ipn/ipnlocal: for IPv6-only nodes, publish IPv6 MagicDNS records of peers

See https://github.com/tailscale/tailscale/issues/2970#issuecomment-931885268

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

ipn/ipnlocal/dnsconfig_test.go

@@ -86,6 +86,40 @@ func TestDNSConfigForNetmap(t *testing.T) {
 				},
 			},
 		},
+		{
+			// An ephemeral node with only an IPv6 address
+			// should get IPv6 records for all its peers,
+			// even if they have IPv4.
+			name: "v6_only_self",
+			nm: &netmap.NetworkMap{
+				Name:      "myname.net",
+				Addresses: ipps("fe75::1"),
+				Peers: []*tailcfg.Node{
+					{
+						Name:      "peera.net",
+						Addresses: ipps("100.102.0.1", "100.102.0.2", "fe75::1001"),
+					},
+					{
+						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{},
+			want: &dns.Config{
+				Routes: map[dnsname.FQDN][]dnstype.Resolver{},
+				Hosts: map[dnsname.FQDN][]netaddr.IP{
+					"b.net.":       ips("fe75::2"),
+					"myname.net.":  ips("fe75::1"),
+					"peera.net.":   ips("fe75::1001"),
+					"v6-only.net.": ips("fe75::3"),
+				},
+			},
+		},
 		{
 			name: "extra_records",
 			nm: &netmap.NetworkMap{

ipn/ipnlocal/local.go

@@ -1817,6 +1817,10 @@ func dnsConfigForNetmap(nm *netmap.NetworkMap, prefs *ipn.Prefs, logf logger.Log
 		Hosts:  map[dnsname.FQDN][]netaddr.IP{},
 	}
 
+	// selfV6Only is whether we only have IPv6 addresses ourselves.
+	selfV6Only := tsaddr.PrefixesContainsFunc(nm.Addresses, tsaddr.PrefixIs6) &&
+		!tsaddr.PrefixesContainsFunc(nm.Addresses, tsaddr.PrefixIs4)
+
 	// Populate MagicDNS records. We do this unconditionally so that
 	// quad-100 can always respond to MagicDNS queries, even if the OS
 	// isn't configured to make MagicDNS resolution truly
@@ -1830,12 +1834,19 @@ 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() })
+		have4 := tsaddr.PrefixesContainsFunc(addrs, tsaddr.PrefixIs4)
 		var ips []netaddr.IP
 		for _, addr := range addrs {
-			// Remove IPv6 addresses for now, as we don't
-			// guarantee that the peer node actually can speak
-			// IPv6 correctly.
+			if selfV6Only {
+				if addr.IP().Is6() {
+					ips = append(ips, addr.IP())
+				}
+				continue
+			}
+			// If this node has an IPv4 address, then
+			// remove peers' IPv6 addresses for now, as we
+			// don't guarantee that the peer node actually
+			// can speak IPv6 correctly.
 			//
 			// https://github.com/tailscale/tailscale/issues/1152
 			// tracks adding the right capability reporting to

net/tsaddr/tsaddr.go

@@ -185,3 +185,9 @@ func IPsContainsFunc(ips []netaddr.IP, f func(netaddr.IP) bool) bool {
 	}
 	return false
 }
+
+// PrefixIs4 reports whether p is an IPv4 prefix.
+func PrefixIs4(p netaddr.IPPrefix) bool { return p.IP().Is4() }
+
+// PrefixIs6 reports whether p is an IPv6 prefix.
+func PrefixIs6(p netaddr.IPPrefix) bool { return p.IP().Is6() }