Allow handling RAs when forwarding is enabled

...to conform with Linux's `accept_ra` sysctl option.

```
  accept_ra - INTEGER
    Accept Router Advertisements; autoconfigure using them.

    It also determines whether or not to transmit Router
    Solicitations. If and only if the functional setting is to
    accept Router Advertisements, Router Solicitations will be
    transmitted.

    Possible values are:
      0 Do not accept Router Advertisements.
      1 Accept Router Advertisements if forwarding is disabled.
      2 Overrule forwarding behaviour. Accept Router Advertisements
        even if forwarding is enabled.

    Functional default: enabled if local forwarding is disabled.
                        disabled if local forwarding is enabled.
```

PiperOrigin-RevId: 372214640

pkg/tcpip/network/ipv6/ndp.go

@@ -48,7 +48,7 @@ const (
 
 	// defaultHandleRAs is the default configuration for whether or not to
 	// handle incoming Router Advertisements as a host.
-	defaultHandleRAs = true
+	defaultHandleRAs = HandlingRAsEnabledWhenForwardingDisabled
 
 	// defaultDiscoverDefaultRouters is the default configuration for
 	// whether or not to discover default routers from incoming Router
@@ -301,6 +301,39 @@ type NDPDispatcher interface {
 	OnDHCPv6Configuration(tcpip.NICID, DHCPv6ConfigurationFromNDPRA)
 }
 
+var _ fmt.Stringer = HandleRAsConfiguration(0)
+
+// HandleRAsConfiguration enumerates when RAs may be handled.
+type HandleRAsConfiguration int
+
+const (
+	// HandlingRAsDisabled indicates that Router Advertisements will not be
+	// handled.
+	HandlingRAsDisabled HandleRAsConfiguration = iota
+
+	// HandlingRAsEnabledWhenForwardingDisabled indicates that router
+	// advertisements will only be handled when forwarding is disabled.
+	HandlingRAsEnabledWhenForwardingDisabled
+
+	// HandlingRAsAlwaysEnabled indicates that Router Advertisements will always
+	// be handled, even when forwarding is enabled.
+	HandlingRAsAlwaysEnabled
+)
+
+// String implements fmt.Stringer.
+func (c HandleRAsConfiguration) String() string {
+	switch c {
+	case HandlingRAsDisabled:
+		return "HandlingRAsDisabled"
+	case HandlingRAsEnabledWhenForwardingDisabled:
+		return "HandlingRAsEnabledWhenForwardingDisabled"
+	case HandlingRAsAlwaysEnabled:
+		return "HandlingRAsAlwaysEnabled"
+	default:
+		return fmt.Sprintf("HandleRAsConfiguration(%d)", c)
+	}
+}
+
 // NDPConfigurations is the NDP configurations for the netstack.
 type NDPConfigurations struct {
 	// The number of Router Solicitation messages to send when the IPv6 endpoint
@@ -318,8 +351,9 @@ type NDPConfigurations struct {
 	// Must be greater than or equal to 0s.
 	MaxRtrSolicitationDelay time.Duration
 
-	// HandleRAs determines whether or not Router Advertisements are processed.
-	HandleRAs bool
+	// HandleRAs is the configuration for when Router Advertisements should be
+	// handled.
+	HandleRAs HandleRAsConfiguration
 
 	// DiscoverDefaultRouters determines whether or not default routers are
 	// discovered from Router Advertisements, as per RFC 4861 section 6. This
@@ -654,8 +688,18 @@ func (ndp *ndpState) handleRA(ip tcpip.Address, ra header.NDPRouterAdvert) {
 	// per-interface basis; it is a protocol-wide configuration, so we check the
 	// protocol's forwarding flag to determine if the IPv6 endpoint is forwarding
 	// packets.
-	if !ndp.configs.HandleRAs || ndp.ep.protocol.Forwarding() {
+	switch ndp.configs.HandleRAs {
+	case HandlingRAsDisabled:
+		ndp.ep.stats.localStats.UnhandledRouterAdvertisements.Increment()
 		return
+	case HandlingRAsEnabledWhenForwardingDisabled:
+		if ndp.ep.protocol.Forwarding() {
+			ndp.ep.stats.localStats.UnhandledRouterAdvertisements.Increment()
+			return
+		}
+	case HandlingRAsAlwaysEnabled:
+	default:
+		panic(fmt.Sprintf("unhandled HandleRAs configuration = %d", ndp.configs.HandleRAs))
 	}
 
 	// Only worry about the DHCPv6 configuration if we have an NDPDispatcher as we

pkg/tcpip/network/ipv6/stats.go

@@ -29,6 +29,10 @@ type Stats struct {
 
 	// ICMP holds ICMPv6 statistics.
 	ICMP tcpip.ICMPv6Stats
+
+	// UnhandledRouterAdvertisements is the number of Router Advertisements that
+	// were observed but not handled.
+	UnhandledRouterAdvertisements *tcpip.StatCounter
 }
 
 // IsNetworkEndpointStats implements stack.NetworkEndpointStats.

pkg/tcpip/stack/stack_test.go

@@ -3022,7 +3022,7 @@ func TestIPv6SourceAddressSelectionScopeAndSameAddress(t *testing.T) {
 			s := stack.New(stack.Options{
 				NetworkProtocols: []stack.NetworkProtocolFactory{ipv6.NewProtocolWithOptions(ipv6.Options{
 					NDPConfigs: ipv6.NDPConfigurations{
-						HandleRAs:                  true,
+						HandleRAs:                  ipv6.HandlingRAsEnabledWhenForwardingDisabled,
 						AutoGenGlobalAddresses:     true,
 						AutoGenTempGlobalAddresses: true,
 					},