Add loopback ignore rule to nat chains (#2190)

This makes sure loopback traffic is not affected by NAT

client/firewall/iptables/router_linux.go

@@ -74,12 +74,12 @@ func (i *routerManager) InsertRoutingRules(pair firewall.RouterPair) error {
 		return nil
 	}
 
-	err = i.insertRoutingRule(firewall.NatFormat, tableNat, chainRTNAT, routingFinalNatJump, pair)
+	err = i.addNATRule(firewall.NatFormat, tableNat, chainRTNAT, routingFinalNatJump, pair)
 	if err != nil {
 		return err
 	}
 
-	err = i.insertRoutingRule(firewall.InNatFormat, tableNat, chainRTNAT, routingFinalNatJump, firewall.GetInPair(pair))
+	err = i.addNATRule(firewall.InNatFormat, tableNat, chainRTNAT, routingFinalNatJump, firewall.GetInPair(pair))
 	if err != nil {
 		return err
 	}
@@ -101,6 +101,7 @@ func (i *routerManager) insertRoutingRule(keyFormat, table, chain, jump string,
 		}
 		delete(i.rules, ruleKey)
 	}
+
 	err = i.iptablesClient.Insert(table, chain, 1, rule...)
 	if err != nil {
 		return fmt.Errorf("error while adding new %s rule for %s: %v", getIptablesRuleType(table), pair.Destination, err)
@@ -317,6 +318,13 @@ func (i *routerManager) createChain(table, newChain string) error {
 			return fmt.Errorf("couldn't create chain %s in %s table, error: %v", newChain, table, err)
 		}
 
+		// Add the loopback return rule to the NAT chain
+		loopbackRule := []string{"-o", "lo", "-j", "RETURN"}
+		err = i.iptablesClient.Insert(table, newChain, 1, loopbackRule...)
+		if err != nil {
+			return fmt.Errorf("failed to add loopback return rule to %s: %v", chainRTNAT, err)
+		}
+
 		err = i.iptablesClient.Append(table, newChain, "-j", "RETURN")
 		if err != nil {
 			return fmt.Errorf("couldn't create chain %s default rule, error: %v", newChain, err)
@@ -326,6 +334,30 @@ func (i *routerManager) createChain(table, newChain string) error {
 	return nil
 }
 
+// addNATRule appends an iptables rule pair to the nat chain
+func (i *routerManager) addNATRule(keyFormat, table, chain, jump string, pair firewall.RouterPair) error {
+	ruleKey := firewall.GenKey(keyFormat, pair.ID)
+	rule := genRuleSpec(jump, pair.Source, pair.Destination)
+	existingRule, found := i.rules[ruleKey]
+	if found {
+		err := i.iptablesClient.DeleteIfExists(table, chain, existingRule...)
+		if err != nil {
+			return fmt.Errorf("error while removing existing NAT rule for %s: %v", pair.Destination, err)
+		}
+		delete(i.rules, ruleKey)
+	}
+
+	// inserting after loopback ignore rule
+	err := i.iptablesClient.Insert(table, chain, 2, rule...)
+	if err != nil {
+		return fmt.Errorf("error while appending new NAT rule for %s: %v", pair.Destination, err)
+	}
+
+	i.rules[ruleKey] = rule
+
+	return nil
+}
+
 // genRuleSpec generates rule specification
 func genRuleSpec(jump, source, destination string) []string {
 	return []string{"-s", source, "-d", destination, "-j", jump}

client/firewall/nftables/manager_linux.go

@@ -95,7 +95,7 @@ func (m *Manager) InsertRoutingRules(pair firewall.RouterPair) error {
 	m.mutex.Lock()
 	defer m.mutex.Unlock()
 
-	return m.router.InsertRoutingRules(pair)
+	return m.router.AddRoutingRules(pair)
 }
 
 func (m *Manager) RemoveRoutingRules(pair firewall.RouterPair) error {

client/firewall/nftables/route_linux.go

@@ -22,6 +22,8 @@ const (
 
 	userDataAcceptForwardRuleSrc = "frwacceptsrc"
 	userDataAcceptForwardRuleDst = "frwacceptdst"
+
+	loopbackInterface = "lo\x00"
 )
 
 // some presets for building nftable rules
@@ -126,6 +128,22 @@ func (r *router) createContainers() error {
 		Type:     nftables.ChainTypeNAT,
 	})
 
+	// Add RETURN rule for loopback interface
+	loRule := &nftables.Rule{
+		Table: r.workTable,
+		Chain: r.chains[chainNameRoutingNat],
+		Exprs: []expr.Any{
+			&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
+			&expr.Cmp{
+				Op:       expr.CmpOpEq,
+				Register: 1,
+				Data:     []byte(loopbackInterface),
+			},
+			&expr.Verdict{Kind: expr.VerdictReturn},
+		},
+	}
+	r.conn.InsertRule(loRule)
+
 	err := r.refreshRulesMap()
 	if err != nil {
 		log.Errorf("failed to clean up rules from FORWARD chain: %s", err)
@@ -138,28 +156,28 @@ func (r *router) createContainers() error {
 	return nil
 }
 
-// InsertRoutingRules inserts a nftable rule pair to the forwarding chain and if enabled, to the nat chain
+// AddRoutingRules appends a nftable rule pair to the forwarding chain and if enabled, to the nat chain
-func (r *router) InsertRoutingRules(pair manager.RouterPair) error {
+func (r *router) AddRoutingRules(pair manager.RouterPair) error {
 	err := r.refreshRulesMap()
 	if err != nil {
 		return err
 	}
 
-	err = r.insertRoutingRule(manager.ForwardingFormat, chainNameRouteingFw, pair, false)
+	err = r.addRoutingRule(manager.ForwardingFormat, chainNameRouteingFw, pair, false)
 	if err != nil {
 		return err
 	}
-	err = r.insertRoutingRule(manager.InForwardingFormat, chainNameRouteingFw, manager.GetInPair(pair), false)
+	err = r.addRoutingRule(manager.InForwardingFormat, chainNameRouteingFw, manager.GetInPair(pair), false)
 	if err != nil {
 		return err
 	}
 
 	if pair.Masquerade {
-		err = r.insertRoutingRule(manager.NatFormat, chainNameRoutingNat, pair, true)
+		err = r.addRoutingRule(manager.NatFormat, chainNameRoutingNat, pair, true)
 		if err != nil {
 			return err
 		}
-		err = r.insertRoutingRule(manager.InNatFormat, chainNameRoutingNat, manager.GetInPair(pair), true)
+		err = r.addRoutingRule(manager.InNatFormat, chainNameRoutingNat, manager.GetInPair(pair), true)
 		if err != nil {
 			return err
 		}
@@ -177,8 +195,8 @@ func (r *router) InsertRoutingRules(pair manager.RouterPair) error {
 	return nil
 }
 
-// insertRoutingRule inserts a nftable rule to the conn client flush queue
+// addRoutingRule inserts a nftable rule to the conn client flush queue
-func (r *router) insertRoutingRule(format, chainName string, pair manager.RouterPair, isNat bool) error {
+func (r *router) addRoutingRule(format, chainName string, pair manager.RouterPair, isNat bool) error {
 	sourceExp := generateCIDRMatcherExpressions(true, pair.Source)
 	destExp := generateCIDRMatcherExpressions(false, pair.Destination)
 
@@ -199,7 +217,7 @@ func (r *router) insertRoutingRule(format, chainName string, pair manager.Router
 		}
 	}
 
-	r.rules[ruleKey] = r.conn.InsertRule(&nftables.Rule{
+	r.rules[ruleKey] = r.conn.AddRule(&nftables.Rule{
 		Table:    r.workTable,
 		Chain:    r.chains[chainName],
 		Exprs:    expression,

client/firewall/nftables/router_linux_test.go

@@ -47,7 +47,7 @@ func TestNftablesManager_InsertRoutingRules(t *testing.T) {
 
 			require.NoError(t, err, "shouldn't return error")
 
-			err = manager.InsertRoutingRules(testCase.InputPair)
+			err = manager.AddRoutingRules(testCase.InputPair)
 			defer func() {
 				_ = manager.RemoveRoutingRules(testCase.InputPair)
 			}()