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>

net/netcheck/netcheck.go

@@ -76,6 +76,7 @@ type Report struct {
 	IPv4        bool // an IPv4 STUN round trip completed
 	IPv6CanSend bool // an IPv6 packet was able to be sent
 	IPv4CanSend bool // an IPv4 packet was able to be sent
+	OSHasIPv6   bool // could bind a socket to ::1
 
 	// MappingVariesByDestIP is whether STUN results depend which
 	// STUN server you're talking to (on IPv4).
@@ -806,6 +807,14 @@ func (c *Client) GetReport(ctx context.Context, dm *tailcfg.DERPMap) (_ *Report,
 		return nil, err
 	}
 
+	// See if IPv6 works at all, or if it's been hard disabled at the
+	// OS level.
+	v6udp, err := netns.Listener(c.logf).ListenPacket(ctx, "udp6", "[::1]:0")
+	if err == nil {
+		rs.report.OSHasIPv6 = true
+		v6udp.Close()
+	}
+
 	// Create a UDP4 socket used for sending to our discovered IPv4 address.
 	rs.pc4Hair, err = netns.Listener(c.logf).ListenPacket(ctx, "udp4", ":0")
 	if err != nil {

net/netcheck/netcheck_test.go

@@ -111,6 +111,9 @@ func TestWorksWhenUDPBlocked(t *testing.T) {
 	// That's not relevant to this test, so just accept what we're
 	// given.
 	want.IPv4CanSend = r.IPv4CanSend
+	// OS IPv6 test is irrelevant here, accept whatever the current
+	// machine has.
+	want.OSHasIPv6 = r.OSHasIPv6
 
 	if !reflect.DeepEqual(r, want) {
 		t.Errorf("mismatch\n got: %+v\nwant: %+v\n", r, want)