Do Source Address Selection when choosing an IPv6 source address

Do Source Address Selection when choosing an IPv6 source address as per RFC 6724
section 5 rules 1-3:
1) Prefer same address
2) Prefer appropriate scope
3) Avoid deprecated addresses.

A later change will update Source Address Selection to follow rules 4-8.

Tests:
Rule 1 & 2: stack.TestIPv6SourceAddressSelectionScopeAndSameAddress,
Rule 3:     stack.TestAutoGenAddrTimerDeprecation,
            stack.TestAutoGenAddrDeprecateFromPI
PiperOrigin-RevId: 289559373

2 files changed, 138 insertions, 1 deletions

diff --git a/pkg/tcpip/header/ipv6.go b/pkg/tcpip/header/ipv6.go
index 135a60b12..83425c614 100644
--- a/pkg/tcpip/header/ipv6.go
+++ b/pkg/tcpip/header/ipv6.go
@@ -333,6 +333,17 @@ func IsV6LinkLocalAddress(addr tcpip.Address) bool {
 	return addr[0] == 0xfe && (addr[1]&0xc0) == 0x80
 }
 
+// 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.
 //
@@ -371,3 +382,35 @@ func LinkLocalAddrWithOpaqueIID(nicName string, dadCounter uint8, secretKey []by
 
 	return tcpip.Address(AppendOpaqueInterfaceIdentifier(lladdrb[:IIDOffsetInIPv6Address], IPv6LinkLocalPrefix.Subnet(), nicName, dadCounter, secretKey))
 }
+
+// IPv6AddressScope is the scope of an IPv6 address.
+type IPv6AddressScope int
+
+const (
+	// LinkLocalScope indicates a link-local address.
+	LinkLocalScope IPv6AddressScope = iota
+
+	// UniqueLocalScope indicates a unique-local address.
+	UniqueLocalScope
+
+	// GlobalScope indicates a global address.
+	GlobalScope
+)
+
+// ScopeForIPv6Address returns the scope for an IPv6 address.
+func ScopeForIPv6Address(addr tcpip.Address) (IPv6AddressScope, *tcpip.Error) {
+	if len(addr) != IPv6AddressSize {
+		return GlobalScope, tcpip.ErrBadAddress
+	}
+
+	switch {
+	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 1994003ed..29f54bc57 100644
--- a/pkg/tcpip/header/ipv6_test.go
+++ b/pkg/tcpip/header/ipv6_test.go
@@ -25,7 +25,13 @@ import (
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
 
-const linkAddr = tcpip.LinkAddress("\x02\x02\x03\x04\x05\x06")
+const (
+	linkAddr         = tcpip.LinkAddress("\x02\x02\x03\x04\x05\x06")
+	linkLocalAddr    = tcpip.Address("\xfe\x80\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	uniqueLocalAddr1 = tcpip.Address("\xfc\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+	uniqueLocalAddr2 = tcpip.Address("\xfd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02")
+	globalAddr       = tcpip.Address("\xa0\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01")
+)
 
 func TestEthernetAdddressToModifiedEUI64(t *testing.T) {
 	expectedIID := [header.IIDSize]byte{0, 2, 3, 255, 254, 4, 5, 6}
@@ -206,3 +212,91 @@ 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 TestScopeForIPv6Address(t *testing.T) {
+	tests := []struct {
+		name  string
+		addr  tcpip.Address
+		scope header.IPv6AddressScope
+		err   *tcpip.Error
+	}{
+		{
+			name:  "Unique Local",
+			addr:  uniqueLocalAddr1,
+			scope: header.UniqueLocalScope,
+			err:   nil,
+		},
+		{
+			name:  "Link Local",
+			addr:  linkLocalAddr,
+			scope: header.LinkLocalScope,
+			err:   nil,
+		},
+		{
+			name:  "Global",
+			addr:  globalAddr,
+			scope: header.GlobalScope,
+			err:   nil,
+		},
+		{
+			name:  "IPv4",
+			addr:  "\x01\x02\x03\x04",
+			scope: header.GlobalScope,
+			err:   tcpip.ErrBadAddress,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got, err := header.ScopeForIPv6Address(test.addr)
+			if err != test.err {
+				t.Errorf("got header.IsV6UniqueLocalAddress(%s) = (_, %v), want = (_, %v)", test.addr, err, test.err)
+			}
+			if got != test.scope {
+				t.Errorf("got header.IsV6UniqueLocalAddress(%s) = (%d, _), want = (%d, _)", test.addr, got, test.scope)
+			}
+		})
+	}
+}