Auto-generate an IPv6 link-local address based on the NIC's MAC Address.

This change adds support for optionally auto-generating an IPv6 link-local
address based on the NIC's MAC Address on NIC enable.

Note, this change will not break existing uses of netstack as the default
configuration for the stack options is set in such a way that a link-local
address will not be auto-generated unless the stack is explicitly configured.
See `stack.Options` for more details. Specifically, see
`stack.Options.AutoGenIPv6LinkLocal`.

Tests: Tests to make sure that the IPb6 link-local address is only
auto-generated if the stack is specifically configured to do so. Also tests to
make sure that an auto-generated address goes through the DAD process.
PiperOrigin-RevId: 276059813

1 files changed, 50 insertions, 3 deletions

diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index dbd304b7e..9be3f6ee4 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -81,6 +81,9 @@ const (
 )
 
 func newNIC(stack *Stack, id tcpip.NICID, name string, ep LinkEndpoint, loopback bool) *NIC {
+	// TODO(b/141011931): Validate a LinkEndpoint (ep) is valid. For
+	// example, make sure that the link address it provides is a valid
+	// unicast ethernet address.
 	nic := &NIC{
 		stack:      stack,
 		id:         id,
@@ -139,11 +142,50 @@ func (n *NIC) enable() *tcpip.Error {
 	// when we perform Duplicate Address Detection, or Router Advertisement
 	// when we do Router Discovery. See RFC 4862, section 5.4.2 and RFC 4861
 	// section 4.2 for more information.
-	if _, ok := n.stack.networkProtocols[header.IPv6ProtocolNumber]; ok {
-		return n.joinGroup(header.IPv6ProtocolNumber, header.IPv6AllNodesMulticastAddress)
+	//
+	// Also auto-generate an IPv6 link-local address based on the NIC's
+	// link address if it is configured to do so. Note, each interface is
+	// required to have IPv6 link-local unicast address, as per RFC 4291
+	// section 2.1.
+	_, ok := n.stack.networkProtocols[header.IPv6ProtocolNumber]
+	if !ok {
+		return nil
 	}
 
-	return nil
+	n.mu.Lock()
+	defer n.mu.Unlock()
+
+	if err := n.joinGroupLocked(header.IPv6ProtocolNumber, header.IPv6AllNodesMulticastAddress); err != nil {
+		return err
+	}
+
+	if !n.stack.autoGenIPv6LinkLocal {
+		return nil
+	}
+
+	l2addr := n.linkEP.LinkAddress()
+
+	// Only attempt to generate the link-local address if we have a
+	// valid MAC address.
+	//
+	// TODO(b/141011931): Validate a LinkEndpoint's link address
+	// (provided by LinkEndpoint.LinkAddress) before reaching this
+	// point.
+	if !header.IsValidUnicastEthernetAddress(l2addr) {
+		return nil
+	}
+
+	addr := header.LinkLocalAddr(l2addr)
+
+	_, err := n.addPermanentAddressLocked(tcpip.ProtocolAddress{
+		Protocol: header.IPv6ProtocolNumber,
+		AddressWithPrefix: tcpip.AddressWithPrefix{
+			Address:   addr,
+			PrefixLen: header.IPv6LinkLocalPrefix.PrefixLen,
+		},
+	}, CanBePrimaryEndpoint)
+
+	return err
 }
 
 // attachLinkEndpoint attaches the NIC to the endpoint, which will enable it
@@ -582,6 +624,11 @@ func (n *NIC) joinGroup(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address
 // exists yet. Otherwise it just increments its count. n MUST be locked before
 // joinGroupLocked is called.
 func (n *NIC) joinGroupLocked(protocol tcpip.NetworkProtocolNumber, addr tcpip.Address) *tcpip.Error {
+	// TODO(b/143102137): When implementing MLD, make sure MLD packets are
+	// not sent unless a valid link-local address is available for use on n
+	// as an MLD packet's source address must be a link-local address as
+	// outlined in RFC 3810 section 5.
+
 	id := NetworkEndpointID{addr}
 	joins := n.mcastJoins[id]
 	if joins == 0 {