wgengine/router: don't assume Linux was built with IP_MULTIPLE_TABLES

Updates #3351
Updates #391

Change-Id: I7e66b686e05f3c970846513679cc62556ebe322a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/router/router_linux.go

@@ -99,7 +99,7 @@ type linuxRouter struct {
 	ipRuleFixLimiter   *rate.Limiter
 
 	// Various feature checks for the network stack.
-	ipRuleAvailable bool
+	ipRuleAvailable bool // whether kernel was built with IP_MULTIPLE_TABLES
 	v6Available     bool
 	v6NATAvailable  bool
 
@@ -165,9 +165,14 @@ func newUserspaceRouterAdvanced(logf logger.Logf, tunname string, linkMon *monit
 	if r.useIPCommand() {
 		r.ipRuleAvailable = (cmd.run("ip", "rule") == nil)
 	} else {
-		// Pretend it is.
+		if rules, err := netlink.RuleList(netlink.FAMILY_V4); err != nil {
+			r.logf("error querying IP rules (does kernel have IP_MULTIPLE_TABLES?): %v", err)
+			r.logf("warning: running without policy routing")
+		} else {
+			r.logf("policy routing available; found %d rules", len(rules))
 			r.ipRuleAvailable = true
 		}
+	}
 
 	return r, nil
 }