Prefer matching labels and longest matching prefix

...when performing source address selection for IPv6.

These are defined in RFC 6724 section 5 rule 6 (prefer matching label)
and rule 8 (use longest matching prefix).

This change also considers ULA of global scope instead of its own scope,
as per RFC 6724 section 3.1:

   Also, note that ULAs are considered as global, not
   site-local, scope but are handled via the prefix policy table as
   discussed in Section 10.6.

Test: stack_test.TestIPv6SourceAddressSelectionScope

Startblock:
  has LGTM from peterjohnston
  and then
  add reviewer brunodalbo
PiperOrigin-RevId: 348580996

2 files changed, 13 insertions, 69 deletions

diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index d522e5f10..5580d6a78 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -18,7 +18,6 @@ import (
 	"crypto/sha256"
 	"encoding/binary"
 	"fmt"
-	"strings"
 
 	"gvisor.dev/gvisor/pkg/tcpip"
 )
@@ -153,13 +152,17 @@ const (
 // IPv6EmptySubnet is the empty IPv6 subnet. It may also be known as the
 // catch-all or wildcard subnet. That is, all IPv6 addresses are considered to
 // be contained within this subnet.
-var IPv6EmptySubnet = func() tcpip.Subnet {
-	subnet, err := tcpip.NewSubnet(IPv6Any, tcpip.AddressMask(IPv6Any))
-	if err != nil {
-		panic(err)
-	}
-	return subnet
-}()
+var IPv6EmptySubnet = tcpip.AddressWithPrefix{
+	Address:   IPv6Any,
+	PrefixLen: 0,
+}.Subnet()
+
+// IPv4MappedIPv6Subnet is the prefix for an IPv4 mapped IPv6 address as defined
+// by RFC 4291 section 2.5.5.
+var IPv4MappedIPv6Subnet = tcpip.AddressWithPrefix{
+	Address:   "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff\x00\x00\x00\x00",
+	PrefixLen: 96,
+}.Subnet()
 
 // IPv6LinkLocalPrefix is the prefix for IPv6 link-local addresses, as defined
 // by RFC 4291 section 2.5.6.
@@ -293,7 +296,7 @@ func IsV4MappedAddress(addr tcpip.Address) bool {
 		return false
 	}
 
-	return strings.HasPrefix(string(addr), "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xff\xff")
+	return IPv4MappedIPv6Subnet.Contains(addr)
 }
 
 // IsV6MulticastAddress determines if the provided address is an IPv6
@@ -399,17 +402,6 @@ func IsV6LinkLocalMulticastAddress(addr tcpip.Address) bool {
 	return IsV6MulticastAddress(addr) && addr[ipv6MulticastAddressScopeByteIdx]&ipv6MulticastAddressScopeMask == ipv6LinkLocalMulticastScope
 }
 
-// IsV6UniqueLocalAddress determines if the provided address is an IPv6
-// unique-local address (within the prefix FC00::/7).
-func IsV6UniqueLocalAddress(addr tcpip.Address) bool {
-	if len(addr) != IPv6AddressSize {
-		return false
-	}
-	// According to RFC 4193 section 3.1, a unique local address has the prefix
-	// FC00::/7.
-	return (addr[0] & 0xfe) == 0xfc
-}
-
 // AppendOpaqueInterfaceIdentifier appends a 64 bit opaque interface identifier
 // (IID) to buf as outlined by RFC 7217 and returns the extended buffer.
 //
@@ -456,9 +448,6 @@ const (
 	// LinkLocalScope indicates a link-local address.
 	LinkLocalScope IPv6AddressScope = iota
 
-	// UniqueLocalScope indicates a unique-local address.
-	UniqueLocalScope
-
 	// GlobalScope indicates a global address.
 	GlobalScope
 )
@@ -476,9 +465,6 @@ func ScopeForIPv6Address(addr tcpip.Address) (IPv6AddressScope, *tcpip.Error) {
 	case IsV6LinkLocalAddress(addr):
 		return LinkLocalScope, nil
 
-	case IsV6UniqueLocalAddress(addr):
-		return UniqueLocalScope, nil
-
 	default:
 		return GlobalScope, nil
 	}
diff --git a/pkg/tcpip/header/ipv6_test.go b/pkg/tcpip/header/ipv6_test.go
index 426a873b1..e3fbd64f3 100644
--- a/pkg/tcpip/header/ipv6_test.go
+++ b/pkg/tcpip/header/ipv6_test.go
@@ -215,48 +215,6 @@ func TestLinkLocalAddrWithOpaqueIID(t *testing.T) {
 	}
 }
 
-func TestIsV6UniqueLocalAddress(t *testing.T) {
-	tests := []struct {
-		name     string
-		addr     tcpip.Address
-		expected bool
-	}{
-		{
-			name:     "Valid Unique 1",
-			addr:     uniqueLocalAddr1,
-			expected: true,
-		},
-		{
-			name:     "Valid Unique 2",
-			addr:     uniqueLocalAddr1,
-			expected: true,
-		},
-		{
-			name:     "Link Local",
-			addr:     linkLocalAddr,
-			expected: false,
-		},
-		{
-			name:     "Global",
-			addr:     globalAddr,
-			expected: false,
-		},
-		{
-			name:     "IPv4",
-			addr:     "\x01\x02\x03\x04",
-			expected: false,
-		},
-	}
-
-	for _, test := range tests {
-		t.Run(test.name, func(t *testing.T) {
-			if got := header.IsV6UniqueLocalAddress(test.addr); got != test.expected {
-				t.Errorf("got header.IsV6UniqueLocalAddress(%s) = %t, want = %t", test.addr, got, test.expected)
-			}
-		})
-	}
-}
-
 func TestIsV6LinkLocalMulticastAddress(t *testing.T) {
 	tests := []struct {
 		name     string
@@ -346,7 +304,7 @@ func TestScopeForIPv6Address(t *testing.T) {
 		{
 			name:  "Unique Local",
 			addr:  uniqueLocalAddr1,
-			scope: header.UniqueLocalScope,
+			scope: header.GlobalScope,
 			err:   nil,
 		},
 		{