wgengine/netstack: make SOCKS5 resolve names to IPv6 if self node when no IPv4

For instance, ephemeral nodes with only IPv6 addresses can now
SOCKS5-dial out to names like "foo" and resolve foo's IPv6 address
rather than foo's IPv4 address and get a "no route"
(*tcpip.ErrNoRoute) error from netstack's dialer.

Per https://github.com/tailscale/tailscale/issues/2268#issuecomment-870027626
which is only part of the isuse.

Updates #2268

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

wgengine/netstack/netstack.go

@@ -177,21 +177,33 @@ type DNSMap map[string]netaddr.IP
 func DNSMapFromNetworkMap(nm *netmap.NetworkMap) DNSMap {
 	ret := make(DNSMap)
 	suffix := nm.MagicDNSSuffix()
-
+	have4 := false
 	if nm.Name != "" && len(nm.Addresses) > 0 {
 		ip := nm.Addresses[0].IP()
 		ret[strings.TrimRight(nm.Name, ".")] = ip
 		if dnsname.HasSuffix(nm.Name, suffix) {
 			ret[dnsname.TrimSuffix(nm.Name, suffix)] = ip
 		}
+		for _, a := range nm.Addresses {
+			if a.IP().Is4() {
+				have4 = true
+			}
+		}
 	}
 	for _, p := range nm.Peers {
-		if p.Name != "" && len(p.Addresses) > 0 {
-			ip := p.Addresses[0].IP()
+		if p.Name == "" {
+			continue
+		}
+		for _, a := range p.Addresses {
+			ip := a.IP()
+			if ip.Is4() && !have4 {
+				continue
+			}
 			ret[strings.TrimRight(p.Name, ".")] = ip
 			if dnsname.HasSuffix(p.Name, suffix) {
 				ret[dnsname.TrimSuffix(p.Name, suffix)] = ip
 			}
+			break
 		}
 	}
 	return ret

wgengine/netstack/netstack_test.go

@@ -0,0 +1,112 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package netstack
+
+import (
+	"reflect"
+	"testing"
+
+	"inet.af/netaddr"
+	"tailscale.com/tailcfg"
+	"tailscale.com/types/netmap"
+)
+
+func TestDNSMapFromNetworkMap(t *testing.T) {
+	pfx := netaddr.MustParseIPPrefix
+	ip := netaddr.MustParseIP
+	tests := []struct {
+		name string
+		nm   *netmap.NetworkMap
+		want DNSMap
+	}{
+		{
+			name: "self",
+			nm: &netmap.NetworkMap{
+				Name: "foo.tailnet",
+				Addresses: []netaddr.IPPrefix{
+					pfx("100.102.103.104/32"),
+					pfx("100::123/128"),
+				},
+			},
+			want: DNSMap{
+				"foo":         ip("100.102.103.104"),
+				"foo.tailnet": ip("100.102.103.104"),
+			},
+		},
+		{
+			name: "self_and_peers",
+			nm: &netmap.NetworkMap{
+				Name: "foo.tailnet",
+				Addresses: []netaddr.IPPrefix{
+					pfx("100.102.103.104/32"),
+					pfx("100::123/128"),
+				},
+				Peers: []*tailcfg.Node{
+					{
+						Name: "a.tailnet",
+						Addresses: []netaddr.IPPrefix{
+							pfx("100.0.0.201/32"),
+							pfx("100::201/128"),
+						},
+					},
+					{
+						Name: "b.tailnet",
+						Addresses: []netaddr.IPPrefix{
+							pfx("100::202/128"),
+						},
+					},
+				},
+			},
+			want: DNSMap{
+				"foo":         ip("100.102.103.104"),
+				"foo.tailnet": ip("100.102.103.104"),
+				"a":           ip("100.0.0.201"),
+				"a.tailnet":   ip("100.0.0.201"),
+				"b":           ip("100::202"),
+				"b.tailnet":   ip("100::202"),
+			},
+		},
+		{
+			name: "self_has_v6_only",
+			nm: &netmap.NetworkMap{
+				Name: "foo.tailnet",
+				Addresses: []netaddr.IPPrefix{
+					pfx("100::123/128"),
+				},
+				Peers: []*tailcfg.Node{
+					{
+						Name: "a.tailnet",
+						Addresses: []netaddr.IPPrefix{
+							pfx("100.0.0.201/32"),
+							pfx("100::201/128"),
+						},
+					},
+					{
+						Name: "b.tailnet",
+						Addresses: []netaddr.IPPrefix{
+							pfx("100::202/128"),
+						},
+					},
+				},
+			},
+			want: DNSMap{
+				"foo":         ip("100::123"),
+				"foo.tailnet": ip("100::123"),
+				"a":           ip("100::201"),
+				"a.tailnet":   ip("100::201"),
+				"b":           ip("100::202"),
+				"b.tailnet":   ip("100::202"),
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := DNSMapFromNetworkMap(tt.nm)
+			if !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("mismatch:\n got %v\nwant %v\n", got, tt.want)
+			}
+		})
+	}
+}