all: detect JetKVM and specialize a handful of things for it

Updates #16524

Change-Id: I183428de8c65d7155d82979d2d33f031c22e3331
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/manager_linux.go

@@ -22,6 +22,7 @@ import (
 	"tailscale.com/types/logger"
 	"tailscale.com/util/clientmetric"
 	"tailscale.com/util/cmpver"
+	"tailscale.com/version/distro"
 )
 
 type kv struct {
@@ -38,6 +39,10 @@ var publishOnce sync.Once
 //
 // The health tracker may be nil; the knobs may be nil and are ignored on this platform.
 func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ *controlknobs.Knobs, interfaceName string) (ret OSConfigurator, err error) {
+	if distro.Get() == distro.JetKVM {
+		return NewNoopManager()
+	}
+
 	env := newOSConfigEnv{
 		fs:                directFS{},
 		dbusPing:          dbusPing,

net/tstun/tun.go

@@ -24,6 +24,9 @@ import (
 // CrateTAP is the hook set by feature/tap.
 var CreateTAP feature.Hook[func(logf logger.Logf, tapName, bridgeName string) (tun.Device, error)]
 
+// modprobeTunHook is a Linux-specific hook to run "/sbin/modprobe tun".
+var modprobeTunHook feature.Hook[func() error]
+
 // New returns a tun.Device for the requested device name, along with
 // the OS-dependent name that was allocated to the device.
 func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
@@ -51,7 +54,22 @@ func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
 		if runtime.GOOS == "plan9" {
 			cleanUpPlan9Interfaces()
 		}
-		dev, err = tun.CreateTUN(tunName, int(DefaultTUNMTU()))
+		// Try to create the TUN device up to two times. If it fails
+		// the first time and we're on Linux, try a desperate
+		// "modprobe tun" to load the tun module and try again.
+		for try := range 2 {
+			dev, err = tun.CreateTUN(tunName, int(DefaultTUNMTU()))
+			if err == nil || !modprobeTunHook.IsSet() {
+				if try > 0 {
+					logf("created TUN device %q after doing `modprobe tun`", tunName)
+				}
+				break
+			}
+			if modprobeTunHook.Get()() != nil {
+				// modprobe failed; no point trying again.
+				break
+			}
+		}
 	}
 	if err != nil {
 		return nil, "", err

net/tstun/tun_linux.go

@@ -17,6 +17,14 @@ import (
 
 func init() {
 	tunDiagnoseFailure = diagnoseLinuxTUNFailure
+	modprobeTunHook.Set(func() error {
+		_, err := modprobeTun()
+		return err
+	})
+}
+
+func modprobeTun() ([]byte, error) {
+	return exec.Command("/sbin/modprobe", "tun").CombinedOutput()
 }
 
 func diagnoseLinuxTUNFailure(tunName string, logf logger.Logf, createErr error) {
@@ -36,7 +44,7 @@ func diagnoseLinuxTUNFailure(tunName string, logf logger.Logf, createErr error)
 	kernel := utsReleaseField(&un)
 	logf("Linux kernel version: %s", kernel)
 
-	modprobeOut, err := exec.Command("/sbin/modprobe", "tun").CombinedOutput()
+	modprobeOut, err := modprobeTun()
 	if err == nil {
 		logf("'modprobe tun' successful")
 		// Either tun is currently loaded, or it's statically