headscale/app.go

package headscale

import (
	"context"
	"crypto/tls"
	"errors"
	"fmt"
	"io"
	"io/fs"
	"net"
	"net/http"
	"net/url"
	"os"
	"os/signal"
	"sort"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/coreos/go-oidc/v3/oidc"
	"github.com/gin-gonic/gin"
	grpc_middleware "github.com/grpc-ecosystem/go-grpc-middleware"
	"github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
	v1 "github.com/juanfont/headscale/gen/go/headscale/v1"
	"github.com/patrickmn/go-cache"
	zerolog "github.com/philip-bui/grpc-zerolog"
	zl "github.com/rs/zerolog"
	"github.com/rs/zerolog/log"
	ginprometheus "github.com/zsais/go-gin-prometheus"
	"golang.org/x/crypto/acme"
	"golang.org/x/crypto/acme/autocert"
	"golang.org/x/oauth2"
	"golang.org/x/sync/errgroup"
	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/credentials/insecure"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/peer"
	"google.golang.org/grpc/reflection"
	"google.golang.org/grpc/status"
	"gorm.io/gorm"
	"inet.af/netaddr"
	"tailscale.com/tailcfg"
	"tailscale.com/types/dnstype"
	"tailscale.com/types/key"
)

const (
	errSTUNAddressNotSet                   = Error("STUN address not set")
	errUnsupportedDatabase                 = Error("unsupported DB")
	errUnsupportedLetsEncryptChallengeType = Error(
		"unknown value for Lets Encrypt challenge type",
	)
)

const (
	AuthPrefix         = "Bearer "
	Postgres           = "postgres"
	Sqlite             = "sqlite3"
	updateInterval     = 5000
	HTTPReadTimeout    = 30 * time.Second
	privateKeyFileMode = 0o600

	registerCacheExpiration = time.Minute * 15
	registerCacheCleanup    = time.Minute * 20

	DisabledClientAuth = "disabled"
	RelaxedClientAuth  = "relaxed"
	EnforcedClientAuth = "enforced"
)

// Config contains the initial Headscale configuration.
type Config struct {
	ServerURL                      string
	Addr                           string
	MetricsAddr                    string
	GRPCAddr                       string
	GRPCAllowInsecure              bool
	EphemeralNodeInactivityTimeout time.Duration
	IPPrefixes                     []netaddr.IPPrefix
	PrivateKeyPath                 string
	BaseDomain                     string

	DERP DERPConfig

	DBtype string
	DBpath string
	DBhost string
	DBport int
	DBname string
	DBuser string
	DBpass string

	TLSLetsEncryptListen        string
	TLSLetsEncryptHostname      string
	TLSLetsEncryptCacheDir      string
	TLSLetsEncryptChallengeType string

	TLSCertPath       string
	TLSKeyPath        string
	TLSClientAuthMode tls.ClientAuthType

	ACMEURL   string
	ACMEEmail string

	DNSConfig *tailcfg.DNSConfig

	UnixSocket           string
	UnixSocketPermission fs.FileMode

	OIDC OIDCConfig

	CLI CLIConfig
}

type OIDCConfig struct {
	Issuer           string
	ClientID         string
	ClientSecret     string
	StripEmaildomain bool
}

type DERPConfig struct {
	ServerEnabled    bool
	ServerRegionID   int
	ServerRegionCode string
	ServerRegionName string
	STUNAddr         string
	URLs             []url.URL
	Paths            []string
	AutoUpdate       bool
	UpdateFrequency  time.Duration
}

type CLIConfig struct {
	Address  string
	APIKey   string
	Timeout  time.Duration
	Insecure bool
}

// Headscale represents the base app of the service.
type Headscale struct {
	cfg        Config
	db         *gorm.DB
	dbString   string
	dbType     string
	dbDebug    bool
	privateKey *key.MachinePrivate

	DERPMap    *tailcfg.DERPMap
	DERPServer *DERPServer

	aclPolicy *ACLPolicy
	aclRules  []tailcfg.FilterRule

	lastStateChange sync.Map

	oidcProvider *oidc.Provider
	oauth2Config *oauth2.Config

	registrationCache *cache.Cache

	ipAllocationMutex sync.Mutex
}

// Look up the TLS constant relative to user-supplied TLS client
// authentication mode. If an unknown mode is supplied, the default
// value, tls.RequireAnyClientCert, is returned. The returned boolean
// indicates if the supplied mode was valid.
func LookupTLSClientAuthMode(mode string) (tls.ClientAuthType, bool) {
	switch mode {
	case DisabledClientAuth:
		// Client cert is _not_ required.
		return tls.NoClientCert, true
	case RelaxedClientAuth:
		// Client cert required, but _not verified_.
		return tls.RequireAnyClientCert, true
	case EnforcedClientAuth:
		// Client cert is _required and verified_.
		return tls.RequireAndVerifyClientCert, true
	default:
		// Return the default when an unknown value is supplied.
		return tls.RequireAnyClientCert, false
	}
}

func NewHeadscale(cfg Config) (*Headscale, error) {
	privKey, err := readOrCreatePrivateKey(cfg.PrivateKeyPath)
	if err != nil {
		return nil, fmt.Errorf("failed to read or create private key: %w", err)
	}

	var dbString string
	switch cfg.DBtype {
	case Postgres:
		dbString = fmt.Sprintf(
			"host=%s port=%d dbname=%s user=%s password=%s sslmode=disable",
			cfg.DBhost,
			cfg.DBport,
			cfg.DBname,
			cfg.DBuser,
			cfg.DBpass,
		)
	case Sqlite:
		dbString = cfg.DBpath
	default:
		return nil, errUnsupportedDatabase
	}

	registrationCache := cache.New(
		registerCacheExpiration,
		registerCacheCleanup,
	)

	app := Headscale{
		cfg:               cfg,
		dbType:            cfg.DBtype,
		dbString:          dbString,
		privateKey:        privKey,
		aclRules:          tailcfg.FilterAllowAll, // default allowall
		registrationCache: registrationCache,
	}

	err = app.initDB()
	if err != nil {
		return nil, err
	}

	if cfg.OIDC.Issuer != "" {
		err = app.initOIDC()
		if err != nil {
			return nil, err
		}
	}

	if app.cfg.DNSConfig != nil && app.cfg.DNSConfig.Proxied { // if MagicDNS
		magicDNSDomains := generateMagicDNSRootDomains(app.cfg.IPPrefixes)
		// we might have routes already from Split DNS
		if app.cfg.DNSConfig.Routes == nil {
			app.cfg.DNSConfig.Routes = make(map[string][]dnstype.Resolver)
		}
		for _, d := range magicDNSDomains {
			app.cfg.DNSConfig.Routes[d.WithoutTrailingDot()] = nil
		}
	}

	if cfg.DERP.ServerEnabled {
		embeddedDERPServer, err := app.NewDERPServer()
		if err != nil {
			return nil, err
		}
		app.DERPServer = embeddedDERPServer
	}

	return &app, nil
}

// Redirect to our TLS url.
func (h *Headscale) redirect(w http.ResponseWriter, req *http.Request) {
	target := h.cfg.ServerURL + req.URL.RequestURI()
	http.Redirect(w, req, target, http.StatusFound)
}

// expireEphemeralNodes deletes ephemeral machine records that have not been
// seen for longer than h.cfg.EphemeralNodeInactivityTimeout.
func (h *Headscale) expireEphemeralNodes(milliSeconds int64) {
	ticker := time.NewTicker(time.Duration(milliSeconds) * time.Millisecond)
	for range ticker.C {
		h.expireEphemeralNodesWorker()
	}
}

func (h *Headscale) expireEphemeralNodesWorker() {
	namespaces, err := h.ListNamespaces()
	if err != nil {
		log.Error().Err(err).Msg("Error listing namespaces")

		return
	}

	for _, namespace := range namespaces {
		machines, err := h.ListMachinesInNamespace(namespace.Name)
		if err != nil {
			log.Error().
				Err(err).
				Str("namespace", namespace.Name).
				Msg("Error listing machines in namespace")

			return
		}

		for _, machine := range machines {
			if machine.AuthKey != nil && machine.LastSeen != nil &&
				machine.AuthKey.Ephemeral &&
				time.Now().
					After(machine.LastSeen.Add(h.cfg.EphemeralNodeInactivityTimeout)) {
				log.Info().
					Str("machine", machine.Hostname).
					Msg("Ephemeral client removed from database")

				err = h.db.Unscoped().Delete(machine).Error
				if err != nil {
					log.Error().
						Err(err).
						Str("machine", machine.Hostname).
						Msg("🤮 Cannot delete ephemeral machine from the database")
				}
			}
		}

		h.setLastStateChangeToNow(namespace.Name)
	}
}

func (h *Headscale) grpcAuthenticationInterceptor(ctx context.Context,
	req interface{},
	info *grpc.UnaryServerInfo,
	handler grpc.UnaryHandler) (interface{}, error) {
	// Check if the request is coming from the on-server client.
	// This is not secure, but it is to maintain maintainability
	// with the "legacy" database-based client
	// It is also neede for grpc-gateway to be able to connect to
	// the server
	client, _ := peer.FromContext(ctx)

	log.Trace().
		Caller().
		Str("client_address", client.Addr.String()).
		Msg("Client is trying to authenticate")

	meta, ok := metadata.FromIncomingContext(ctx)
	if !ok {
		log.Error().
			Caller().
			Str("client_address", client.Addr.String()).
			Msg("Retrieving metadata is failed")

		return ctx, status.Errorf(
			codes.InvalidArgument,
			"Retrieving metadata is failed",
		)
	}

	authHeader, ok := meta["authorization"]
	if !ok {
		log.Error().
			Caller().
			Str("client_address", client.Addr.String()).
			Msg("Authorization token is not supplied")

		return ctx, status.Errorf(
			codes.Unauthenticated,
			"Authorization token is not supplied",
		)
	}

	token := authHeader[0]

	if !strings.HasPrefix(token, AuthPrefix) {
		log.Error().
			Caller().
			Str("client_address", client.Addr.String()).
			Msg(`missing "Bearer " prefix in "Authorization" header`)

		return ctx, status.Error(
			codes.Unauthenticated,
			`missing "Bearer " prefix in "Authorization" header`,
		)
	}

	valid, err := h.ValidateAPIKey(strings.TrimPrefix(token, AuthPrefix))
	if err != nil {
		log.Error().
			Caller().
			Err(err).
			Str("client_address", client.Addr.String()).
			Msg("failed to validate token")

		return ctx, status.Error(codes.Internal, "failed to validate token")
	}

	if !valid {
		log.Info().
			Str("client_address", client.Addr.String()).
			Msg("invalid token")

		return ctx, status.Error(codes.Unauthenticated, "invalid token")
	}

	return handler(ctx, req)
}

func (h *Headscale) httpAuthenticationMiddleware(ctx *gin.Context) {
	log.Trace().
		Caller().
		Str("client_address", ctx.ClientIP()).
		Msg("HTTP authentication invoked")

	authHeader := ctx.GetHeader("authorization")

	if !strings.HasPrefix(authHeader, AuthPrefix) {
		log.Error().
			Caller().
			Str("client_address", ctx.ClientIP()).
			Msg(`missing "Bearer " prefix in "Authorization" header`)
		ctx.AbortWithStatus(http.StatusUnauthorized)

		return
	}

	valid, err := h.ValidateAPIKey(strings.TrimPrefix(authHeader, AuthPrefix))
	if err != nil {
		log.Error().
			Caller().
			Err(err).
			Str("client_address", ctx.ClientIP()).
			Msg("failed to validate token")

		ctx.AbortWithStatus(http.StatusInternalServerError)

		return
	}

	if !valid {
		log.Info().
			Str("client_address", ctx.ClientIP()).
			Msg("invalid token")

		ctx.AbortWithStatus(http.StatusUnauthorized)

		return
	}

	ctx.Next()
}

// ensureUnixSocketIsAbsent will check if the given path for headscales unix socket is clear
// and will remove it if it is not.
func (h *Headscale) ensureUnixSocketIsAbsent() error {
	// File does not exist, all fine
	if _, err := os.Stat(h.cfg.UnixSocket); errors.Is(err, os.ErrNotExist) {
		return nil
	}

	return os.Remove(h.cfg.UnixSocket)
}

func (h *Headscale) createPrometheusRouter() *gin.Engine {
	promRouter := gin.Default()

	prometheus := ginprometheus.NewPrometheus("gin")
	prometheus.Use(promRouter)

	return promRouter
}

func (h *Headscale) createRouter(grpcMux *runtime.ServeMux) *gin.Engine {
	router := gin.Default()

	router.GET(
		"/health",
		func(c *gin.Context) { c.JSON(http.StatusOK, gin.H{"healthy": "ok"}) },
	)
	router.GET("/key", h.KeyHandler)
	router.GET("/register", h.RegisterWebAPI)
	router.POST("/machine/:id/map", h.PollNetMapHandler)
	router.POST("/machine/:id", h.RegistrationHandler)
	router.GET("/oidc/register/:mkey", h.RegisterOIDC)
	router.GET("/oidc/callback", h.OIDCCallback)
	router.GET("/apple", h.AppleConfigMessage)
	router.GET("/apple/:platform", h.ApplePlatformConfig)
	router.GET("/windows", h.WindowsConfigMessage)
	router.GET("/windows/tailscale.reg", h.WindowsRegConfig)
	router.GET("/swagger", SwaggerUI)
	router.GET("/swagger/v1/openapiv2.json", SwaggerAPIv1)

	if h.cfg.DERP.ServerEnabled {
		router.Any("/derp", h.DERPHandler)
		router.Any("/derp/probe", h.DERPProbeHandler)
		router.Any("/bootstrap-dns", h.DERPBootstrapDNSHandler)
	}

	api := router.Group("/api")
	api.Use(h.httpAuthenticationMiddleware)
	{
		api.Any("/v1/*any", gin.WrapF(grpcMux.ServeHTTP))
	}

	router.NoRoute(stdoutHandler)

	return router
}

// Serve launches a GIN server with the Headscale API.
func (h *Headscale) Serve() error {
	var err error

	// Fetch an initial DERP Map before we start serving
	h.DERPMap = GetDERPMap(h.cfg.DERP)

	if h.cfg.DERP.ServerEnabled {
		// When embedded DERP is enabled we always need a STUN server
		if h.cfg.DERP.STUNAddr == "" {
			return errSTUNAddressNotSet
		}

		h.DERPMap.Regions[h.DERPServer.region.RegionID] = &h.DERPServer.region
		go h.ServeSTUN()
	}

	if h.cfg.DERP.AutoUpdate {
		derpMapCancelChannel := make(chan struct{})
		defer func() { derpMapCancelChannel <- struct{}{} }()
		go h.scheduledDERPMapUpdateWorker(derpMapCancelChannel)
	}

	go h.expireEphemeralNodes(updateInterval)

	if zl.GlobalLevel() == zl.TraceLevel {
		zerolog.RespLog = true
	} else {
		zerolog.RespLog = false
	}

	// Prepare group for running listeners
	errorGroup := new(errgroup.Group)

	ctx := context.Background()
	ctx, cancel := context.WithCancel(ctx)
	defer cancel()

	//
	//
	// Set up LOCAL listeners
	//

	err = h.ensureUnixSocketIsAbsent()
	if err != nil {
		return fmt.Errorf("unable to remove old socket file: %w", err)
	}

	socketListener, err := net.Listen("unix", h.cfg.UnixSocket)
	if err != nil {
		return fmt.Errorf("failed to set up gRPC socket: %w", err)
	}

	// Change socket permissions
	if err := os.Chmod(h.cfg.UnixSocket, h.cfg.UnixSocketPermission); err != nil {
		return fmt.Errorf("failed change permission of gRPC socket: %w", err)
	}

	// Handle common process-killing signals so we can gracefully shut down:
	sigc := make(chan os.Signal, 1)
	signal.Notify(sigc, os.Interrupt, syscall.SIGTERM)
	go func(c chan os.Signal) {
		// Wait for a SIGINT or SIGKILL:
		sig := <-c
		log.Printf("Caught signal %s: shutting down.", sig)
		// Stop listening (and unlink the socket if unix type):
		socketListener.Close()
		// And we're done:
		os.Exit(0)
	}(sigc)

	grpcGatewayMux := runtime.NewServeMux()

	// Make the grpc-gateway connect to grpc over socket
	grpcGatewayConn, err := grpc.Dial(
		h.cfg.UnixSocket,
		[]grpc.DialOption{
			grpc.WithTransportCredentials(insecure.NewCredentials()),
			grpc.WithContextDialer(GrpcSocketDialer),
		}...,
	)
	if err != nil {
		return err
	}

	// Connect to the gRPC server over localhost to skip
	// the authentication.
	err = v1.RegisterHeadscaleServiceHandler(ctx, grpcGatewayMux, grpcGatewayConn)
	if err != nil {
		return err
	}

	// Start the local gRPC server without TLS and without authentication
	grpcSocket := grpc.NewServer(zerolog.UnaryInterceptor())

	v1.RegisterHeadscaleServiceServer(grpcSocket, newHeadscaleV1APIServer(h))
	reflection.Register(grpcSocket)

	errorGroup.Go(func() error { return grpcSocket.Serve(socketListener) })

	//
	//
	// Set up REMOTE listeners
	//

	tlsConfig, err := h.getTLSSettings()
	if err != nil {
		log.Error().Err(err).Msg("Failed to set up TLS configuration")

		return err
	}

	//
	//
	// gRPC setup
	//

	// We are sadly not able to run gRPC and HTTPS (2.0) on the same
	// port because the connection mux does not support matching them
	// since they are so similar. There is multiple issues open and we
	// can revisit this if changes:
	// https://github.com/soheilhy/cmux/issues/68
	// https://github.com/soheilhy/cmux/issues/91

	if tlsConfig != nil || h.cfg.GRPCAllowInsecure {
		log.Info().Msgf("Enabling remote gRPC at %s", h.cfg.GRPCAddr)

		grpcOptions := []grpc.ServerOption{
			grpc.UnaryInterceptor(
				grpc_middleware.ChainUnaryServer(
					h.grpcAuthenticationInterceptor,
					zerolog.NewUnaryServerInterceptor(),
				),
			),
		}

		if tlsConfig != nil {
			grpcOptions = append(grpcOptions,
				grpc.Creds(credentials.NewTLS(tlsConfig)),
			)
		} else {
			log.Warn().Msg("gRPC is running without security")
		}

		grpcServer := grpc.NewServer(grpcOptions...)

		v1.RegisterHeadscaleServiceServer(grpcServer, newHeadscaleV1APIServer(h))
		reflection.Register(grpcServer)

		grpcListener, err := net.Listen("tcp", h.cfg.GRPCAddr)
		if err != nil {
			return fmt.Errorf("failed to bind to TCP address: %w", err)
		}

		errorGroup.Go(func() error { return grpcServer.Serve(grpcListener) })

		log.Info().
			Msgf("listening and serving gRPC on: %s", h.cfg.GRPCAddr)
	}

	//
	//
	// HTTP setup
	//

	router := h.createRouter(grpcGatewayMux)

	httpServer := &http.Server{
		Addr:        h.cfg.Addr,
		Handler:     router,
		ReadTimeout: HTTPReadTimeout,
		// Go does not handle timeouts in HTTP very well, and there is
		// no good way to handle streaming timeouts, therefore we need to
		// keep this at unlimited and be careful to clean up connections
		// https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts/#aboutstreaming
		WriteTimeout: 0,
	}

	var httpListener net.Listener
	if tlsConfig != nil {
		httpServer.TLSConfig = tlsConfig
		httpListener, err = tls.Listen("tcp", h.cfg.Addr, tlsConfig)
	} else {
		httpListener, err = net.Listen("tcp", h.cfg.Addr)
	}
	if err != nil {
		return fmt.Errorf("failed to bind to TCP address: %w", err)
	}

	errorGroup.Go(func() error { return httpServer.Serve(httpListener) })

	log.Info().
		Msgf("listening and serving HTTP on: %s", h.cfg.Addr)

	promRouter := h.createPrometheusRouter()

	promHTTPServer := &http.Server{
		Addr:         h.cfg.MetricsAddr,
		Handler:      promRouter,
		ReadTimeout:  HTTPReadTimeout,
		WriteTimeout: 0,
	}

	var promHTTPListener net.Listener
	promHTTPListener, err = net.Listen("tcp", h.cfg.MetricsAddr)

	if err != nil {
		return fmt.Errorf("failed to bind to TCP address: %w", err)
	}

	errorGroup.Go(func() error { return promHTTPServer.Serve(promHTTPListener) })

	log.Info().
		Msgf("listening and serving metrics on: %s", h.cfg.MetricsAddr)

	return errorGroup.Wait()
}

func (h *Headscale) getTLSSettings() (*tls.Config, error) {
	var err error
	if h.cfg.TLSLetsEncryptHostname != "" {
		if !strings.HasPrefix(h.cfg.ServerURL, "https://") {
			log.Warn().
				Msg("Listening with TLS but ServerURL does not start with https://")
		}

		certManager := autocert.Manager{
			Prompt:     autocert.AcceptTOS,
			HostPolicy: autocert.HostWhitelist(h.cfg.TLSLetsEncryptHostname),
			Cache:      autocert.DirCache(h.cfg.TLSLetsEncryptCacheDir),
			Client: &acme.Client{
				DirectoryURL: h.cfg.ACMEURL,
			},
			Email: h.cfg.ACMEEmail,
		}

		switch h.cfg.TLSLetsEncryptChallengeType {
		case "TLS-ALPN-01":
			// Configuration via autocert with TLS-ALPN-01 (https://tools.ietf.org/html/rfc8737)
			// The RFC requires that the validation is done on port 443; in other words, headscale
			// must be reachable on port 443.
			return certManager.TLSConfig(), nil

		case "HTTP-01":
			// Configuration via autocert with HTTP-01. This requires listening on
			// port 80 for the certificate validation in addition to the headscale
			// service, which can be configured to run on any other port.
			go func() {
				log.Fatal().
					Caller().
					Err(http.ListenAndServe(h.cfg.TLSLetsEncryptListen, certManager.HTTPHandler(http.HandlerFunc(h.redirect)))).
					Msg("failed to set up a HTTP server")
			}()

			return certManager.TLSConfig(), nil

		default:
			return nil, errUnsupportedLetsEncryptChallengeType
		}
	} else if h.cfg.TLSCertPath == "" {
		if !strings.HasPrefix(h.cfg.ServerURL, "http://") {
			log.Warn().Msg("Listening without TLS but ServerURL does not start with http://")
		}

		return nil, err
	} else {
		if !strings.HasPrefix(h.cfg.ServerURL, "https://") {
			log.Warn().Msg("Listening with TLS but ServerURL does not start with https://")
		}

		log.Info().Msg(fmt.Sprintf(
			"Client authentication (mTLS) is \"%s\". See the docs to learn about configuring this setting.",
			h.cfg.TLSClientAuthMode))

		tlsConfig := &tls.Config{
			ClientAuth:   h.cfg.TLSClientAuthMode,
			NextProtos:   []string{"http/1.1"},
			Certificates: make([]tls.Certificate, 1),
			MinVersion:   tls.VersionTLS12,
		}

		tlsConfig.Certificates[0], err = tls.LoadX509KeyPair(h.cfg.TLSCertPath, h.cfg.TLSKeyPath)

		return tlsConfig, err
	}
}

func (h *Headscale) setLastStateChangeToNow(namespace string) {
	now := time.Now().UTC()
	lastStateUpdate.WithLabelValues("", "headscale").Set(float64(now.Unix()))
	h.lastStateChange.Store(namespace, now)
}

func (h *Headscale) getLastStateChange(namespaces ...string) time.Time {
	times := []time.Time{}

	for _, namespace := range namespaces {
		if wrapped, ok := h.lastStateChange.Load(namespace); ok {
			lastChange, _ := wrapped.(time.Time)

			times = append(times, lastChange)
		}
	}

	sort.Slice(times, func(i, j int) bool {
		return times[i].After(times[j])
	})

	log.Trace().Msgf("Latest times %#v", times)

	if len(times) == 0 {
		return time.Now().UTC()
	} else {
		return times[0]
	}
}

func stdoutHandler(ctx *gin.Context) {
	body, _ := io.ReadAll(ctx.Request.Body)

	log.Trace().
		Interface("header", ctx.Request.Header).
		Interface("proto", ctx.Request.Proto).
		Interface("url", ctx.Request.URL).
		Bytes("body", body).
		Msg("Request did not match")
}

func readOrCreatePrivateKey(path string) (*key.MachinePrivate, error) {
	privateKey, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		log.Info().Str("path", path).Msg("No private key file at path, creating...")

		machineKey := key.NewMachine()

		machineKeyStr, err := machineKey.MarshalText()
		if err != nil {
			return nil, fmt.Errorf(
				"failed to convert private key to string for saving: %w",
				err,
			)
		}
		err = os.WriteFile(path, machineKeyStr, privateKeyFileMode)
		if err != nil {
			return nil, fmt.Errorf(
				"failed to save private key to disk: %w",
				err,
			)
		}

		return &machineKey, nil
	} else if err != nil {
		return nil, fmt.Errorf("failed to read private key file: %w", err)
	}

	trimmedPrivateKey := strings.TrimSpace(string(privateKey))
	privateKeyEnsurePrefix := PrivateKeyEnsurePrefix(trimmedPrivateKey)

	var machineKey key.MachinePrivate
	if err = machineKey.UnmarshalText([]byte(privateKeyEnsurePrefix)); err != nil {
		log.Info().
			Str("path", path).
			Msg("This might be due to a legacy (headscale pre-0.12) private key. " +
				"If the key is in WireGuard format, delete the key and restart headscale. " +
				"A new key will automatically be generated. All Tailscale clients will have to be restarted")

		return nil, fmt.Errorf("failed to parse private key: %w", err)
	}

	return &machineKey, nil
}