cmd/tailscale: fix "up" warning about netfilter-mode on Synology

Fixes #6811

Change-Id: Ia43723e6ebedc9b01729897cec271c462b16e9ae
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/up.go

@@ -330,7 +330,11 @@ func prefsFromUpArgs(upArgs upArgsT, warnf logger.Logf, st *ipnstate.Status, goo
 	prefs.ControlURL = upArgs.server
 	prefs.WantRunning = true
 	prefs.RouteAll = upArgs.acceptRoutes
-
+	if distro.Get() == distro.Synology {
+		// ipn.NewPrefs returns a non-zero Netfilter default. But Synology only
+		// supports "off" mode.
+		prefs.NetfilterMode = preftype.NetfilterOff
+	}
 	if upArgs.exitNodeIP != "" {
 		if err := prefs.SetExitNodeIP(upArgs.exitNodeIP, st); err != nil {
 			var e ipn.ExitNodeLocalIPError
@@ -879,6 +883,10 @@ func checkForAccidentalSettingReverts(newPrefs, curPrefs *ipn.Prefs, env upCheck
 			// Issue 3176. Old prefs had 'RouteAll: true' on disk, so ignore that.
 			continue
 		}
+		if flagName == "netfilter-mode" && valNew == preftype.NetfilterOn && env.goos == "linux" && env.distro == distro.Synology {
+			// Issue 6811. Ignore on Synology.
+			continue
+		}
 		missing = append(missing, fmtFlagValueArg(flagName, valCur))
 	}
 	if len(missing) == 0 {