Support generating opaque interface identifiers as defined by RFC 7217

Support generating opaque interface identifiers as defined by RFC 7217 for
auto-generated IPv6 link-local addresses. Opaque interface identifiers will also
be used for IPv6 addresses auto-generated via SLAAC in a later change.

Note, this change does not handle retries in response to DAD conflicts yet.
That will also come in a later change.

Tests: Test that when configured to generated opaque IIDs, they are properly
generated as outlined by RFC 7217.
PiperOrigin-RevId: 288035349

7 files changed, 384 insertions, 15 deletions

diff --git a/pkg/tcpip/header/BUILD b/pkg/tcpip/header/BUILD
index f1d837196..f2061c778 100644
--- a/pkg/tcpip/header/BUILD
+++ b/pkg/tcpip/header/BUILD
@@ -44,6 +44,7 @@ go_test(
     ],
     deps = [
         ":header",
+        "//pkg/rand",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
         "@com_github_google_go-cmp//cmp:go_default_library",
diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index fc671e439..135a60b12 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -15,6 +15,7 @@
 package header
 
 import (
+	"crypto/sha256"
 	"encoding/binary"
 	"strings"
 
@@ -102,6 +103,11 @@ const (
 	// bytes including and after the IIDOffsetInIPv6Address-th byte are
 	// for the IID.
 	IIDOffsetInIPv6Address = 8
+
+	// OpaqueIIDSecretKeyMinBytes is the recommended minimum number of bytes
+	// for the secret key used to generate an opaque interface identifier as
+	// outlined by RFC 7217.
+	OpaqueIIDSecretKeyMinBytes = 16
 )
 
 // IPv6EmptySubnet is the empty IPv6 subnet. It may also be known as the
@@ -326,3 +332,42 @@ func IsV6LinkLocalAddress(addr tcpip.Address) bool {
 	}
 	return addr[0] == 0xfe && (addr[1]&0xc0) == 0x80
 }
+
+// AppendOpaqueInterfaceIdentifier appends a 64 bit opaque interface identifier
+// (IID) to buf as outlined by RFC 7217 and returns the extended buffer.
+//
+// The opaque IID is generated from the cryptographic hash of the concatenation
+// of the prefix, NIC's name, DAD counter (DAD retry counter) and the secret
+// key. The secret key SHOULD be at least OpaqueIIDSecretKeyMinBytes bytes and
+// MUST be generated to a pseudo-random number. See RFC 4086 for randomness
+// requirements for security.
+//
+// If buf has enough capacity for the IID (IIDSize bytes), a new underlying
+// array for the buffer will not be allocated.
+func AppendOpaqueInterfaceIdentifier(buf []byte, prefix tcpip.Subnet, nicName string, dadCounter uint8, secretKey []byte) []byte {
+	// As per RFC 7217 section 5, the opaque identifier can be generated as a
+	// cryptographic hash of the concatenation of each of the function parameters.
+	// Note, we omit the optional Network_ID field.
+	h := sha256.New()
+	// h.Write never returns an error.
+	h.Write([]byte(prefix.ID()[:IIDOffsetInIPv6Address]))
+	h.Write([]byte(nicName))
+	h.Write([]byte{dadCounter})
+	h.Write(secretKey)
+
+	var sumBuf [sha256.Size]byte
+	sum := h.Sum(sumBuf[:0])
+
+	return append(buf, sum[:IIDSize]...)
+}
+
+// LinkLocalAddrWithOpaqueIID computes the default IPv6 link-local address with
+// an opaque IID.
+func LinkLocalAddrWithOpaqueIID(nicName string, dadCounter uint8, secretKey []byte) tcpip.Address {
+	lladdrb := [IPv6AddressSize]byte{
+		0: 0xFE,
+		1: 0x80,
+	}
+
+	return tcpip.Address(AppendOpaqueInterfaceIdentifier(lladdrb[:IIDOffsetInIPv6Address], IPv6LinkLocalPrefix.Subnet(), nicName, dadCounter, secretKey))
+}
diff --git a/pkg/tcpip/header/ipv6_test.go b/pkg/tcpip/header/ipv6_test.go
index 42c5c6fc1..cd1862e42 100644
--- a/pkg/tcpip/header/ipv6_test.go
+++ b/pkg/tcpip/header/ipv6_test.go
@@ -15,9 +15,12 @@
 package header_test
 
 import (
+	"bytes"
+	"crypto/sha256"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
+	"gvisor.dev/gvisor/pkg/rand"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
@@ -43,3 +46,163 @@ func TestLinkLocalAddr(t *testing.T) {
 		t.Errorf("got LinkLocalAddr(%s) = %s, want = %s", linkAddr, got, want)
 	}
 }
+
+func TestAppendOpaqueInterfaceIdentifier(t *testing.T) {
+	var secretKeyBuf [header.OpaqueIIDSecretKeyMinBytes * 2]byte
+	if n, err := rand.Read(secretKeyBuf[:]); err != nil {
+		t.Fatalf("rand.Read(_): %s", err)
+	} else if want := header.OpaqueIIDSecretKeyMinBytes * 2; n != want {
+		t.Fatalf("expected rand.Read to read %d bytes, read %d bytes", want, n)
+	}
+
+	tests := []struct {
+		name       string
+		prefix     tcpip.Subnet
+		nicName    string
+		dadCounter uint8
+		secretKey  []byte
+	}{
+		{
+			name:       "SecretKey of minimum size",
+			prefix:     header.IPv6LinkLocalPrefix.Subnet(),
+			nicName:    "eth0",
+			dadCounter: 0,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes],
+		},
+		{
+			name: "SecretKey of less than minimum size",
+			prefix: func() tcpip.Subnet {
+				addrWithPrefix := tcpip.AddressWithPrefix{
+					Address:   "\x01\x02\x03\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
+					PrefixLen: header.IIDOffsetInIPv6Address * 8,
+				}
+				return addrWithPrefix.Subnet()
+			}(),
+			nicName:    "eth10",
+			dadCounter: 1,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes/2],
+		},
+		{
+			name: "SecretKey of more than minimum size",
+			prefix: func() tcpip.Subnet {
+				addrWithPrefix := tcpip.AddressWithPrefix{
+					Address:   "\x01\x02\x03\x04\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
+					PrefixLen: header.IIDOffsetInIPv6Address * 8,
+				}
+				return addrWithPrefix.Subnet()
+			}(),
+			nicName:    "eth11",
+			dadCounter: 2,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes*2],
+		},
+		{
+			name: "Nil SecretKey",
+			prefix: func() tcpip.Subnet {
+				addrWithPrefix := tcpip.AddressWithPrefix{
+					Address:   "\x01\x02\x03\x05\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00",
+					PrefixLen: header.IIDOffsetInIPv6Address * 8,
+				}
+				return addrWithPrefix.Subnet()
+			}(),
+			nicName:    "eth12",
+			dadCounter: 3,
+			secretKey:  nil,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			h := sha256.New()
+			h.Write([]byte(test.prefix.ID()[:header.IIDOffsetInIPv6Address]))
+			h.Write([]byte(test.nicName))
+			h.Write([]byte{test.dadCounter})
+			if k := test.secretKey; k != nil {
+				h.Write(k)
+			}
+			var hashSum [sha256.Size]byte
+			h.Sum(hashSum[:0])
+			want := hashSum[:header.IIDSize]
+
+			// Passing a nil buffer should result in a new buffer returned with the
+			// IID.
+			if got := header.AppendOpaqueInterfaceIdentifier(nil, test.prefix, test.nicName, test.dadCounter, test.secretKey); !bytes.Equal(got, want) {
+				t.Errorf("got AppendOpaqueInterfaceIdentifier(nil, %s, %s, %d, %x) = %x, want = %x", test.prefix, test.nicName, test.dadCounter, test.secretKey, got, want)
+			}
+
+			// Passing a buffer with sufficient capacity for the IID should populate
+			// the buffer provided.
+			var iidBuf [header.IIDSize]byte
+			if got := header.AppendOpaqueInterfaceIdentifier(iidBuf[:0], test.prefix, test.nicName, test.dadCounter, test.secretKey); !bytes.Equal(got, want) {
+				t.Errorf("got AppendOpaqueInterfaceIdentifier(iidBuf[:0], %s, %s, %d, %x) = %x, want = %x", test.prefix, test.nicName, test.dadCounter, test.secretKey, got, want)
+			}
+			if got := iidBuf[:]; !bytes.Equal(got, want) {
+				t.Errorf("got iidBuf = %x, want = %x", got, want)
+			}
+		})
+	}
+}
+
+func TestLinkLocalAddrWithOpaqueIID(t *testing.T) {
+	var secretKeyBuf [header.OpaqueIIDSecretKeyMinBytes * 2]byte
+	if n, err := rand.Read(secretKeyBuf[:]); err != nil {
+		t.Fatalf("rand.Read(_): %s", err)
+	} else if want := header.OpaqueIIDSecretKeyMinBytes * 2; n != want {
+		t.Fatalf("expected rand.Read to read %d bytes, read %d bytes", want, n)
+	}
+
+	prefix := header.IPv6LinkLocalPrefix.Subnet()
+
+	tests := []struct {
+		name       string
+		prefix     tcpip.Subnet
+		nicName    string
+		dadCounter uint8
+		secretKey  []byte
+	}{
+		{
+			name:       "SecretKey of minimum size",
+			nicName:    "eth0",
+			dadCounter: 0,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes],
+		},
+		{
+			name:       "SecretKey of less than minimum size",
+			nicName:    "eth10",
+			dadCounter: 1,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes/2],
+		},
+		{
+			name:       "SecretKey of more than minimum size",
+			nicName:    "eth11",
+			dadCounter: 2,
+			secretKey:  secretKeyBuf[:header.OpaqueIIDSecretKeyMinBytes*2],
+		},
+		{
+			name:       "Nil SecretKey",
+			nicName:    "eth12",
+			dadCounter: 3,
+			secretKey:  nil,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			addrBytes := [header.IPv6AddressSize]byte{
+				0: 0xFE,
+				1: 0x80,
+			}
+
+			want := tcpip.Address(header.AppendOpaqueInterfaceIdentifier(
+				addrBytes[:header.IIDOffsetInIPv6Address],
+				prefix,
+				test.nicName,
+				test.dadCounter,
+				test.secretKey,
+			))
+
+			if got := header.LinkLocalAddrWithOpaqueIID(test.nicName, test.dadCounter, test.secretKey); got != want {
+				t.Errorf("got LinkLocalAddrWithOpaqueIID(%s, %d, %x) = %s, want = %s", test.nicName, test.dadCounter, test.secretKey, got, want)
+			}
+		})
+	}
+}
diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index 69077669a..b8f9517d0 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -59,6 +59,7 @@ go_test(
     ],
     deps = [
         ":stack",
+        "//pkg/rand",
         "//pkg/tcpip",
         "//pkg/tcpip/buffer",
         "//pkg/tcpip/checker",
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index ddd014658..3bed0af3c 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -178,20 +178,24 @@ func (n *NIC) enable() *tcpip.Error {
 		return nil
 	}
 
-	l2addr := n.linkEP.LinkAddress()
+	var addr tcpip.Address
+	if oIID := n.stack.opaqueIIDOpts; oIID.NICNameFromID != nil {
+		addr = header.LinkLocalAddrWithOpaqueIID(oIID.NICNameFromID(n.ID()), 0, oIID.SecretKey)
+	} else {
+		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
+		}
 
-	// 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)
 	}
 
-	addr := header.LinkLocalAddr(l2addr)
-
 	_, err := n.addPermanentAddressLocked(tcpip.ProtocolAddress{
 		Protocol: header.IPv6ProtocolNumber,
 		AddressWithPrefix: tcpip.AddressWithPrefix{
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 7a9600679..c6e6becf3 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -352,6 +352,33 @@ func (u *uniqueIDGenerator) UniqueID() uint64 {
 	return atomic.AddUint64((*uint64)(u), 1)
 }
 
+// NICNameFromID is a function that returns a stable name for the specified NIC,
+// even if the NIC ID changes over time.
+type NICNameFromID func(tcpip.NICID) string
+
+// OpaqueInterfaceIdentifierOptions holds the options related to the generation
+// of opaque interface indentifiers (IIDs) as defined by RFC 7217.
+type OpaqueInterfaceIdentifierOptions struct {
+	// NICNameFromID is a function that returns a stable name for a specified NIC,
+	// even if the NIC ID changes over time.
+	//
+	// Must be specified to generate the opaque IID.
+	NICNameFromID NICNameFromID
+
+	// SecretKey is a pseudo-random number used as the secret key when generating
+	// opaque IIDs as defined by RFC 7217. The key SHOULD be at least
+	// header.OpaqueIIDSecretKeyMinBytes bytes and MUST follow minimum randomness
+	// requirements for security as outlined by RFC 4086. SecretKey MUST NOT
+	// change between program runs, unless explicitly changed.
+	//
+	// OpaqueInterfaceIdentifierOptions takes ownership of SecretKey. SecretKey
+	// MUST NOT be modified after Stack is created.
+	//
+	// May be nil, but a nil value is highly discouraged to maintain
+	// some level of randomness between nodes.
+	SecretKey []byte
+}
+
 // Stack is a networking stack, with all supported protocols, NICs, and route
 // table.
 type Stack struct {
@@ -422,6 +449,10 @@ type Stack struct {
 
 	// uniqueIDGenerator is a generator of unique identifiers.
 	uniqueIDGenerator UniqueID
+
+	// opaqueIIDOpts hold the options for generating opaque interface identifiers
+	// (IIDs) as outlined by RFC 7217.
+	opaqueIIDOpts OpaqueInterfaceIdentifierOptions
 }
 
 // UniqueID is an abstract generator of unique identifiers.
@@ -479,6 +510,10 @@ type Options struct {
 	// RawFactory produces raw endpoints. Raw endpoints are enabled only if
 	// this is non-nil.
 	RawFactory RawFactory
+
+	// OpaqueIIDOpts hold the options for generating opaque interface identifiers
+	// (IIDs) as outlined by RFC 7217.
+	OpaqueIIDOpts OpaqueInterfaceIdentifierOptions
 }
 
 // TransportEndpointInfo holds useful information about a transport endpoint
@@ -549,6 +584,7 @@ func New(opts Options) *Stack {
 		autoGenIPv6LinkLocal: opts.AutoGenIPv6LinkLocal,
 		uniqueIDGenerator:    opts.UniqueID,
 		ndpDisp:              opts.NDPDisp,
+		opaqueIIDOpts:        opts.OpaqueIIDOpts,
 	}
 
 	// Add specified network protocols.
diff --git a/pkg/tcpip/stack/stack_test.go b/pkg/tcpip/stack/stack_test.go
index 8fc034ca1..e18dfea83 100644
--- a/pkg/tcpip/stack/stack_test.go
+++ b/pkg/tcpip/stack/stack_test.go
@@ -27,6 +27,7 @@ import (
 	"time"
 
 	"github.com/google/go-cmp/cmp"
+	"gvisor.dev/gvisor/pkg/rand"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/buffer"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
@@ -1894,55 +1895,67 @@ func TestNICForwarding(t *testing.T) {
 }
 
 // TestNICAutoGenAddr tests the auto-generation of IPv6 link-local addresses
-// (or lack there-of if disabled (default)). Note, DAD will be disabled in
-// these tests.
+// using the modified EUI-64 of the NIC's MAC address (or lack there-of if
+// disabled (default)). Note, DAD will be disabled in these tests.
 func TestNICAutoGenAddr(t *testing.T) {
 	tests := []struct {
 		name      string
 		autoGen   bool
 		linkAddr  tcpip.LinkAddress
+		iidOpts   stack.OpaqueInterfaceIdentifierOptions
 		shouldGen bool
 	}{
 		{
 			"Disabled",
 			false,
 			linkAddr1,
+			stack.OpaqueInterfaceIdentifierOptions{
+				NICNameFromID: func(nicID tcpip.NICID) string {
+					return fmt.Sprintf("nic%d", nicID)
+				},
+			},
 			false,
 		},
 		{
 			"Enabled",
 			true,
 			linkAddr1,
+			stack.OpaqueInterfaceIdentifierOptions{},
 			true,
 		},
 		{
 			"Nil MAC",
 			true,
 			tcpip.LinkAddress([]byte(nil)),
+			stack.OpaqueInterfaceIdentifierOptions{},
 			false,
 		},
 		{
 			"Empty MAC",
 			true,
 			tcpip.LinkAddress(""),
+			stack.OpaqueInterfaceIdentifierOptions{},
 			false,
 		},
 		{
 			"Invalid MAC",
 			true,
 			tcpip.LinkAddress("\x01\x02\x03"),
+			stack.OpaqueInterfaceIdentifierOptions{},
 			false,
 		},
 		{
 			"Multicast MAC",
 			true,
 			tcpip.LinkAddress("\x01\x02\x03\x04\x05\x06"),
+			stack.OpaqueInterfaceIdentifierOptions{},
 			false,
 		},
 		{
 			"Unspecified MAC",
 			true,
 			tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00"),
+			stack.OpaqueInterfaceIdentifierOptions{},
 			false,
 		},
 	}
@@ -1951,6 +1964,112 @@ func TestNICAutoGenAddr(t *testing.T) {
 		t.Run(test.name, func(t *testing.T) {
 			opts := stack.Options{
 				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				OpaqueIIDOpts:    test.iidOpts,
+			}
+
+			if test.autoGen {
+				// Only set opts.AutoGenIPv6LinkLocal when test.autoGen is true because
+				// opts.AutoGenIPv6LinkLocal should be false by default.
+				opts.AutoGenIPv6LinkLocal = true
+			}
+
+			e := channel.New(10, 1280, test.linkAddr)
+			s := stack.New(opts)
+			if err := s.CreateNIC(1, e); err != nil {
+				t.Fatalf("CreateNIC(_) = %s", err)
+			}
+
+			addr, err := s.GetMainNICAddress(1, header.IPv6ProtocolNumber)
+			if err != nil {
+				t.Fatalf("stack.GetMainNICAddress(_, _) err = %s", err)
+			}
+
+			if test.shouldGen {
+				// Should have auto-generated an address and resolved immediately (DAD
+				// is disabled).
+				if want := (tcpip.AddressWithPrefix{Address: header.LinkLocalAddr(test.linkAddr), PrefixLen: header.IPv6LinkLocalPrefix.PrefixLen}); addr != want {
+					t.Fatalf("got stack.GetMainNICAddress(_, _) = %s, want = %s", addr, want)
+				}
+			} else {
+				// Should not have auto-generated an address.
+				if want := (tcpip.AddressWithPrefix{}); addr != want {
+					t.Fatalf("got stack.GetMainNICAddress(_, _) = (%s, nil), want = (%s, nil)", addr, want)
+				}
+			}
+		})
+	}
+}
+
+// TestNICAutoGenAddrWithOpaque tests the auto-generation of IPv6 link-local
+// addresses with opaque interface identifiers. Link Local addresses should
+// always be generated with opaque IIDs if configured to use them, even if the
+// NIC has an invalid MAC address.
+func TestNICAutoGenAddrWithOpaque(t *testing.T) {
+	var secretKey [header.OpaqueIIDSecretKeyMinBytes]byte
+	n, err := rand.Read(secretKey[:])
+	if err != nil {
+		t.Fatalf("rand.Read(_): %s", err)
+	}
+	if n != header.OpaqueIIDSecretKeyMinBytes {
+		t.Fatalf("expected rand.Read to read %d bytes, read %d bytes", header.OpaqueIIDSecretKeyMinBytes, n)
+	}
+
+	iidOpts := stack.OpaqueInterfaceIdentifierOptions{
+		NICNameFromID: func(nicID tcpip.NICID) string {
+			return fmt.Sprintf("nic%d", nicID)
+		},
+		SecretKey: secretKey[:],
+	}
+
+	tests := []struct {
+		name     string
+		autoGen  bool
+		linkAddr tcpip.LinkAddress
+	}{
+		{
+			"Disabled",
+			false,
+			linkAddr1,
+		},
+		{
+			"Enabled",
+			true,
+			linkAddr1,
+		},
+		// These are all cases where we would not have generated a
+		// link-local address if opaque IIDs were disabled.
+		{
+			"Nil MAC",
+			true,
+			tcpip.LinkAddress([]byte(nil)),
+		},
+		{
+			"Empty MAC",
+			true,
+			tcpip.LinkAddress(""),
+		},
+		{
+			"Invalid MAC",
+			true,
+			tcpip.LinkAddress("\x01\x02\x03"),
+		},
+		{
+			"Multicast MAC",
+			true,
+			tcpip.LinkAddress("\x01\x02\x03\x04\x05\x06"),
+		},
+		{
+			"Unspecified MAC",
+			true,
+			tcpip.LinkAddress("\x00\x00\x00\x00\x00\x00"),
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			opts := stack.Options{
+				NetworkProtocols: []stack.NetworkProtocol{ipv6.NewProtocol()},
+				OpaqueIIDOpts:    iidOpts,
 			}
 
 			if test.autoGen {
@@ -1972,10 +2091,10 @@ func TestNICAutoGenAddr(t *testing.T) {
 				t.Fatalf("stack.GetMainNICAddress(_, _) err = %s", err)
 			}
 
-			if test.shouldGen {
+			if test.autoGen {
 				// Should have auto-generated an address and
 				// resolved immediately (DAD is disabled).
-				if want := (tcpip.AddressWithPrefix{Address: header.LinkLocalAddr(test.linkAddr), PrefixLen: header.IPv6LinkLocalPrefix.PrefixLen}); addr != want {
+				if want := (tcpip.AddressWithPrefix{Address: header.LinkLocalAddrWithOpaqueIID("nic1", 0, secretKey[:]), PrefixLen: header.IPv6LinkLocalPrefix.PrefixLen}); addr != want {
 					t.Fatalf("got stack.GetMainNICAddress(_, _) = %s, want = %s", addr, want)
 				}
 			} else {