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tailscale/wgengine/router/dns_networkmanager.go

210 lines
6.8 KiB
Go

// 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.
// +build linux
package router
import (
"bufio"
"bytes"
"context"
"encoding/binary"
"fmt"
"os"
"os/exec"
"github.com/godbus/dbus/v5"
)
type nmSettings map[string]map[string]dbus.Variant
// nmIsActive determines if NetworkManager is currently managing system DNS settings.
func nmIsActive() bool {
// This is somewhat tricky because NetworkManager supports a number
// of DNS configuration modes. In all cases, we expect it to be installed
// and /etc/resolv.conf to contain a mention of NetworkManager in the comments.
_, err := exec.LookPath("NetworkManager")
if err != nil {
return false
}
f, err := os.Open("/etc/resolv.conf")
if err != nil {
return false
}
defer f.Close()
scanner := bufio.NewScanner(f)
for scanner.Scan() {
line := scanner.Bytes()
// Look for the word "NetworkManager" until comments end.
if len(line) > 0 && line[0] != '#' {
return false
}
if bytes.Contains(line, []byte("NetworkManager")) {
return true
}
}
return false
}
// dnsNetworkManagerUp updates the DNS config for the Tailscale interface
// through the NetworkManager DBus API.
func dnsNetworkManagerUp(config DNSConfig, interfaceName string) error {
ctx, cancel := context.WithTimeout(context.Background(), dnsReconfigTimeout)
defer cancel()
conn, err := dbus.SystemBus()
if err != nil {
return fmt.Errorf("connecting to system bus: %w", err)
}
defer conn.Close()
// This is how we get at the DNS settings:
// org.freedesktop.NetworkManager
// ⇩
// org.freedesktop.NetworkManager.Device
// (describes a network interface)
// ⇩
// org.freedesktop.NetworkManager.Connection.Active
// (active instance of a connection initialized from settings)
// ⇩
// org.freedesktop.NetworkManager.Connection
// (connection settings)
// contains {dns, dns-priority, dns-search}
//
// Ref: https://developer.gnome.org/NetworkManager/stable/settings-ipv4.html.
nm := conn.Object(
"org.freedesktop.NetworkManager",
dbus.ObjectPath("/org/freedesktop/NetworkManager"),
)
var devicePath dbus.ObjectPath
err = nm.CallWithContext(
ctx, "org.freedesktop.NetworkManager.GetDeviceByIpIface", 0,
interfaceName,
).Store(&devicePath)
if err != nil {
return fmt.Errorf("GetDeviceByIpIface: %w", err)
}
device := conn.Object("org.freedesktop.NetworkManager", devicePath)
var activeConnPath dbus.ObjectPath
err = device.CallWithContext(
ctx, "org.freedesktop.DBus.Properties.Get", 0,
"org.freedesktop.NetworkManager.Device", "ActiveConnection",
).Store(&activeConnPath)
if err != nil {
return fmt.Errorf("getting ActiveConnection: %w", err)
}
activeConn := conn.Object("org.freedesktop.NetworkManager", activeConnPath)
var connPath dbus.ObjectPath
err = activeConn.CallWithContext(
ctx, "org.freedesktop.DBus.Properties.Get", 0,
"org.freedesktop.NetworkManager.Connection.Active", "Connection",
).Store(&connPath)
if err != nil {
return fmt.Errorf("getting Connection: %w", err)
}
connection := conn.Object("org.freedesktop.NetworkManager", connPath)
// Note: strictly speaking, the following is not safe.
//
// It appears that the way to update connection settings
// in NetworkManager is to get an entire connection settings object,
// modify the fields we are interested in, then submit the modified object.
//
// This is unfortunate: if the network state changes in the meantime
// (most relevantly to us, if routes change), we will overwrite those changes.
//
// That said, fortunately, this should have no real effect, as Tailscale routes
// do not seem to show up in NetworkManager at all,
// so they are presumably immune from being tampered with.
var settings nmSettings
err = connection.CallWithContext(
ctx, "org.freedesktop.NetworkManager.Settings.Connection.GetSettings", 0,
).Store(&settings)
if err != nil {
return fmt.Errorf("getting Settings: %w", err)
}
// Frustratingly, NetworkManager represents IPv4 addresses as uint32s,
// although IPv6 addresses are represented as byte arrays.
// Perform the conversion here.
var (
dnsv4 []uint32
dnsv6 [][]byte
)
for _, ip := range config.Nameservers {
b := ip.As16()
if ip.Is4() {
dnsv4 = append(dnsv4, binary.BigEndian.Uint32(b[12:]))
} else {
dnsv6 = append(dnsv6, b[:])
}
}
ipv4Map := settings["ipv4"]
ipv4Map["dns"] = dbus.MakeVariant(dnsv4)
ipv4Map["dns-search"] = dbus.MakeVariant(config.Domains)
// dns-priority = -1 ensures that we have priority
// over other interfaces, except those exploiting this same trick.
// Ref: https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1211110/comments/92.
ipv4Map["dns-priority"] = dbus.MakeVariant(-1)
// In principle, we should not need set this to true,
// as our interface does not configure any automatic DNS settings (presumably via DHCP).
// All the same, better to be safe.
ipv4Map["ignore-auto-dns"] = dbus.MakeVariant(true)
ipv6Map := settings["ipv6"]
// This is a hack.
// Methods "disabled", "ignore", "link-local" (IPv6 default) prevent us from setting DNS.
// It seems that our only recourse is "manual" or "auto".
// "manual" requires addresses, so we use "auto", which will assign us a random IPv6 /64.
ipv6Map["method"] = dbus.MakeVariant("auto")
// Our IPv6 config is a fake, so it should never become the default route.
ipv6Map["never-default"] = dbus.MakeVariant(true)
// Moreover, we should ignore all autoconfigured routes (hopefully none), as they are bogus.
ipv6Map["ignore-auto-routes"] = dbus.MakeVariant(true)
// Finally, set the actual DNS config.
ipv6Map["dns"] = dbus.MakeVariant(dnsv6)
ipv6Map["dns-search"] = dbus.MakeVariant(config.Domains)
ipv6Map["dns-priority"] = dbus.MakeVariant(-1)
ipv6Map["ignore-auto-dns"] = dbus.MakeVariant(true)
// deprecatedProperties are the properties in interface settings
// that are deprecated by NetworkManager.
//
// In practice, this means that they are returned for reading,
// but submitting a settings object with them present fails
// with hard-to-diagnose errors. They must be removed.
deprecatedProperties := []string{
"addresses", "routes",
}
for _, property := range deprecatedProperties {
delete(ipv4Map, property)
delete(ipv6Map, property)
}
err = connection.CallWithContext(
ctx, "org.freedesktop.NetworkManager.Settings.Connection.UpdateUnsaved", 0, settings,
).Store()
if err != nil {
return fmt.Errorf("setting Settings: %w", err)
}
return nil
}
// dnsNetworkManagerDown undoes the changes made by dnsNetworkManagerUp.
func dnsNetworkManagerDown(interfaceName string) error {
return dnsNetworkManagerUp(DNSConfig{Nameservers: nil, Domains: nil}, interfaceName)
}