wgengine: only use AmbientCaps on DSM7+

Signed-off-by: Maisem Ali <maisem@tailscale.com>
(cherry picked from commit 27799a1a96)

wgengine/router/router_linux.go

@@ -156,7 +156,7 @@ func newUserspaceRouter(logf logger.Logf, tunDev tun.Device, linkMon *monitor.Mo
 	}
 
 	cmd := osCommandRunner{
-		ambientCapNetAdmin: distro.Get() == distro.Synology,
+		ambientCapNetAdmin: useAmbientCaps(),
 	}
 
 	return newUserspaceRouterAdvanced(logf, tunname, linkMon, ipt4, ipt6, cmd, supportsV6, supportsV6NAT)
@@ -185,6 +185,17 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, linkMon *monit
 	return r, nil
 }
 
+func useAmbientCaps() bool {
+	if distro.Get() != distro.Synology {
+		return false
+	}
+	v, err := strconv.Atoi(os.Getenv("SYNOPKG_DSM_VERSION_MAJOR"))
+	if err != nil {
+		return false
+	}
+	return v >= 7
+}
+
 // onIPRuleDeleted is the callback from the link monitor for when an IP policy
 // rule is deleted. See Issue 1591.
 //