Join all routers group when forwarding is enabled

See comments inline code for rationale.

Test: ip_test.TestJoinLeaveAllRoutersGroup
PiperOrigin-RevId: 367449434

1 files changed, 52 insertions, 5 deletions

diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 1a5661ca4..6dc6fc9bd 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -150,6 +150,38 @@ func (p *protocol) forgetEndpoint(nicID tcpip.NICID) {
 	delete(p.mu.eps, nicID)
 }
 
+// transitionForwarding transitions the endpoint's forwarding status to
+// forwarding.
+//
+// Must only be called when the forwarding status changes.
+func (e *endpoint) transitionForwarding(forwarding bool) {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if forwarding {
+		// There does not seem to be an RFC requirement for a node to join the all
+		// routers multicast address but
+		// https://www.iana.org/assignments/multicast-addresses/multicast-addresses.xhtml
+		// specifies the address as a group for all routers on a subnet so we join
+		// the group here.
+		if err := e.joinGroupLocked(header.IPv4AllRoutersGroup); err != nil {
+			// joinGroupLocked only returns an error if the group address is not a
+			// valid IPv4 multicast address.
+			panic(fmt.Sprintf("e.joinGroupLocked(%s): %s", header.IPv4AllRoutersGroup, err))
+		}
+
+		return
+	}
+
+	switch err := e.leaveGroupLocked(header.IPv4AllRoutersGroup).(type) {
+	case nil:
+	case *tcpip.ErrBadLocalAddress:
+		// The endpoint may have already left the multicast group.
+	default:
+		panic(fmt.Sprintf("e.leaveGroupLocked(%s): %s", header.IPv4AllRoutersGroup, err))
+	}
+}
+
 // Enable implements stack.NetworkEndpoint.
 func (e *endpoint) Enable() tcpip.Error {
 	e.mu.Lock()
@@ -226,7 +258,7 @@ func (e *endpoint) disableLocked() {
 	}
 
 	// The endpoint may have already left the multicast group.
-	switch err := e.leaveGroupLocked(header.IPv4AllSystems); err.(type) {
+	switch err := e.leaveGroupLocked(header.IPv4AllSystems).(type) {
 	case nil, *tcpip.ErrBadLocalAddress:
 	default:
 		panic(fmt.Sprintf("unexpected error when leaving group = %s: %s", header.IPv4AllSystems, err))
@@ -1168,12 +1200,27 @@ func (p *protocol) Forwarding() bool {
 	return uint8(atomic.LoadUint32(&p.forwarding)) == 1
 }
 
+// setForwarding sets the forwarding status for the protocol.
+//
+// Returns true if the forwarding status was updated.
+func (p *protocol) setForwarding(v bool) bool {
+	if v {
+		return atomic.CompareAndSwapUint32(&p.forwarding, 0 /* old */, 1 /* new */)
+	}
+	return atomic.CompareAndSwapUint32(&p.forwarding, 1 /* old */, 0 /* new */)
+}
+
 // SetForwarding implements stack.ForwardingNetworkProtocol.
 func (p *protocol) SetForwarding(v bool) {
-	if v {
-		atomic.StoreUint32(&p.forwarding, 1)
-	} else {
-		atomic.StoreUint32(&p.forwarding, 0)
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	if !p.setForwarding(v) {
+		return
+	}
+
+	for _, ep := range p.mu.eps {
+		ep.transitionForwarding(v)
 	}
 }