ipn/ipnlocal: add MagicDNS records for IPv6-only nodes

Updates #2268

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/dnsconfig_test.go

@@ -68,6 +68,10 @@ func TestDNSConfigForNetmap(t *testing.T) {
 						Name:      "b.net",
 						Addresses: ipps("100.102.0.1", "100.102.0.2", "fe75::2"),
 					},
+					{
+						Name:      "v6-only.net",
+						Addresses: ipps("fe75::3"), // no IPv4, so we don't ignore IPv6
+					},
 				},
 			},
 			prefs: &ipn.Prefs{},
@@ -77,6 +81,7 @@ func TestDNSConfigForNetmap(t *testing.T) {
 					"b.net.":       ips("100.102.0.1", "100.102.0.2"),
 					"myname.net.":  ips("100.101.101.101"),
 					"peera.net.":   ips("100.102.0.1", "100.102.0.2"),
+					"v6-only.net.": ips("fe75::3"),
 				},
 			},
 		},

ipn/ipnlocal/local.go

@@ -1798,6 +1798,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 		if err != nil {
 			return // TODO: propagate error?
 		}
+		have4 := tsaddr.PrefixesContainsFunc(addrs, func(p netaddr.IPPrefix) bool { return p.IP().Is4() })
 		var ips []netaddr.IP
 		for _, addr := range addrs {
 			// Remove IPv6 addresses for now, as we don't
@@ -1807,7 +1808,7 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 			// https://github.com/tailscale/tailscale/issues/1152
 			// tracks adding the right capability reporting to
 			// enable AAAA in MagicDNS.
-			if addr.IP().Is6() {
+			if addr.IP().Is6() && have4 {
 				continue
 			}
 			ips = append(ips, addr.IP())

net/tsaddr/tsaddr.go

@@ -164,3 +164,24 @@ func NewContainsIPFunc(addrs []netaddr.IPPrefix) func(ip netaddr.IP) bool {
 	}
 	return func(ip netaddr.IP) bool { return m[ip] }
 }
+
+// PrefixesContainsFunc reports whether f is true for any IPPrefix in
+// ipp.
+func PrefixesContainsFunc(ipp []netaddr.IPPrefix, f func(netaddr.IPPrefix) bool) bool {
+	for _, v := range ipp {
+		if f(v) {
+			return true
+		}
+	}
+	return false
+}
+
+// IPsContainsFunc reports whether f is true for any IP in ips.
+func IPsContainsFunc(ips []netaddr.IP, f func(netaddr.IP) bool) bool {
+	for _, v := range ips {
+		if f(v) {
+			return true
+		}
+	}
+	return false
+}