Support parsing Prf field in RAs

This change prepares for a later change which actually handles the
Prf field in RAs to discover default routers with preference values,
as per RFC 4191.

Updates #6172.

Test: header_test.TestNDPRouterAdvert
PiperOrigin-RevId: 379421710

2 files changed, 143 insertions, 23 deletions

diff --git a/pkg/tcpip/header/ndp_router_advert.go b/pkg/tcpip/header/ndp_router_advert.go
index bf7610863..7e2f0c797 100644
--- a/pkg/tcpip/header/ndp_router_advert.go
+++ b/pkg/tcpip/header/ndp_router_advert.go
@@ -19,12 +19,72 @@ import (
 	"time"
 )
 
+// NDPRoutePreference is the preference values for default routers or
+// more-specific routes.
+//
+// As per RFC 4191 section 2.1,
+//
+//   Default router preferences and preferences for more-specific routes
+//   are encoded the same way.
+//
+//   Preference values are encoded as a two-bit signed integer, as
+//   follows:
+//
+//      01      High
+//      00      Medium (default)
+//      11      Low
+//      10      Reserved - MUST NOT be sent
+//
+//   Note that implementations can treat the value as a two-bit signed
+//   integer.
+//
+//   Having just three values reinforces that they are not metrics and
+//   more values do not appear to be necessary for reasonable scenarios.
+type NDPRoutePreference uint8
+
+const (
+	// HighRoutePreference indicates a high preference, as per
+	// RFC 4191 section 2.1.
+	HighRoutePreference NDPRoutePreference = 0b01
+
+	// MediumRoutePreference indicates a medium preference, as per
+	// RFC 4191 section 2.1.
+	//
+	// This is the default preference value.
+	MediumRoutePreference = 0b00
+
+	// LowRoutePreference indicates a low preference, as per
+	// RFC 4191 section 2.1.
+	LowRoutePreference = 0b11
+
+	// ReservedRoutePreference is a reserved preference value, as per
+	// RFC 4191 section 2.1.
+	//
+	// It MUST NOT be sent.
+	ReservedRoutePreference = 0b10
+)
+
 // NDPRouterAdvert is an NDP Router Advertisement message. It will only contain
 // the body of an ICMPv6 packet.
 //
-// See RFC 4861 section 4.2 for more details.
+// See RFC 4861 section 4.2 and RFC 4191 section 2.2 for more details.
 type NDPRouterAdvert []byte
 
+// As per RFC 4191 section 2.2,
+//
+//       0                   1                   2                   3
+//       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+//      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//      |     Type      |     Code      |          Checksum             |
+//      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//      | Cur Hop Limit |M|O|H|Prf|Resvd|       Router Lifetime         |
+//      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//      |                         Reachable Time                        |
+//      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//      |                          Retrans Timer                        |
+//      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+//      |   Options ...
+//      +-+-+-+-+-+-+-+-+-+-+-+-
 const (
 	// NDPRAMinimumSize is the minimum size of a valid NDP Router
 	// Advertisement message (body of an ICMPv6 packet).
@@ -47,6 +107,14 @@ const (
 	// within the bit-field/flags byte of an NDPRouterAdvert.
 	ndpRAOtherConfFlagMask = (1 << 6)
 
+	// ndpDefaultRouterPreferenceShift is the shift of the Prf (Default Router
+	// Preference) field within the flags byte of an NDPRouterAdvert.
+	ndpDefaultRouterPreferenceShift = 3
+
+	// ndpDefaultRouterPreferenceMask is the mask of the Prf (Default Router
+	// Preference) field within the flags byte of an NDPRouterAdvert.
+	ndpDefaultRouterPreferenceMask = (0b11 << ndpDefaultRouterPreferenceShift)
+
 	// ndpRARouterLifetimeOffset is the start of the 2-byte Router Lifetime
 	// field within an NDPRouterAdvert.
 	ndpRARouterLifetimeOffset = 2
@@ -80,6 +148,11 @@ func (b NDPRouterAdvert) OtherConfFlag() bool {
 	return b[ndpRAFlagsOffset]&ndpRAOtherConfFlagMask != 0
 }
 
+// DefaultRouterPreference returns the Default Router Preference field.
+func (b NDPRouterAdvert) DefaultRouterPreference() NDPRoutePreference {
+	return NDPRoutePreference((b[ndpRAFlagsOffset] & ndpDefaultRouterPreferenceMask) >> ndpDefaultRouterPreferenceShift)
+}
+
 // RouterLifetime returns the lifetime associated with the default router. A
 // value of 0 means the source of the Router Advertisement is not a default
 // router and SHOULD NOT appear on the default router list. Note, a value of 0
diff --git a/pkg/tcpip/header/ndp_test.go b/pkg/tcpip/header/ndp_test.go
index 1b5093e58..8fd1f7d13 100644
--- a/pkg/tcpip/header/ndp_test.go
+++ b/pkg/tcpip/header/ndp_test.go
@@ -126,36 +126,83 @@ func TestNDPNeighborAdvert(t *testing.T) {
 }
 
 func TestNDPRouterAdvert(t *testing.T) {
-	b := []byte{
-		64, 128, 1, 2,
-		3, 4, 5, 6,
-		7, 8, 9, 10,
+	tests := []struct {
+		hopLimit                        uint8
+		managedFlag, otherConfFlag      bool
+		prf                             NDPRoutePreference
+		routerLifetimeS                 uint16
+		reachableTimeMS, retransTimerMS uint32
+	}{
+		{
+			hopLimit:        1,
+			managedFlag:     false,
+			otherConfFlag:   true,
+			prf:             HighRoutePreference,
+			routerLifetimeS: 2,
+			reachableTimeMS: 3,
+			retransTimerMS:  4,
+		},
+		{
+			hopLimit:        64,
+			managedFlag:     true,
+			otherConfFlag:   false,
+			prf:             LowRoutePreference,
+			routerLifetimeS: 258,
+			reachableTimeMS: 78492,
+			retransTimerMS:  13213,
+		},
 	}
 
-	ra := NDPRouterAdvert(b)
+	for i, test := range tests {
+		t.Run(fmt.Sprintf("%d", i), func(t *testing.T) {
+			flags := uint8(0)
+			if test.managedFlag {
+				flags |= 1 << 7
+			}
+			if test.otherConfFlag {
+				flags |= 1 << 6
+			}
+			flags |= uint8(test.prf) << 3
 
-	if got := ra.CurrHopLimit(); got != 64 {
-		t.Errorf("got ra.CurrHopLimit = %d, want = 64", got)
-	}
+			b := []byte{
+				test.hopLimit, flags, 1, 2,
+				3, 4, 5, 6,
+				7, 8, 9, 10,
+			}
+			binary.BigEndian.PutUint16(b[2:], test.routerLifetimeS)
+			binary.BigEndian.PutUint32(b[4:], test.reachableTimeMS)
+			binary.BigEndian.PutUint32(b[8:], test.retransTimerMS)
 
-	if got := ra.ManagedAddrConfFlag(); !got {
-		t.Errorf("got ManagedAddrConfFlag = false, want = true")
-	}
+			ra := NDPRouterAdvert(b)
 
-	if got := ra.OtherConfFlag(); got {
-		t.Errorf("got OtherConfFlag = true, want = false")
-	}
+			if got := ra.CurrHopLimit(); got != test.hopLimit {
+				t.Errorf("got ra.CurrHopLimit() = %d, want = %d", got, test.hopLimit)
+			}
 
-	if got, want := ra.RouterLifetime(), time.Second*258; got != want {
-		t.Errorf("got ra.RouterLifetime = %d, want = %d", got, want)
-	}
+			if got := ra.ManagedAddrConfFlag(); got != test.managedFlag {
+				t.Errorf("got ManagedAddrConfFlag() = %t, want = %t", got, test.managedFlag)
+			}
 
-	if got, want := ra.ReachableTime(), time.Millisecond*50595078; got != want {
-		t.Errorf("got ra.ReachableTime = %d, want = %d", got, want)
-	}
+			if got := ra.OtherConfFlag(); got != test.otherConfFlag {
+				t.Errorf("got OtherConfFlag() = %t, want = %t", got, test.otherConfFlag)
+			}
+
+			if got := ra.DefaultRouterPreference(); got != test.prf {
+				t.Errorf("got DefaultRouterPreference() = %d, want = %d", got, test.prf)
+			}
 
-	if got, want := ra.RetransTimer(), time.Millisecond*117967114; got != want {
-		t.Errorf("got ra.RetransTimer = %d, want = %d", got, want)
+			if got, want := ra.RouterLifetime(), time.Second*time.Duration(test.routerLifetimeS); got != want {
+				t.Errorf("got ra.RouterLifetime() = %d, want = %d", got, want)
+			}
+
+			if got, want := ra.ReachableTime(), time.Millisecond*time.Duration(test.reachableTimeMS); got != want {
+				t.Errorf("got ra.ReachableTime() = %d, want = %d", got, want)
+			}
+
+			if got, want := ra.RetransTimer(), time.Millisecond*time.Duration(test.retransTimerMS); got != want {
+				t.Errorf("got ra.RetransTimer() = %d, want = %d", got, want)
+			}
+		})
 	}
 }