1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
|
// Copyright (c) 2020 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 dns
import (
"context"
"errors"
"fmt"
"io/ioutil"
"os"
"os/exec"
"time"
"github.com/godbus/dbus/v5"
"inet.af/netaddr"
"tailscale.com/types/logger"
"tailscale.com/util/cmpver"
)
type kv struct {
k, v string
}
func (kv kv) String() string {
return fmt.Sprintf("%s=%s", kv.k, kv.v)
}
func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurator, err error) {
var debug []kv
dbg := func(k, v string) {
debug = append(debug, kv{k, v})
}
defer func() {
if ret != nil {
dbg("ret", fmt.Sprintf("%T", ret))
}
logf("dns: %v", debug)
}()
bs, err := ioutil.ReadFile("/etc/resolv.conf")
if os.IsNotExist(err) {
dbg("rc", "missing")
return newDirectManager(), nil
}
if err != nil {
return nil, fmt.Errorf("reading /etc/resolv.conf: %w", err)
}
switch resolvOwner(bs) {
case "systemd-resolved":
dbg("rc", "resolved")
// Some systems, for reasons known only to them, have a
// resolv.conf that has the word "systemd-resolved" in its
// header, but doesn't actually point to resolved. We mustn't
// try to program resolved in that case.
// https://github.com/tailscale/tailscale/issues/2136
if err := resolvedIsActuallyResolver(); err != nil {
dbg("resolved", "not-in-use")
return newDirectManager(), nil
}
if err := dbusPing("org.freedesktop.resolve1", "/org/freedesktop/resolve1"); err != nil {
dbg("resolved", "no")
return newDirectManager(), nil
}
if err := dbusPing("org.freedesktop.NetworkManager", "/org/freedesktop/NetworkManager/DnsManager"); err != nil {
dbg("nm", "no")
return newResolvedManager(logf, interfaceName)
}
dbg("nm", "yes")
if err := nmIsUsingResolved(); err != nil {
dbg("nm-resolved", "no")
return newResolvedManager(logf, interfaceName)
}
dbg("nm-resolved", "yes")
// Version of NetworkManager before 1.26.6 programmed resolved
// incorrectly, such that NM's settings would always take
// precedence over other settings set by other resolved
// clients.
//
// If we're dealing with such a version, we have to set our
// DNS settings through NM to have them take.
//
// However, versions 1.26.6 later both fixed the resolved
// programming issue _and_ started ignoring DNS settings for
// "unmanaged" interfaces - meaning NM 1.26.6 and later
// actively ignore DNS configuration we give it. So, for those
// NM versions, we can and must use resolved directly.
//
// Even more fun, even-older versions of NM won't let us set
// DNS settings if the interface isn't managed by NM, with a
// hard failure on DBus requests. Empirically, NM 1.22 does
// this. Based on the versions popular distros shipped, we
// conservatively decree that only 1.26.0 through 1.26.5 are
// "safe" to use for our purposes. This roughly matches
// distros released in the latter half of 2020.
//
// In a perfect world, we'd avoid this by replacing
// configuration out from under NM entirely (e.g. using
// directManager to overwrite resolv.conf), but in a world
// where resolved runs, we need to get correct configuration
// into resolved regardless of what's in resolv.conf (because
// resolved can also be queried over dbus, or via an NSS
// module that bypasses /etc/resolv.conf). Given that we must
// get correct configuration into resolved, we have no choice
// but to use NM, and accept the loss of IPv6 configuration
// that comes with it (see
// https://github.com/tailscale/tailscale/issues/1699,
// https://github.com/tailscale/tailscale/pull/1945)
safe, err := nmVersionBetween("1.26.0", "1.26.5")
if err != nil {
// Failed to figure out NM's version, can't make a correct
// decision.
return nil, fmt.Errorf("checking NetworkManager version: %v", err)
}
if safe {
dbg("nm-safe", "yes")
return newNMManager(interfaceName)
}
dbg("nm-safe", "no")
return newResolvedManager(logf, interfaceName)
case "resolvconf":
dbg("rc", "resolvconf")
if _, err := exec.LookPath("resolvconf"); err != nil {
dbg("resolvconf", "no")
return newDirectManager(), nil
}
dbg("resolvconf", "yes")
return newResolvconfManager(logf)
case "NetworkManager":
// You'd think we would use newNMManager somewhere in
// here. However, as explained in
// https://github.com/tailscale/tailscale/issues/1699 , using
// NetworkManager for DNS configuration carries with it the
// cost of losing IPv6 configuration on the Tailscale network
// interface. So, when we can avoid it, we bypass
// NetworkManager by replacing resolv.conf directly.
//
// If you ever try to put NMManager back here, keep in mind
// that versions >=1.26.6 will ignore DNS configuration
// anyway, so you still need a fallback path that uses
// directManager.
dbg("rc", "nm")
return newDirectManager(), nil
default:
dbg("rc", "unknown")
return newDirectManager(), nil
}
}
func nmVersionBetween(first, last string) (bool, error) {
conn, err := dbus.SystemBus()
if err != nil {
// DBus probably not running.
return false, err
}
nm := conn.Object("org.freedesktop.NetworkManager", dbus.ObjectPath("/org/freedesktop/NetworkManager"))
v, err := nm.GetProperty("org.freedesktop.NetworkManager.Version")
if err != nil {
return false, err
}
version, ok := v.Value().(string)
if !ok {
return false, fmt.Errorf("unexpected type %T for NM version", v.Value())
}
outside := cmpver.Compare(version, first) < 0 || cmpver.Compare(version, last) > 0
return !outside, nil
}
func nmIsUsingResolved() error {
conn, err := dbus.SystemBus()
if err != nil {
// DBus probably not running.
return err
}
nm := conn.Object("org.freedesktop.NetworkManager", dbus.ObjectPath("/org/freedesktop/NetworkManager/DnsManager"))
v, err := nm.GetProperty("org.freedesktop.NetworkManager.DnsManager.Mode")
if err != nil {
return fmt.Errorf("getting NM mode: %w", err)
}
mode, ok := v.Value().(string)
if !ok {
return fmt.Errorf("unexpected type %T for NM DNS mode", v.Value())
}
if mode != "systemd-resolved" {
return errors.New("NetworkManager is not using systemd-resolved for DNS")
}
return nil
}
func resolvedIsActuallyResolver() error {
cfg, err := newDirectManager().readResolvConf()
if err != nil {
return err
}
if len(cfg.Nameservers) != 1 || cfg.Nameservers[0] != netaddr.IPv4(127, 0, 0, 53) {
return errors.New("resolv.conf doesn't point to systemd-resolved")
}
return nil
}
func dbusPing(name, objectPath string) error {
ctx, cancel := context.WithTimeout(context.Background(), time.Second)
defer cancel()
conn, err := dbus.SystemBus()
if err != nil {
// DBus probably not running.
return err
}
obj := conn.Object(name, dbus.ObjectPath(objectPath))
call := obj.CallWithContext(ctx, "org.freedesktop.DBus.Peer.Ping", 0)
return call.Err
}
|
