net/netcheck: test for OS IPv6 support as well as connectivity.

This lets us distinguish "no IPv6 because the device's ISP doesn't
offer IPv6" from "IPv6 is unavailable/disabled in the OS".

Signed-off-by: David Anderson <danderson@tailscale.com>

tailcfg/tailcfg.go

@@ -496,10 +496,14 @@ type NetInfo struct {
 	// It reports true even if there's no NAT involved.
 	HairPinning opt.Bool
 
-	// WorkingIPv6 is whether IPv6 works.
+	// WorkingIPv6 is whether the host has IPv6 internet connectivity.
 	WorkingIPv6 opt.Bool
 
-	// WorkingUDP is whether UDP works.
+	// OSHasIPv6 is whether the OS supports IPv6 at all, regardless of
+	// whether IPv6 internet connectivity is available.
+	OSHasIPv6 opt.Bool
+
+	// WorkingUDP is whether the host has UDP internet connectivity.
 	WorkingUDP opt.Bool
 
 	// HavePortMap is whether we have an existing portmap open
@@ -590,6 +594,7 @@ func (ni *NetInfo) BasicallyEqual(ni2 *NetInfo) bool {
 	return ni.MappingVariesByDestIP == ni2.MappingVariesByDestIP &&
 		ni.HairPinning == ni2.HairPinning &&
 		ni.WorkingIPv6 == ni2.WorkingIPv6 &&
+		ni.OSHasIPv6 == ni2.OSHasIPv6 &&
 		ni.WorkingUDP == ni2.WorkingUDP &&
 		ni.HavePortMap == ni2.HavePortMap &&
 		ni.UPnP == ni2.UPnP &&

tailcfg/tailcfg_clone.go

@@ -154,6 +154,7 @@ var _NetInfoCloneNeedsRegeneration = NetInfo(struct {
 	MappingVariesByDestIP opt.Bool
 	HairPinning           opt.Bool
 	WorkingIPv6           opt.Bool
+	OSHasIPv6             opt.Bool
 	WorkingUDP            opt.Bool
 	HavePortMap           bool
 	UPnP                  opt.Bool

tailcfg/tailcfg_test.go

@@ -500,6 +500,7 @@ func TestNetInfoFields(t *testing.T) {
 		"MappingVariesByDestIP",
 		"HairPinning",
 		"WorkingIPv6",
+		"OSHasIPv6",
 		"WorkingUDP",
 		"HavePortMap",
 		"UPnP",

tailcfg/tailcfg_view.go

@@ -338,6 +338,7 @@ func (v *NetInfoView) UnmarshalJSON(b []byte) error {
 func (v NetInfoView) MappingVariesByDestIP() opt.Bool { return v.ж.MappingVariesByDestIP }
 func (v NetInfoView) HairPinning() opt.Bool           { return v.ж.HairPinning }
 func (v NetInfoView) WorkingIPv6() opt.Bool           { return v.ж.WorkingIPv6 }
+func (v NetInfoView) OSHasIPv6() opt.Bool             { return v.ж.OSHasIPv6 }
 func (v NetInfoView) WorkingUDP() opt.Bool            { return v.ж.WorkingUDP }
 func (v NetInfoView) HavePortMap() bool               { return v.ж.HavePortMap }
 func (v NetInfoView) UPnP() opt.Bool                  { return v.ж.UPnP }
@@ -354,6 +355,7 @@ var _NetInfoViewNeedsRegeneration = NetInfo(struct {
 	MappingVariesByDestIP opt.Bool
 	HairPinning           opt.Bool
 	WorkingIPv6           opt.Bool
+	OSHasIPv6             opt.Bool
 	WorkingUDP            opt.Bool
 	HavePortMap           bool
 	UPnP                  opt.Bool