lightning-terminal/rpc_proxy.go

package terminal

import (
	"context"
	"crypto/tls"
	"encoding/base64"
	"encoding/hex"
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"strings"
	"sync/atomic"
	"time"

	"github.com/improbable-eng/grpc-web/go/grpcweb"
	"github.com/lightninglabs/lightning-terminal/litrpc"
	"github.com/lightninglabs/lightning-terminal/perms"
	"github.com/lightninglabs/lightning-terminal/session"
	"github.com/lightningnetwork/lnd/lncfg"
	"github.com/lightningnetwork/lnd/macaroons"
	grpcProxy "github.com/mwitkow/grpc-proxy/proxy"
	"google.golang.org/grpc"
	"google.golang.org/grpc/backoff"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/status"
	"google.golang.org/grpc/test/bufconn"
	"gopkg.in/macaroon.v2"
)

const (
	contentTypeGrpc = "application/grpc"

	// HeaderMacaroon is the HTTP header field name that is used to send
	// the macaroon.
	HeaderMacaroon = "Macaroon"
)

// ErrWaitingToStart is returned if Lit's rpcProxy is not yet ready to handle
// calls.
var ErrWaitingToStart = fmt.Errorf("waiting for the RPC server to start")

// proxyErr is an error type that adds more context to an error occurring in the
// proxy.
type proxyErr struct {
	wrapped      error
	proxyContext string
}

// Error returns the error message as a string, including the proxy's context.
func (e *proxyErr) Error() string {
	return fmt.Sprintf("proxy error with context %s: %v", e.proxyContext,
		e.wrapped)
}

// Unwrap returns the wrapped error.
func (e *proxyErr) Unwrap() error {
	return e.wrapped
}

// newRpcProxy creates a new RPC proxy that can take any native gRPC, grpc-web
// or REST request and delegate (and convert if necessary) it to the correct
// component.
func newRpcProxy(cfg *Config, validator macaroons.MacaroonValidator,
	superMacValidator session.SuperMacaroonValidator,
	permsMgr *perms.Manager, bufListener *bufconn.Listener) *rpcProxy {

	// The gRPC web calls are protected by HTTP basic auth which is defined
	// by base64(username:password). Because we only have a password, we
	// just use base64(password:password).
	basicAuth := base64.StdEncoding.EncodeToString([]byte(fmt.Sprintf(
		"%s:%s", cfg.UIPassword, cfg.UIPassword,
	)))

	// Set up the final gRPC server that will serve gRPC web to the browser
	// and translate all incoming gRPC web calls into native gRPC that are
	// then forwarded to lnd's RPC interface. GRPC web has a few kinks that
	// need to be addressed with a custom director that just takes care of a
	// few HTTP header fields.
	p := &rpcProxy{
		cfg:               cfg,
		basicAuth:         basicAuth,
		permsMgr:          permsMgr,
		macValidator:      validator,
		superMacValidator: superMacValidator,
		bufListener:       bufListener,
	}
	p.grpcServer = grpc.NewServer(
		// From the grpxProxy doc: This codec is *crucial* to the
		// functioning of the proxy.
		grpc.CustomCodec(grpcProxy.Codec()), // nolint:staticcheck
		grpc.ChainStreamInterceptor(p.StreamServerInterceptor),
		grpc.ChainUnaryInterceptor(p.UnaryServerInterceptor),
		grpc.UnknownServiceHandler(
			grpcProxy.TransparentHandler(p.makeDirector(true)),
		),
	)

	// Create the gRPC web proxy that wraps the just created grpcServer and
	// converts the browser's gRPC web calls into native gRPC.
	options := []grpcweb.Option{
		grpcweb.WithWebsockets(true),
		grpcweb.WithWebsocketPingInterval(2 * time.Minute),
		grpcweb.WithCorsForRegisteredEndpointsOnly(false),
	}
	p.grpcWebProxy = grpcweb.WrapServer(p.grpcServer, options...)
	return p
}

// rpcProxy is an RPC proxy server that can take any native gRPC, grpc-web or
// REST request and delegate it to the correct component. Any grpc-web request
// is first converted into a native gRPC request. The gRPC call is then handed
// to our local gRPC server that has all in-process RPC servers registered. If
// the call is meant for a component that is registered as running in-process,
// it is handled there. If not, the director will forward the call to either a
// local or remote lnd instance.
//
//	any RPC or grpc-web call
//	    |
//	    V
//	+---+----------------------+
//	| grpc-web proxy           |
//	+---+----------------------+
//	    |
//	    v native gRPC call with basic auth
//	+---+----------------------+
//	| interceptors             |
//	+---+----------------------+
//	    |
//	    v native gRPC call with macaroon
//	+---+----------------------+
//	| gRPC server              |
//	+---+----------------------+
//	    |
//	    v unknown authenticated call, gRPC server is just a wrapper
//	+---+----------------------+
//	| director                 |
//	+---+----------------------+
//	    |
//	    v authenticated call
//	+---+----------------------+ call to lnd or integrated daemon
//	| lnd (remote or local)    +---------------+
//	| faraday remote           |               |
//	| loop remote              |    +----------v----------+
//	| pool remote              |    | lnd local subserver |
//	+--------------------------+    |  - faraday          |
//	                                |  - loop             |
//	                                |  - pool             |
//	                                +---------------------+
type rpcProxy struct {
	litrpc.UnimplementedProxyServer

	// started is set to 1 once the rpcProxy has successfully started. It
	// must only ever be used atomically.
	started int32

	cfg       *Config
	basicAuth string
	permsMgr  *perms.Manager

	macValidator      macaroons.MacaroonValidator
	superMacValidator session.SuperMacaroonValidator
	bufListener       *bufconn.Listener

	superMacaroon string

	lndConn     *grpc.ClientConn
	faradayConn *grpc.ClientConn
	loopConn    *grpc.ClientConn
	poolConn    *grpc.ClientConn

	grpcServer   *grpc.Server
	grpcWebProxy *grpcweb.WrappedGrpcServer
}

// Start creates initial connection to lnd.
func (p *rpcProxy) Start() error {
	var err error

	// Setup the connection to lnd.
	host, _, tlsPath, _, _ := p.cfg.lndConnectParams()

	// We use a bufconn to connect to lnd in integrated mode.
	if p.cfg.LndMode == ModeIntegrated {
		p.lndConn, err = dialBufConnBackend(p.bufListener)
	} else {
		p.lndConn, err = dialBackend("lnd", host, tlsPath)
	}
	if err != nil {
		return fmt.Errorf("could not dial lnd: %v", err)
	}

	// Make sure we can connect to all the daemons that are configured to be
	// running in remote mode.
	if p.cfg.faradayRemote {
		p.faradayConn, err = dialBackend(
			"faraday", p.cfg.Remote.Faraday.RPCServer,
			lncfg.CleanAndExpandPath(
				p.cfg.Remote.Faraday.TLSCertPath,
			),
		)
		if err != nil {
			return fmt.Errorf("could not dial remote faraday: %v",
				err)
		}
	}

	if p.cfg.loopRemote {
		p.loopConn, err = dialBackend(
			"loop", p.cfg.Remote.Loop.RPCServer,
			lncfg.CleanAndExpandPath(p.cfg.Remote.Loop.TLSCertPath),
		)
		if err != nil {
			return fmt.Errorf("could not dial remote loop: %v", err)
		}
	}

	if p.cfg.poolRemote {
		p.poolConn, err = dialBackend(
			"pool", p.cfg.Remote.Pool.RPCServer,
			lncfg.CleanAndExpandPath(p.cfg.Remote.Pool.TLSCertPath),
		)
		if err != nil {
			return fmt.Errorf("could not dial remote pool: %v", err)
		}
	}

	atomic.CompareAndSwapInt32(&p.started, 0, 1)

	return nil
}

// hasStarted returns true if the rpcProxy has started and is ready to handle
// requests.
func (p *rpcProxy) hasStarted() bool {
	return atomic.LoadInt32(&p.started) == 1
}

// Stop shuts down the lnd connection.
func (p *rpcProxy) Stop() error {
	p.grpcServer.Stop()

	if p.lndConn != nil {
		if err := p.lndConn.Close(); err != nil {
			log.Errorf("Error closing lnd connection: %v", err)
			return err
		}
	}

	if p.faradayConn != nil {
		if err := p.faradayConn.Close(); err != nil {
			log.Errorf("Error closing faraday connection: %v", err)
			return err
		}
	}

	if p.loopConn != nil {
		if err := p.loopConn.Close(); err != nil {
			log.Errorf("Error closing loop connection: %v", err)
			return err
		}
	}

	if p.poolConn != nil {
		if err := p.poolConn.Close(); err != nil {
			log.Errorf("Error closing pool connection: %v", err)
			return err
		}
	}

	return nil
}

// StopDaemon will send a shutdown request to the interrupt handler, triggering
// a graceful shutdown of the daemon.
//
// NOTE: this is part of the litrpc.ProxyServiceServer interface.
func (p *rpcProxy) StopDaemon(_ context.Context,
	_ *litrpc.StopDaemonRequest) (*litrpc.StopDaemonResponse, error) {

	log.Infof("StopDaemon rpc request received")

	interceptor.RequestShutdown()

	return &litrpc.StopDaemonResponse{}, nil
}

// GetInfo returns general information concerning the LiTd node.
//
// NOTE: this is part of the litrpc.ProxyServiceServer interface.
func (p *rpcProxy) GetInfo(_ context.Context, _ *litrpc.GetInfoRequest) (
	*litrpc.GetInfoResponse, error) {

	return &litrpc.GetInfoResponse{
		Version: Version(),
	}, nil
}

// isHandling checks if the specified request is something to be handled by lnd
// or any of the attached sub daemons. If true is returned, the call was handled
// by the RPC proxy and the caller MUST NOT handle it again. If false is
// returned, the request was not handled and the caller MUST handle it.
func (p *rpcProxy) isHandling(resp http.ResponseWriter,
	req *http.Request) bool {

	// gRPC web requests are easy to identify. Send them to the gRPC
	// web proxy.
	if p.grpcWebProxy.IsGrpcWebRequest(req) ||
		p.grpcWebProxy.IsGrpcWebSocketRequest(req) {

		log.Infof("Handling gRPC web request: %s", req.URL.Path)
		p.grpcWebProxy.ServeHTTP(resp, req)

		return true
	}

	// Normal gRPC requests are also easy to identify. These we can
	// send directly to the lnd proxy's gRPC server.
	if isGrpcRequest(req) {
		log.Infof("Handling gRPC request: %s", req.URL.Path)
		p.grpcServer.ServeHTTP(resp, req)

		return true
	}

	return false
}

// makeDirector is a function that returns a director that directs an incoming
// request to the correct backend, depending on what kind of authentication
// information is attached to the request.
func (p *rpcProxy) makeDirector(allowLitRPC bool) func(ctx context.Context,
	requestURI string) (context.Context, *grpc.ClientConn, error) {

	return func(ctx context.Context, requestURI string) (context.Context,
		*grpc.ClientConn, error) {

		// If this header is present in the request from the web client,
		// the actual connection to the backend will not be established.
		// https://github.com/improbable-eng/grpc-web/issues/568
		md, _ := metadata.FromIncomingContext(ctx)
		mdCopy := md.Copy()
		delete(mdCopy, "connection")

		outCtx := metadata.NewOutgoingContext(ctx, mdCopy)

		// Is there a basic auth or super macaroon set?
		authHeaders := md.Get("authorization")
		macHeader := md.Get(HeaderMacaroon)
		switch {
		case len(authHeaders) == 1 && !p.cfg.DisableUI:
			macBytes, err := p.basicAuthToMacaroon(
				authHeaders[0], requestURI, nil,
			)
			if err != nil {
				return outCtx, nil, err
			}
			if len(macBytes) > 0 {
				mdCopy.Set(HeaderMacaroon, hex.EncodeToString(
					macBytes,
				))
			}

		case len(macHeader) == 1 && session.IsSuperMacaroon(macHeader[0]):
			// If we have a macaroon, and it's a super macaroon,
			// then we need to convert it into the actual daemon
			// macaroon if they're running in remote mode.
			macBytes, err := p.convertSuperMacaroon(
				ctx, macHeader[0], requestURI,
			)
			if err != nil {
				return outCtx, nil, err
			}
			if len(macBytes) > 0 {
				mdCopy.Set(HeaderMacaroon, hex.EncodeToString(
					macBytes,
				))
			}
		}

		// Direct the call to the correct backend. All gRPC calls end up
		// here since our gRPC server instance doesn't have any handlers
		// registered itself. So all daemon calls that are remote are
		// forwarded to them directly. Everything else will go to lnd
		// since it must either be an lnd call or something that'll be
		// handled by the integrated daemons that are hooking into lnd's
		// gRPC server.
		switch {
		case p.permsMgr.IsFaradayURI(requestURI) && p.cfg.faradayRemote:
			return outCtx, p.faradayConn, nil

		case p.permsMgr.IsLoopURI(requestURI) && p.cfg.loopRemote:
			return outCtx, p.loopConn, nil

		case p.permsMgr.IsPoolURI(requestURI) && p.cfg.poolRemote:
			return outCtx, p.poolConn, nil

		// Calls to LiT session RPC aren't allowed in some cases.
		case p.permsMgr.IsLitURI(requestURI) && !allowLitRPC:
			return outCtx, nil, status.Errorf(
				codes.Unimplemented, "unknown service %s",
				requestURI,
			)

		default:
			return outCtx, p.lndConn, nil
		}
	}
}

// UnaryServerInterceptor is a gRPC interceptor that checks whether the
// request is authorized by the included macaroons.
func (p *rpcProxy) UnaryServerInterceptor(ctx context.Context, req interface{},
	info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{},
	error) {

	if !p.hasStarted() {
		return nil, ErrWaitingToStart
	}

	uriPermissions, ok := p.permsMgr.URIPermissions(info.FullMethod)
	if !ok {
		return nil, fmt.Errorf("%s: unknown permissions "+
			"required for method", info.FullMethod)
	}

	// For now, basic authentication is just a quick fix until we
	// have proper macaroon support implemented in the UI. We allow
	// gRPC web requests to have it and "convert" the auth into a
	// proper macaroon now.
	newCtx, err := p.convertBasicAuth(ctx, info.FullMethod, nil)
	if err != nil {
		// Make sure we handle the case where the super macaroon
		// is still empty on startup.
		if pErr, ok := err.(*proxyErr); ok &&
			pErr.proxyContext == "supermacaroon" {

			return nil, fmt.Errorf("super macaroon error: "+
				"%v", pErr)
		}
		return nil, err
	}

	// With the basic auth converted to a macaroon if necessary,
	// let's now validate the macaroon.
	err = p.macValidator.ValidateMacaroon(
		newCtx, uriPermissions, info.FullMethod,
	)
	if err != nil {
		return nil, err
	}

	return handler(ctx, req)
}

// StreamServerInterceptor is a GRPC interceptor that checks whether the
// request is authorized by the included macaroons.
func (p *rpcProxy) StreamServerInterceptor(srv interface{},
	ss grpc.ServerStream, info *grpc.StreamServerInfo,
	handler grpc.StreamHandler) error {

	if !p.hasStarted() {
		return ErrWaitingToStart
	}

	uriPermissions, ok := p.permsMgr.URIPermissions(info.FullMethod)
	if !ok {
		return fmt.Errorf("%s: unknown permissions required "+
			"for method", info.FullMethod)
	}

	// For now, basic authentication is just a quick fix until we
	// have proper macaroon support implemented in the UI. We allow
	// gRPC web requests to have it and "convert" the auth into a
	// proper macaroon now.
	ctx, err := p.convertBasicAuth(
		ss.Context(), info.FullMethod, nil,
	)
	if err != nil {
		// Make sure we handle the case where the super macaroon
		// is still empty on startup.
		if pErr, ok := err.(*proxyErr); ok &&
			pErr.proxyContext == "supermacaroon" {

			return fmt.Errorf("super macaroon error: "+
				"%v", pErr)
		}
		return err
	}

	// With the basic auth converted to a macaroon if necessary,
	// let's now validate the macaroon.
	err = p.macValidator.ValidateMacaroon(
		ctx, uriPermissions, info.FullMethod,
	)
	if err != nil {
		return err
	}

	return handler(srv, ss)
}

// convertBasicAuth tries to convert the HTTP authorization header into a
// macaroon based authentication header.
func (p *rpcProxy) convertBasicAuth(ctx context.Context,
	requestURI string, ctxErr error) (context.Context, error) {

	// If the UI is disabled, then there is no UI password and so the
	// request is required to have a macaroon in it.
	if p.cfg.DisableUI {
		return ctx, ctxErr
	}

	md, ok := metadata.FromIncomingContext(ctx)
	if !ok {
		return ctx, ctxErr
	}

	authHeaders := md.Get("authorization")
	if len(authHeaders) == 0 {
		// No basic auth provided, we don't add a macaroon and let the
		// gRPC security interceptor reject the request.
		return ctx, ctxErr
	}

	macBytes, err := p.basicAuthToMacaroon(
		authHeaders[0], requestURI, ctxErr,
	)
	if err != nil || len(macBytes) == 0 {
		return ctx, err
	}

	md.Set(HeaderMacaroon, hex.EncodeToString(macBytes))
	return metadata.NewIncomingContext(ctx, md), nil
}

// basicAuthToMacaroon checks that the incoming request context has the expected
// and valid basic authentication header then attaches the correct macaroon to
// the context so it can be forwarded to the actual gRPC server.
func (p *rpcProxy) basicAuthToMacaroon(basicAuth, requestURI string,
	ctxErr error) ([]byte, error) {

	// The user specified an authorization header so this is very likely a
	// gRPC Web call from the UI. But we only attach the macaroon if the
	// auth is correct. That way an attacker doesn't know that basic auth
	// is even allowed as the error message will only be the macaroon error
	// from the lnd backend.
	authHeaderParts := strings.Split(basicAuth, " ")
	if len(authHeaderParts) != 2 {
		return nil, ctxErr
	}
	if authHeaderParts[1] != p.basicAuth {
		return nil, ctxErr
	}

	var (
		macPath string
		macData []byte
	)
	switch {
	case p.permsMgr.IsLndURI(requestURI):
		_, _, _, macPath, macData = p.cfg.lndConnectParams()

	case p.permsMgr.IsFaradayURI(requestURI):
		if p.cfg.faradayRemote {
			macPath = p.cfg.Remote.Faraday.MacaroonPath
		} else {
			macPath = p.cfg.Faraday.MacaroonPath
		}

	case p.permsMgr.IsLoopURI(requestURI):
		if p.cfg.loopRemote {
			macPath = p.cfg.Remote.Loop.MacaroonPath
		} else {
			macPath = p.cfg.Loop.MacaroonPath
		}

	case p.permsMgr.IsPoolURI(requestURI):
		if p.cfg.poolRemote {
			macPath = p.cfg.Remote.Pool.MacaroonPath
		} else {
			macPath = p.cfg.Pool.MacaroonPath
		}

	case p.permsMgr.IsLitURI(requestURI):
		macPath = p.cfg.MacaroonPath

	default:
		return nil, fmt.Errorf("unknown gRPC web request: %v",
			requestURI)
	}

	switch {
	// If we have a super macaroon, we can use that one directly since it
	// will contain all permissions we need.
	case len(p.superMacaroon) > 0:
		superMacData, err := hex.DecodeString(p.superMacaroon)

		// Make sure we can avoid running into an empty macaroon here if
		// something went wrong with the decoding process (if we're
		// still starting up).
		if err != nil {
			return nil, &proxyErr{
				proxyContext: "supermacaroon",
				wrapped: fmt.Errorf("couldn't decode "+
					"super macaroon: %v", err),
			}
		}

		return superMacData, nil

	// If we have macaroon data directly, just encode them. This could be
	// for initial requests to lnd while we don't have the super macaroon
	// just yet (or are actually calling to bake the super macaroon).
	case len(macData) > 0:
		return macData, nil

	// The fall back is to read the macaroon from a path. This is in remote
	// mode.
	case len(macPath) > 0:
		// Now that we know which macaroon to load, do it and attach it
		// to the request context.
		return readMacaroon(lncfg.CleanAndExpandPath(macPath))
	}

	return nil, &proxyErr{
		proxyContext: "auth",
		wrapped:      fmt.Errorf("unknown macaroon to use"),
	}
}

// convertSuperMacaroon converts a super macaroon into a daemon specific
// macaroon, but only if the super macaroon contains all required permissions
// and the target daemon is actually running in remote mode.
func (p *rpcProxy) convertSuperMacaroon(ctx context.Context, macHex string,
	fullMethod string) ([]byte, error) {

	requiredPermissions, ok := p.permsMgr.URIPermissions(fullMethod)
	if !ok {
		return nil, fmt.Errorf("%s: unknown permissions required for "+
			"method", fullMethod)
	}

	// We have a super macaroon, from here on out we'll return errors if
	// something isn't the way we expect it to be.
	macBytes, err := hex.DecodeString(macHex)
	if err != nil {
		return nil, err
	}

	// Make sure the super macaroon is valid and contains all the required
	// permissions.
	err = p.superMacValidator(
		ctx, macBytes, requiredPermissions, fullMethod,
	)
	if err != nil {
		return nil, err
	}

	// Is this actually a request that goes to a daemon that is running
	// remotely?
	switch {
	case p.permsMgr.IsFaradayURI(fullMethod) && p.cfg.faradayRemote:
		return readMacaroon(lncfg.CleanAndExpandPath(
			p.cfg.Remote.Faraday.MacaroonPath,
		))

	case p.permsMgr.IsLoopURI(fullMethod) && p.cfg.loopRemote:
		return readMacaroon(lncfg.CleanAndExpandPath(
			p.cfg.Remote.Loop.MacaroonPath,
		))

	case p.permsMgr.IsPoolURI(fullMethod) && p.cfg.poolRemote:
		return readMacaroon(lncfg.CleanAndExpandPath(
			p.cfg.Remote.Pool.MacaroonPath,
		))
	}

	return nil, nil
}

// dialBufConnBackend dials an in-memory connection to an RPC listener and
// ignores any TLS certificate mismatches.
func dialBufConnBackend(listener *bufconn.Listener) (*grpc.ClientConn, error) {
	tlsConfig := credentials.NewTLS(&tls.Config{
		InsecureSkipVerify: true,
	})
	conn, err := grpc.Dial(
		"",
		grpc.WithContextDialer(
			func(context.Context, string) (net.Conn, error) {
				return listener.Dial()
			},
		),
		grpc.WithTransportCredentials(tlsConfig),

		// From the grpcProxy doc: This codec is *crucial* to the
		// functioning of the proxy.
		grpc.WithCodec(grpcProxy.Codec()), // nolint
		grpc.WithTransportCredentials(tlsConfig),
		grpc.WithDefaultCallOptions(maxMsgRecvSize),
		grpc.WithConnectParams(grpc.ConnectParams{
			Backoff:           backoff.DefaultConfig,
			MinConnectTimeout: defaultConnectTimeout,
		}),
	)

	return conn, err
}

// dialBackend connects to a gRPC backend through the given address and uses the
// given TLS certificate to authenticate the connection.
func dialBackend(name, dialAddr, tlsCertPath string) (*grpc.ClientConn, error) {
	var opts []grpc.DialOption
	tlsConfig, err := credentials.NewClientTLSFromFile(tlsCertPath, "")
	if err != nil {
		return nil, fmt.Errorf("could not read %s TLS cert %s: %v",
			name, tlsCertPath, err)
	}

	opts = append(
		opts,

		// From the grpcProxy doc: This codec is *crucial* to the
		// functioning of the proxy.
		grpc.WithCodec(grpcProxy.Codec()), // nolint
		grpc.WithTransportCredentials(tlsConfig),
		grpc.WithDefaultCallOptions(maxMsgRecvSize),
		grpc.WithConnectParams(grpc.ConnectParams{
			Backoff:           backoff.DefaultConfig,
			MinConnectTimeout: defaultConnectTimeout,
		}),
	)

	log.Infof("Dialing %s gRPC server at %s", name, dialAddr)
	cc, err := grpc.Dial(dialAddr, opts...)
	if err != nil {
		return nil, fmt.Errorf("failed dialing %s backend: %v", name,
			err)
	}
	return cc, nil
}

// readMacaroon tries to read the macaroon file at the specified path and create
// gRPC dial options from it.
func readMacaroon(macPath string) ([]byte, error) {
	// Load the specified macaroon file.
	macBytes, err := ioutil.ReadFile(macPath)
	if err != nil {
		return nil, fmt.Errorf("unable to read macaroon path: %v", err)
	}

	// Make sure it actually is a macaroon by parsing it.
	mac := &macaroon.Macaroon{}
	if err := mac.UnmarshalBinary(macBytes); err != nil {
		return nil, fmt.Errorf("unable to decode macaroon: %v", err)
	}

	// It's a macaroon alright, let's return the binary data now.
	return macBytes, nil
}

// isGrpcRequest determines if a request is a gRPC request by checking that the
// "content-type" is "application/grpc" and that the protocol is HTTP/2.
func isGrpcRequest(req *http.Request) bool {
	contentType := req.Header.Get("content-type")
	return req.ProtoMajor == 2 &&
		strings.HasPrefix(contentType, contentTypeGrpc)
}