lnrpc: lets encrypt

This commit enables lnd to request and renew a Let's Encrypt
certificate. This certificate is used both for the grpc as well as the
rest listeners. It allows clients to connect without having a copy of
the (public) server certificate.

Co-authored-by: Vegard Engen <vegard@engen.priv.no>

cmd/lncli/main.go

@@ -5,6 +5,7 @@
 package main
 
 import (
+	"crypto/tls"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -74,13 +75,24 @@ func getClientConn(ctx *cli.Context, skipMacaroons bool) *grpc.ClientConn {
 		fatal(fmt.Errorf("could not load global options: %v", err))
 	}
 
-	// Load the specified TLS certificate and build transport credentials
-	// with it.
+	// Load the specified TLS certificate.
 	certPool, err := profile.cert()
 	if err != nil {
 		fatal(fmt.Errorf("could not create cert pool: %v", err))
 	}
-	creds := credentials.NewClientTLSFromCert(certPool, "")
+
+	// Build transport credentials from the certificate pool. If there is no
+	// certificate pool, we expect the server to use a non-self-signed
+	// certificate such as a certificate obtained from Let's Encrypt.
+	var creds credentials.TransportCredentials
+	if certPool != nil {
+		creds = credentials.NewClientTLSFromCert(certPool, "")
+	} else {
+		// Fallback to the system pool. Using an empty tls config is an
+		// alternative to x509.SystemCertPool(). That call is not
+		// supported on Windows.
+		creds = credentials.NewTLS(&tls.Config{})
+	}
 
 	// Create a dial options array.
 	opts := []grpc.DialOption{

cmd/lncli/profile.go

@@ -36,6 +36,10 @@ type profileEntry struct {
 
 // cert returns the profile's TLS certificate as a x509 certificate pool.
 func (e *profileEntry) cert() (*x509.CertPool, error) {
+	if e.TLSCert == "" {
+		return nil, nil
+	}
+
 	cp := x509.NewCertPool()
 	if !cp.AppendCertsFromPEM([]byte(e.TLSCert)) {
 		return nil, fmt.Errorf("credentials: failed to append " +
@@ -113,11 +117,16 @@ func profileFromContext(ctx *cli.Context, store bool) (*profileEntry, error) {
 		return nil, err
 	}
 
-	// Load the certificate file now. We store it as plain PEM directly.
-	tlsCert, err := ioutil.ReadFile(tlsCertPath)
-	if err != nil {
-		return nil, fmt.Errorf("could not load TLS cert file %s: %v",
-			tlsCertPath, err)
+	// Load the certificate file now, if specified. We store it as plain PEM
+	// directly.
+	var tlsCert []byte
+	if lnrpc.FileExists(tlsCertPath) {
+		var err error
+		tlsCert, err = ioutil.ReadFile(tlsCertPath)
+		if err != nil {
+			return nil, fmt.Errorf("could not load TLS cert file "+
+				"%s: %v", tlsCertPath, err)
+		}
 	}
 
 	// Now load and possibly encrypt the macaroon file.

config.go

@@ -64,6 +64,8 @@ const (
 	defaultMaxLogFileSize                = 10
 	defaultMinBackoff                    = time.Second
 	defaultMaxBackoff                    = time.Hour
+	defaultLetsEncryptDirname            = "letsencrypt"
+	defaultLetsEncryptPort               = 80
 
 	defaultTorSOCKSPort            = 9050
 	defaultTorDNSHost              = "soa.nodes.lightning.directory"
@@ -127,8 +129,9 @@ var (
 
 	defaultTowerDir = filepath.Join(defaultDataDir, defaultTowerSubDirname)
 
-	defaultTLSCertPath = filepath.Join(DefaultLndDir, defaultTLSCertFilename)
-	defaultTLSKeyPath  = filepath.Join(DefaultLndDir, defaultTLSKeyFilename)
+	defaultTLSCertPath    = filepath.Join(DefaultLndDir, defaultTLSCertFilename)
+	defaultTLSKeyPath     = filepath.Join(DefaultLndDir, defaultTLSKeyFilename)
+	defaultLetsEncryptDir = filepath.Join(DefaultLndDir, defaultLetsEncryptDirname)
 
 	defaultBtcdDir         = btcutil.AppDataDir("btcd", false)
 	defaultBtcdRPCCertFile = filepath.Join(defaultBtcdDir, "rpc.cert")
@@ -179,6 +182,10 @@ type Config struct {
 	MaxLogFileSize  int           `long:"maxlogfilesize" description:"Maximum logfile size in MB"`
 	AcceptorTimeout time.Duration `long:"acceptortimeout" description:"Time after which an RPCAcceptor will time out and return false if it hasn't yet received a response"`
 
+	LetsEncryptDir    string `long:"letsencryptdir" description:"The directory to store Let's Encrypt certificates within"`
+	LetsEncryptPort   int    `long:"letsencryptport" description:"The port on which lnd will listen for Let's Encrypt challenges. Let's Encrypt will always try to contact on port 80. Often non-root processes are not allowed to bind to ports lower than 1024. This configuration option allows a different port to be used, but must be used in combination with port forwarding from port 80."`
+	LetsEncryptDomain string `long:"letsencryptdomain" description:"Request a Let's Encrypt certificate for this domain. Note that the certicate is only requested and stored when the first rpc connection comes in."`
+
 	// We'll parse these 'raw' string arguments into real net.Addrs in the
 	// loadConfig function. We need to expose the 'raw' strings so the
 	// command line library can access them.
@@ -318,6 +325,8 @@ func DefaultConfig() Config {
 		DebugLevel:      defaultLogLevel,
 		TLSCertPath:     defaultTLSCertPath,
 		TLSKeyPath:      defaultTLSKeyPath,
+		LetsEncryptDir:  defaultLetsEncryptDir,
+		LetsEncryptPort: defaultLetsEncryptPort,
 		LogDir:          defaultLogDir,
 		MaxLogFiles:     defaultMaxLogFiles,
 		MaxLogFileSize:  defaultMaxLogFileSize,
@@ -520,6 +529,9 @@ func ValidateConfig(cfg Config, usageMessage string) (*Config, error) {
 	lndDir := CleanAndExpandPath(cfg.LndDir)
 	if lndDir != DefaultLndDir {
 		cfg.DataDir = filepath.Join(lndDir, defaultDataDirname)
+		cfg.LetsEncryptDir = filepath.Join(
+			lndDir, defaultLetsEncryptDirname,
+		)
 		cfg.TLSCertPath = filepath.Join(lndDir, defaultTLSCertFilename)
 		cfg.TLSKeyPath = filepath.Join(lndDir, defaultTLSKeyFilename)
 		cfg.LogDir = filepath.Join(lndDir, defaultLogDirname)
@@ -558,6 +570,7 @@ func ValidateConfig(cfg Config, usageMessage string) (*Config, error) {
 	cfg.DataDir = CleanAndExpandPath(cfg.DataDir)
 	cfg.TLSCertPath = CleanAndExpandPath(cfg.TLSCertPath)
 	cfg.TLSKeyPath = CleanAndExpandPath(cfg.TLSKeyPath)
+	cfg.LetsEncryptDir = CleanAndExpandPath(cfg.LetsEncryptDir)
 	cfg.AdminMacPath = CleanAndExpandPath(cfg.AdminMacPath)
 	cfg.ReadMacPath = CleanAndExpandPath(cfg.ReadMacPath)
 	cfg.InvoiceMacPath = CleanAndExpandPath(cfg.InvoiceMacPath)

lnd.go

@@ -23,6 +23,7 @@ import (
 	"github.com/btcsuite/btcwallet/wallet"
 	proxy "github.com/grpc-ecosystem/grpc-gateway/runtime"
 	"github.com/lightninglabs/neutrino"
+	"golang.org/x/crypto/acme/autocert"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 	"gopkg.in/macaroon-bakery.v2/bakery"
@@ -264,13 +265,15 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 	defer cleanUp()
 
 	// Only process macaroons if --no-macaroons isn't set.
-	tlsCfg, restCreds, restProxyDest, err := getTLSConfig(cfg)
+	tlsCfg, restCreds, restProxyDest, cleanUp, err := getTLSConfig(cfg)
 	if err != nil {
 		err := fmt.Errorf("unable to load TLS credentials: %v", err)
 		ltndLog.Error(err)
 		return err
 	}
 
+	defer cleanUp()
+
 	serverCreds := credentials.NewTLS(tlsCfg)
 	serverOpts := []grpc.ServerOption{grpc.Creds(serverCreds)}
 
@@ -748,7 +751,7 @@ func Main(cfg *Config, lisCfg ListenerCfg, shutdownChan <-chan struct{}) error {
 // getTLSConfig returns a TLS configuration for the gRPC server and credentials
 // and a proxy destination for the REST reverse proxy.
 func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
-	string, error) {
+	string, func(), error) {
 
 	// Ensure we create TLS key and certificate if they don't exist.
 	if !fileExists(cfg.TLSCertPath) && !fileExists(cfg.TLSKeyPath) {
@@ -759,7 +762,7 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 			cfg.TLSDisableAutofill, cert.DefaultAutogenValidity,
 		)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 		rpcsLog.Infof("Done generating TLS certificates")
 	}
@@ -768,7 +771,7 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 		cfg.TLSCertPath, cfg.TLSKeyPath,
 	)
 	if err != nil {
-		return nil, nil, "", err
+		return nil, nil, "", nil, err
 	}
 
 	// We check whether the certifcate we have on disk match the IPs and
@@ -782,7 +785,7 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 			cfg.TLSExtraDomains, cfg.TLSDisableAutofill,
 		)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 	}
 
@@ -794,12 +797,12 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 
 		err := os.Remove(cfg.TLSCertPath)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 
 		err = os.Remove(cfg.TLSKeyPath)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 
 		rpcsLog.Infof("Renewing TLS certificates...")
@@ -809,7 +812,7 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 			cfg.TLSDisableAutofill, cert.DefaultAutogenValidity,
 		)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 		rpcsLog.Infof("Done renewing TLS certificates")
 
@@ -818,14 +821,15 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 			cfg.TLSCertPath, cfg.TLSKeyPath,
 		)
 		if err != nil {
-			return nil, nil, "", err
+			return nil, nil, "", nil, err
 		}
 	}
 
 	tlsCfg := cert.TLSConfFromCert(certData)
+
 	restCreds, err := credentials.NewClientTLSFromFile(cfg.TLSCertPath, "")
 	if err != nil {
-		return nil, nil, "", err
+		return nil, nil, "", nil, err
 	}
 
 	restProxyDest := cfg.RPCListeners[0].String()
@@ -841,7 +845,65 @@ func getTLSConfig(cfg *Config) (*tls.Config, *credentials.TransportCredentials,
 		)
 	}
 
-	return tlsCfg, &restCreds, restProxyDest, nil
+	// If Let's Encrypt is enabled, instantiate autocert to request/renew
+	// the certificates.
+	cleanUp := func() {}
+	if cfg.LetsEncryptDomain != "" {
+		ltndLog.Infof("Using Let's Encrypt certificate for domain %v",
+			cfg.LetsEncryptDomain)
+
+		manager := autocert.Manager{
+			Cache:      autocert.DirCache(cfg.LetsEncryptDir),
+			Prompt:     autocert.AcceptTOS,
+			HostPolicy: autocert.HostWhitelist(cfg.LetsEncryptDomain),
+		}
+
+		addr := fmt.Sprintf(":%v", cfg.LetsEncryptPort)
+		srv := &http.Server{
+			Addr:    addr,
+			Handler: manager.HTTPHandler(nil),
+		}
+		shutdownCompleted := make(chan struct{})
+		cleanUp = func() {
+			err := srv.Shutdown(context.Background())
+			if err != nil {
+				ltndLog.Errorf("Autocert listener shutdown "+
+					" error: %v", err)
+
+				return
+			}
+			<-shutdownCompleted
+			ltndLog.Infof("Autocert challenge listener stopped")
+		}
+
+		go func() {
+			ltndLog.Infof("Autocert challenge listener started "+
+				"at %v", addr)
+
+			err := srv.ListenAndServe()
+			if err != http.ErrServerClosed {
+				ltndLog.Errorf("autocert http: %v", err)
+			}
+			close(shutdownCompleted)
+		}()
+
+		getCertificate := func(h *tls.ClientHelloInfo) (
+			*tls.Certificate, error) {
+
+			lecert, err := manager.GetCertificate(h)
+			if err != nil {
+				ltndLog.Errorf("GetCertificate: %v", err)
+				return &certData, nil
+			}
+
+			return lecert, err
+		}
+
+		// The self-signed tls.cert remains available as fallback.
+		tlsCfg.GetCertificate = getCertificate
+	}
+
+	return tlsCfg, &restCreds, restProxyDest, cleanUp, nil
 }
 
 // fileExists reports whether the named file or directory exists.

sample-lnd.conf

@@ -50,6 +50,20 @@
 ; or want to expose the node at a domain.
 ; externalhosts=my-node-domain.com
 
+; Sets the directory to store Let's Encrypt certificates within
+; letsencryptdir=~/.lnd/letsencrypt
+
+; Sets the port on which lnd will listen for Let's Encrypt challenges. Let's 
+; Encrypt will always try to contact on port 80. Often non-root processes are
+; not allowed to bind to ports lower than 1024. This configuration option allows
+; a different port to be used, but must be used in combination with port
+; forwarding from port 80.
+; letsencryptport=8080
+
+; Request a Let's Encrypt certificate for this domain. Note that the certicate
+; is only requested and stored when the first rpc connection comes in.
+; letsencryptdomain=example.com
+
 ; Disable macaroon authentication. Macaroons are used are bearer credentials to
 ; authenticate all RPC access. If one wishes to opt out of macaroons, uncomment
 ; the line below.

server_test.go

@@ -119,10 +119,11 @@ func TestTLSAutoRegeneration(t *testing.T) {
 		TLSKeyPath:   keyPath,
 		RPCListeners: rpcListeners,
 	}
-	_, _, _, err = getTLSConfig(cfg)
+	_, _, _, cleanUp, err := getTLSConfig(cfg)
 	if err != nil {
 		t.Fatalf("couldn't retrieve TLS config")
 	}
+	defer cleanUp()
 
 	// Grab the certificate to test that getTLSConfig did its job correctly
 	// and generated a new cert.