cmd/tailscale/cli, ipn/ipnlocal: give SSH tips when off/unconfigured

Updates #3802

Change-Id: I6b9a3175f68a6daa670f912561f2c2ececc07770
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -404,7 +404,7 @@ func updatePrefs(prefs, curPrefs *ipn.Prefs, env upCheckEnv) (simpleUp bool, jus
 	return simpleUp, justEditMP, nil
 }
 
-func runUp(ctx context.Context, args []string) error {
+func runUp(ctx context.Context, args []string) (retErr error) {
 	if len(args) > 0 {
 		fatalf("too many non-flag arguments: %q", args)
 	}
@@ -481,6 +481,12 @@ func runUp(ctx context.Context, args []string) error {
 		}
 	}
 
+	defer func() {
+		if retErr == nil {
+			checkSSHUpWarnings(ctx)
+		}
+	}()
+
 	simpleUp, justEditMP, err := updatePrefs(prefs, curPrefs, env)
 	if err != nil {
 		fatalf("%s", err)
@@ -676,6 +682,28 @@ func runUp(ctx context.Context, args []string) error {
 	}
 }
 
+func checkSSHUpWarnings(ctx context.Context) {
+	if !upArgs.runSSH {
+		return
+	}
+	st, err := localClient.Status(ctx)
+	if err != nil {
+		// Ignore. Don't spam more.
+		return
+	}
+	if len(st.Health) == 0 {
+		return
+	}
+	if len(st.Health) == 1 && strings.Contains(st.Health[0], "SSH") {
+		printf("%s\n", st.Health[0])
+		return
+	}
+	printf("# Health check:\n")
+	for _, m := range st.Health {
+		printf("    - %s\n", m)
+	}
+}
+
 func printUpDoneJSON(state ipn.State, errorString string) {
 	js := &upOutputJSON{BackendState: state.String(), Error: errorString}
 	data, err := json.MarshalIndent(js, "", "  ")

ipn/ipnlocal/local.go

@@ -416,6 +416,9 @@ func (b *LocalBackend) updateStatus(sb *ipnstate.StatusBuilder, extraLocked func
 				s.Health = append(s.Health, err.Error())
 			}
 		}
+		if m := b.sshOnButUnusableHealthCheckMessageLocked(); m != "" {
+			s.Health = append(s.Health, m)
+		}
 		if b.netMap != nil {
 			s.CertDomains = append([]string(nil), b.netMap.DNS.CertDomains...)
 			s.MagicDNSSuffix = b.netMap.MagicDNSSuffix()
@@ -1840,11 +1843,21 @@ func (b *LocalBackend) CheckPrefs(p *ipn.Prefs) error {
 }
 
 func (b *LocalBackend) checkPrefsLocked(p *ipn.Prefs) error {
+	var errs []error
 	if p.Hostname == "badhostname.tailscale." {
 		// Keep this one just for testing.
-		return errors.New("bad hostname [test]")
+		errs = append(errs, errors.New("bad hostname [test]"))
+	}
+	if err := b.checkSSHPrefsLocked(p); err != nil {
+		errs = append(errs, err)
+	}
+	return multierr.New(errs...)
+}
+
+func (b *LocalBackend) checkSSHPrefsLocked(p *ipn.Prefs) error {
+	if !p.RunSSH {
+		return nil
 	}
-	if p.RunSSH {
 	switch runtime.GOOS {
 	case "linux":
 		if distro.Get() == distro.Synology && !envknob.UseWIPCode() {
@@ -1865,14 +1878,53 @@ func (b *LocalBackend) checkPrefsLocked(p *ipn.Prefs) error {
 	if !canSSH {
 		return errors.New("The Tailscale SSH server has been administratively disabled.")
 	}
-		if b.netMap != nil && b.netMap.SSHPolicy == nil &&
-			envknob.SSHPolicyFile() == "" && !envknob.SSHIgnoreTailnetPolicy() {
-			return errors.New("Unable to enable local Tailscale SSH server; not enabled/configured on Tailnet.")
+	if envknob.SSHIgnoreTailnetPolicy() || envknob.SSHPolicyFile() != "" {
+		return nil
+	}
+	if b.netMap != nil {
+		if !hasCapability(b.netMap, tailcfg.CapabilitySSH) {
+			if b.isDefaultServerLocked() {
+				return errors.New("Unable to enable local Tailscale SSH server; not enabled on Tailnet. See https://tailscale.com/s/ssh")
+			}
+			return errors.New("Unable to enable local Tailscale SSH server; not enabled on Tailnet.")
 		}
 	}
 	return nil
 }
 
+func (b *LocalBackend) sshOnButUnusableHealthCheckMessageLocked() (healthMessage string) {
+	if b.prefs == nil || !b.prefs.RunSSH {
+		return ""
+	}
+	if envknob.SSHIgnoreTailnetPolicy() || envknob.SSHPolicyFile() != "" {
+		return "development SSH policy in use"
+	}
+	nm := b.netMap
+	if nm == nil {
+		return ""
+	}
+	if nm.SSHPolicy != nil && len(nm.SSHPolicy.Rules) > 0 {
+		return ""
+	}
+	isDefault := b.isDefaultServerLocked()
+	isAdmin := hasCapability(nm, tailcfg.CapabilityAdmin)
+
+	if !isAdmin {
+		return "Tailscale SSH enabled, but access controls don't allow anyone to access this device. Ask your admin to update your tailnet's ACLs to allow access."
+	}
+	if !isDefault {
+		return "Tailscale SSH enabled, but access controls don't allow anyone to access this device. Update your tailnet's ACLs to allow access."
+	}
+	return "Tailscale SSH enabled, but access controls don't allow anyone to access this device. Update your tailnet's ACLs at https://tailscale.com/s/ssh-policy"
+}
+
+func (b *LocalBackend) isDefaultServerLocked() bool {
+	if b.prefs == nil {
+		return true // assume true until set otherwise
+	}
+	return b.prefs.ControlURLOrDefault() == ipn.DefaultControlURL
+}
+
 func (b *LocalBackend) EditPrefs(mp *ipn.MaskedPrefs) (*ipn.Prefs, error) {
 	b.mu.Lock()
 	p0 := b.prefs.Clone()