
+		return errors.New("missing domain name")
+	}
+	for _, d := range st.CertDomains {
+		if d == domain {
+			return nil
+		}
+	}
+	// Transitional way while server doesn't yet populate CertDomains: also permit the client
+	// attempting Self.DNSName.
+	okay := st.CertDomains[:len(st.CertDomains):len(st.CertDomains)]
+	if st.Self != nil {
+		if v := strings.Trim(st.Self.DNSName, "."); v != "" {
+			if v == domain {
+				return nil
+			}
+			okay = append(okay, v)
+		}
+	}
+	switch len(okay) {
+	case 0:
+		return errors.New("your Tailscale account does not support getting TLS certs")
+	case 1:
+		return fmt.Errorf("invalid domain %q; only %q is permitted", domain, okay[0])
+	default:
+		return fmt.Errorf("invalid domain %q; must be one of %q", domain, okay)
+	}
+}

ipn/localapi/disabled_stubs.go

@@ -0,0 +1,17 @@
+// 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.
+
+//go:build ios || android
+// +build ios android
+
+package localapi
+
+import (
+	"net/http"
+	"runtime"
+)
+
+func (h *Handler) serveCert(w http.ResponseWriter, r *http.Request) {
+	http.Error(w, "disabled on "+runtime.GOOS, http.StatusNotFound)
+}

ipn/localapi/localapi.go

@@ -57,6 +57,12 @@ type Handler struct {
 	b            *ipnlocal.LocalBackend
 	logf         logger.Logf
 	backendLogID string
+
+	// certMu guards all cert/ACME operations, so concurrent
+	// requests for certs don't slam ACME. The first will go
+	// through and populate the on-disk cache and the rest should
+	// use that.
+	certMu sync.Mutex
 }
 
 func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -83,6 +89,10 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		h.serveFilePut(w, r)
 		return
 	}
+	if strings.HasPrefix(r.URL.Path, "/localapi/v0/cert/") {
+		h.serveCert(w, r)
+		return
+	}
 	switch r.URL.Path {
 	case "/localapi/v0/whois":
 		h.serveWhoIs(w, r)