lightning-terminal/rpc_proxy.go

package terminal

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

	"github.com/improbable-eng/grpc-web/go/grpcweb"
	"github.com/lightninglabs/lightning-terminal/litrpc"
	litmac "github.com/lightninglabs/lightning-terminal/macaroons"
	"github.com/lightninglabs/lightning-terminal/perms"
	litstatus "github.com/lightninglabs/lightning-terminal/status"
	"github.com/lightninglabs/lightning-terminal/subservers"
	"github.com/lightningnetwork/lnd/lncfg"
	"github.com/lightningnetwork/lnd/lnrpc"
	"github.com/lightningnetwork/lnd/macaroons"
	grpcProxy "github.com/mwitkow/grpc-proxy/proxy"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/status"
	"gopkg.in/macaroon.v2"
)

const (
	contentTypeGrpc = "application/grpc"

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

	lndBakeMacMethod      = "/lnrpc.Lightning/BakeMacaroon"
	litBakeSuperMacMethod = "/litrpc.Proxy/BakeSuperMacaroon"
)

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

	// ErrUnknownRequest is an error returned when the request URI is
	// unknown if the permissions for the request are unknown.
	ErrUnknownRequest = fmt.Errorf("unknown request")
)

// 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 litmac.SuperMacaroonValidator,
	permsMgr *perms.Manager, subServerMgr *subservers.Manager,
	statusMgr *litstatus.Manager, getLNDClient lndBasicClientFn) *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,
		subServerMgr:      subServerMgr,
		statusMgr:         statusMgr,
		getBasicLNDClient: getLNDClient,
	}
	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
	subServerMgr      *subservers.Manager
	statusMgr         *litstatus.Manager
	getBasicLNDClient lndBasicClientFn

	bakeSuperMac bakeSuperMac

	macValidator      macaroons.MacaroonValidator
	superMacValidator litmac.SuperMacaroonValidator

	superMacaroon string

	lndConn *grpc.ClientConn

	grpcServer   *grpc.Server
	grpcWebProxy *grpcweb.WrappedGrpcServer
}

// bakeSuperMac can be used to bake a new super macaroon.
type bakeSuperMac func(ctx context.Context, rootKeyID uint32,
	readOnly bool) (string, error)

// lndBasicClientFn can be used to obtain access to an lnrpc.LightningClient if
// it is available.
type lndBasicClientFn func() (lnrpc.LightningClient, error)

// Start creates initial connection to lnd.
func (p *rpcProxy) Start(lndConn *grpc.ClientConn,
	bakeSuperMac bakeSuperMac) error {

	p.lndConn = lndConn
	p.bakeSuperMac = bakeSuperMac

	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()

	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(),
		CommitHash: CommitHash + Dirty,
	}, nil
}

// BakeSuperMacaroon bakes a new macaroon that includes permissions for
// all the active daemons that LiT is connected to.
//
// NOTE: this is part of the litrpc.ProxyServiceServer interface.
func (p *rpcProxy) BakeSuperMacaroon(ctx context.Context,
	req *litrpc.BakeSuperMacaroonRequest) (
	*litrpc.BakeSuperMacaroonResponse, error) {

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

	superMac, err := p.bakeSuperMac(ctx, req.RootKeyIdSuffix, req.ReadOnly)
	if err != nil {
		return nil, err
	}

	return &litrpc.BakeSuperMacaroonResponse{
		Macaroon: superMac,
	}, 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 &&
			litmac.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,
				))
			}
		}

		if err := p.checkSubSystemStarted(requestURI); err != nil {
			return outCtx, nil, err
		}

		// 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.
		handled, conn, err := p.subServerMgr.GetRemoteConn(requestURI)
		if err != nil {
			return outCtx, nil, status.Error(
				codes.Unavailable, err.Error(),
			)
		}
		if handled {
			return outCtx, conn, nil
		}

		// Calls to LiT session RPC aren't allowed in some cases.
		isLit := p.permsMgr.IsSubServerURI(subservers.LIT, requestURI)
		if isLit && !allowLitRPC {
			return outCtx, nil, status.Errorf(
				codes.Unimplemented, "unknown service %s",
				requestURI,
			)
		}

		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) {

	uriPermissions, ok := p.permsMgr.URIPermissions(info.FullMethod)
	if !ok {
		return nil, ErrUnknownRequest
	}

	if err := p.checkSubSystemStarted(info.FullMethod); err != nil {
		return nil, err
	}

	// 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
	}

	// We special case the handling of a call to LiT's BakeSuperMacaroon
	// method if we are in stateless init mode. If a user has called this
	// method, they are either calling with an LND macaroon or a baked
	// super macaroon.
	if p.cfg.statelessInitMode &&
		info.FullMethod == litBakeSuperMacMethod {

		// Fetch permissions that are required for LND's BakeMacaroon
		// method. Since this will only be called in stateless-init mode
		// which is only possible in integrated mode, we can be sure
		// that the permissions returned here are the up-to-date
		// permissions for the LND method.
		requiredPerms, ok := p.permsMgr.URIPermissions(lndBakeMacMethod)
		if !ok {
			return nil, fmt.Errorf("unknown permissions for %s",
				lndBakeMacMethod)
		}
		permissions := make(
			[]*lnrpc.MacaroonPermission, len(requiredPerms),
		)
		for idx, perm := range requiredPerms {
			permissions[idx] = &lnrpc.MacaroonPermission{
				Entity: perm.Entity,
				Action: perm.Action,
			}
		}

		// Next, we extract the macaroon provided by the user.
		macHex, err := macaroons.RawMacaroonFromContext(ctx)
		if err != nil {
			return nil, err
		}

		macBytes, err := hex.DecodeString(macHex)
		if err != nil {
			return nil, err
		}

		lndClient, err := p.getBasicLNDClient()
		if err != nil {
			return nil, err
		}

		// Using LiT's existing connection to LND, we verify that the
		// provided macaroon contains the required permissions for the
		// LND BakeMacaroon method.
		resp, err := lndClient.CheckMacaroonPermissions(
			ctx, &lnrpc.CheckMacPermRequest{
				Macaroon:    macBytes,
				Permissions: permissions,
				FullMethod:  lndBakeMacMethod,
			},
		)

		// There are two valid possible outcomes depending on if the
		// user used LND's macaroon which contains the permissions
		// required for LND's BakeMacaroon method or if they used a
		// super macaroon that contains the necessary permissions for
		// LiT's BakeSuperMacaroon method. If the former is the case,
		// and the macaroon is valid, then we can now directly call the
		// handler without further checking the macaroon.
		if err == nil && resp.Valid {
			// Call LiT's BakeSuperMacaroon function.
			return handler(ctx, req)
		}

		// If we do get an error from the above call, then the later
		// case described above might be true: the call might have
		// been performed with a macaroon that has the permissions for
		// LiT's BakeSuperMacaroon method. In that case, we log the
		// error, but we pass the call on to LiT's macaroon validator
		// as normal.
		log.Warnf("The call to LiT's %s method did not contain a "+
			"macaroon with the permissions required for LND's %s "+
			"method: %v. The call will instead be verified "+
			"against LiT's macaroon validator to check if it has "+
			"direct permissions for LiT's %s method.",
			litBakeSuperMacMethod, lndBakeMacMethod, err,
			litBakeSuperMacMethod)
	}

	// 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 {

	uriPermissions, ok := p.permsMgr.URIPermissions(info.FullMethod)
	if !ok {
		return ErrUnknownRequest
	}

	if err := p.checkSubSystemStarted(info.FullMethod); err != nil {
		return err
	}

	// 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 macData []byte
	handled, macPath := p.subServerMgr.MacaroonPath(requestURI)

	switch {
	case handled:

	case p.permsMgr.IsSubServerURI(subservers.LND, requestURI):
		_, _, _, macPath, macData = p.cfg.lndConnectParams()

	case p.permsMgr.IsSubServerURI(subservers.LIT, requestURI):
		macPath = p.cfg.MacaroonPath

	default:
		return nil, ErrUnknownRequest
	}

	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, ErrUnknownRequest
	}

	// 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?
	handled, macBytes, err := p.subServerMgr.ReadRemoteMacaroon(fullMethod)
	if handled {
		return macBytes, err
	}

	return nil, nil
}

// checkSubSystemStarted checks if the subsystem responsible for handling the
// given URI has started.
func (p *rpcProxy) checkSubSystemStarted(requestURI string) error {
	// A request to Lit's status and proxy services is always allowed.
	if isStatusReq(requestURI) || isProxyReq(requestURI) {
		return nil
	}

	// Check if the rpcProxy has started yet.
	if !p.hasStarted() {
		return ErrWaitingToStart
	}

	handled, system := p.subServerMgr.Handles(requestURI)
	switch {
	case handled:

	case isAccountsReq(requestURI):
		system = subservers.ACCOUNTS

	case p.permsMgr.IsSubServerURI(subservers.LIT, requestURI):
		system = subservers.LIT

	case p.permsMgr.IsSubServerURI(subservers.LND, requestURI):
		system = subservers.LND

	default:
		return ErrUnknownRequest
	}

	// Check with the status manager to see if the sub-server is ready to
	// handle the request.
	ready, disabled, err := p.statusMgr.IsSystemReady(system, requestURI)
	if err != nil {
		return err
	}

	if disabled {
		return fmt.Errorf("%s has been disabled", system)
	}

	if !ready {
		return fmt.Errorf("%s is not ready for: %s", system, requestURI)
	}

	return 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)
}

// isStatusReq returns true if the given request is intended for the
// litrpc.Status service.
func isStatusReq(uri string) bool {
	return strings.HasPrefix(
		uri, fmt.Sprintf("/%s", litrpc.Status_ServiceDesc.ServiceName),
	)
}

// isProxyReq returns true if the given request is intended for the litrpc.Proxy
// service.
func isProxyReq(uri string) bool {
	return strings.HasPrefix(
		uri, fmt.Sprintf("/%s", litrpc.Proxy_ServiceDesc.ServiceName),
	)
}

// isAccountsReq returns true if the given request is intended for the
// litrpc.Accounts service.
func isAccountsReq(uri string) bool {
	return strings.HasPrefix(
		uri, fmt.Sprintf(
			"/%s", litrpc.Accounts_ServiceDesc.ServiceName,
		),
	)
}