[client] Exclude loopback from NAT (#2747)

client/firewall/iptables/router_linux.go

@@ -433,10 +433,12 @@ func (r *router) removeNatRule(pair firewall.RouterPair) error {
 
 func genRuleSpec(jump string, source, destination netip.Prefix, intf string, inverse bool) []string {
 	intdir := "-i"
+	lointdir := "-o"
 	if inverse {
 		intdir = "-o"
+		lointdir = "-i"
 	}
-	return []string{intdir, intf, "-s", source.String(), "-d", destination.String(), "-j", jump}
+	return []string{intdir, intf, "!", lointdir, "lo", "-s", source.String(), "-d", destination.String(), "-j", jump}
 }
 
 func genRouteFilteringRuleSpec(params routeFilteringRuleParams) []string {

client/firewall/nftables/router_linux.go

@@ -425,11 +425,15 @@ func (r *router) addNatRule(pair firewall.RouterPair) error {
 	destExp := generateCIDRMatcherExpressions(false, pair.Destination)
 
 	dir := expr.MetaKeyIIFNAME
+	notDir := expr.MetaKeyOIFNAME
 	if pair.Inverse {
 		dir = expr.MetaKeyOIFNAME
+		notDir = expr.MetaKeyIIFNAME
 	}
 
+	lo := ifname("lo")
 	intf := ifname(r.wgIface.Name())
+
 	exprs := []expr.Any{
 		&expr.Meta{
 			Key:      dir,
@@ -440,6 +444,17 @@ func (r *router) addNatRule(pair firewall.RouterPair) error {
 			Register: 1,
 			Data:     intf,
 		},
+
+		// We need to exclude the loopback interface as this changes the ebpf proxy port
+		&expr.Meta{
+			Key:      notDir,
+			Register: 1,
+		},
+		&expr.Cmp{
+			Op:       expr.CmpOpNeq,
+			Register: 1,
+			Data:     lo,
+		},
 	}
 
 	exprs = append(exprs, sourceExp...)

client/firewall/nftables/router_linux_test.go

@@ -69,6 +69,12 @@ func TestNftablesManager_AddNatRule(t *testing.T) {
 						Register: 1,
 						Data:     ifname(ifaceMock.Name()),
 					},
+					&expr.Meta{Key: expr.MetaKeyOIFNAME, Register: 1},
+					&expr.Cmp{
+						Op:       expr.CmpOpNeq,
+						Register: 1,
+						Data:     ifname("lo"),
+					},
 				)
 
 				natRuleKey := firewall.GenKey(firewall.NatFormat, testCase.InputPair)
@@ -97,6 +103,12 @@ func TestNftablesManager_AddNatRule(t *testing.T) {
 						Register: 1,
 						Data:     ifname(ifaceMock.Name()),
 					},
+					&expr.Meta{Key: expr.MetaKeyIIFNAME, Register: 1},
+					&expr.Cmp{
+						Op:       expr.CmpOpNeq,
+						Register: 1,
+						Data:     ifname("lo"),
+					},
 				)
 
 				inNatRuleKey := firewall.GenKey(firewall.NatFormat, firewall.GetInversePair(testCase.InputPair))