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tailscale/net/dns/resolved.go

416 lines
13 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.
//go:build linux
// +build linux
package dns
import (
"context"
"errors"
"fmt"
"net"
"strings"
"time"
"github.com/godbus/dbus/v5"
"golang.org/x/sys/unix"
"tailscale.com/health"
"tailscale.com/logtail/backoff"
"tailscale.com/net/netaddr"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
)
// resolvedListenAddr is the listen address of the resolved stub resolver.
//
// We only consider resolved to be the system resolver if the stub resolver is;
// that is, if this address is the sole nameserver in /etc/resolved.conf.
// In other cases, resolved may be managing the system DNS configuration directly.
// Then the nameserver list will be a concatenation of those for all
// the interfaces that register their interest in being a default resolver with
//
// SetLinkDomains([]{{"~.", true}, ...})
//
// which includes at least the interface with the default route, i.e. not us.
// This does not work for us: there is a possibility of getting NXDOMAIN
// from the other nameservers before we are asked or get a chance to respond.
// We consider this case as lacking resolved support and fall through to dnsDirect.
//
// While it may seem that we need to read a config option to get at this,
// this address is, in fact, hard-coded into resolved.
var resolvedListenAddr = netaddr.IPv4(127, 0, 0, 53)
var errNotReady = errors.New("interface not ready")
// DBus entities we talk to.
//
// DBus is an RPC bus. In particular, the bus we're talking to is the
// system-wide bus (there is also a per-user session bus for
// user-specific applications).
//
// Daemons connect to the bus, and advertise themselves under a
// well-known object name. That object exposes paths, and each path
// implements one or more interfaces that contain methods, properties,
// and signals.
//
// Clients connect to the bus and walk that same hierarchy to invoke
// RPCs, get/set properties, or listen for signals.
const (
dbusResolvedObject = "org.freedesktop.resolve1"
dbusResolvedPath dbus.ObjectPath = "/org/freedesktop/resolve1"
dbusResolvedInterface = "org.freedesktop.resolve1.Manager"
dbusPath dbus.ObjectPath = "/org/freedesktop/DBus"
dbusInterface = "org.freedesktop.DBus"
dbusOwnerSignal = "NameOwnerChanged" // broadcast when a well-known name's owning process changes.
)
type resolvedLinkNameserver struct {
Family int32
Address []byte
}
type resolvedLinkDomain struct {
Domain string
RoutingOnly bool
}
// changeRequest tracks latest OSConfig and related error responses to update.
type changeRequest struct {
config OSConfig // configs OSConfigs, one per each SetDNS call
res chan<- error // response channel
}
// resolvedManager is an OSConfigurator which uses the systemd-resolved DBus API.
type resolvedManager struct {
ctx context.Context
cancel func() // terminate the context, for close
logf logger.Logf
ifidx int
configCR chan changeRequest // tracks OSConfigs changes and error responses
}
func newResolvedManager(logf logger.Logf, interfaceName string) (*resolvedManager, error) {
iface, err := net.InterfaceByName(interfaceName)
if err != nil {
return nil, err
}
ctx, cancel := context.WithCancel(context.Background())
logf = logger.WithPrefix(logf, "dns: ")
mgr := &resolvedManager{
ctx: ctx,
cancel: cancel,
logf: logf,
ifidx: iface.Index,
configCR: make(chan changeRequest),
}
go mgr.run(ctx)
return mgr, nil
}
func (m *resolvedManager) SetDNS(config OSConfig) error {
errc := make(chan error, 1)
defer close(errc)
select {
case <-m.ctx.Done():
return m.ctx.Err()
case m.configCR <- changeRequest{config, errc}:
}
select {
case <-m.ctx.Done():
return m.ctx.Err()
case err := <-errc:
if err != nil {
m.logf("failed to configure resolved: %v", err)
}
return err
}
}
func (m *resolvedManager) run(ctx context.Context) {
var (
conn *dbus.Conn
signals chan *dbus.Signal
rManager dbus.BusObject // rManager is the Resolved DBus connection
)
bo := backoff.NewBackoff("resolved-dbus", m.logf, 30*time.Second)
needsReconnect := make(chan bool, 1)
defer func() {
if conn != nil {
conn.Close()
}
}()
// Reconnect the systemBus if disconnected.
reconnect := func() error {
var err error
signals = make(chan *dbus.Signal, 16)
conn, err = dbus.SystemBus()
if err != nil {
m.logf("dbus connection error: %v", err)
} else {
m.logf("[v1] dbus connected")
}
if err != nil {
// Backoff increases time between reconnect attempts.
go func() {
bo.BackOff(ctx, err)
needsReconnect <- true
}()
return err
}
rManager = conn.Object(dbusResolvedObject, dbus.ObjectPath(dbusResolvedPath))
// Only receive the DBus signals we need to resync our config on
// resolved restart. Failure to set filters isn't a fatal error,
// we'll just receive all broadcast signals and have to ignore
// them on our end.
if err = conn.AddMatchSignal(dbus.WithMatchObjectPath(dbusPath), dbus.WithMatchInterface(dbusInterface), dbus.WithMatchMember(dbusOwnerSignal), dbus.WithMatchArg(0, dbusResolvedObject)); err != nil {
m.logf("[v1] Setting DBus signal filter failed: %v", err)
}
conn.Signal(signals)
// Reset backoff and SetNSOSHealth after successful on reconnect.
bo.BackOff(ctx, nil)
health.SetDNSOSHealth(nil)
return nil
}
// Create initial systemBus connection.
reconnect()
lastConfig := OSConfig{}
for {
select {
case <-ctx.Done():
if rManager == nil {
return
}
// RevertLink resets all per-interface settings on systemd-resolved to defaults.
// When ctx goes away systemd-resolved auto reverts.
// Keeping for potential use in future refactor.
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".RevertLink", 0, m.ifidx); call.Err != nil {
m.logf("[v1] RevertLink: %v", call.Err)
return
}
return
case configCR := <-m.configCR:
// Track and update sync with latest config change.
lastConfig = configCR.config
if rManager == nil {
configCR.res <- fmt.Errorf("resolved DBus does not have a connection")
continue
}
err := m.setConfigOverDBus(ctx, rManager, configCR.config)
configCR.res <- err
case <-needsReconnect:
if err := reconnect(); err != nil {
m.logf("[v1] SystemBus reconnect error %T", err)
}
continue
case signal, ok := <-signals:
// If signal ends and is nil then program tries to reconnect.
if !ok {
if err := reconnect(); err != nil {
m.logf("[v1] SystemBus reconnect error %T", err)
}
continue
}
// In theory the signal was filtered by DBus, but if
// AddMatchSignal in the constructor failed, we may be
// getting other spam.
if signal.Path != dbusPath || signal.Name != dbusInterface+"."+dbusOwnerSignal {
continue
}
if lastConfig.IsZero() {
continue
}
// signal.Body is a []any of 3 strings: bus name, previous owner, new owner.
if len(signal.Body) != 3 {
m.logf("[unexpected] DBus NameOwnerChanged len(Body) = %d, want 3")
}
if name, ok := signal.Body[0].(string); !ok || name != dbusResolvedObject {
continue
}
newOwner, ok := signal.Body[2].(string)
if !ok {
m.logf("[unexpected] DBus NameOwnerChanged.new_owner is a %T, not a string", signal.Body[2])
}
if newOwner == "" {
// systemd-resolved left the bus, no current owner,
// nothing to do.
continue
}
// The resolved bus name has a new owner, meaning resolved
// restarted. Reprogram current config.
m.logf("systemd-resolved restarted, syncing DNS config")
err := m.setConfigOverDBus(ctx, rManager, lastConfig)
// Set health while holding the lock, because this will
// graciously serialize the resync's health outcome with a
// concurrent SetDNS call.
health.SetDNSOSHealth(err)
if err != nil {
m.logf("failed to configure systemd-resolved: %v", err)
}
}
}
}
// setConfigOverDBus updates resolved DBus config and is only called from the run goroutine.
func (m *resolvedManager) setConfigOverDBus(ctx context.Context, rManager dbus.BusObject, config OSConfig) error {
ctx, cancel := context.WithTimeout(ctx, reconfigTimeout)
defer cancel()
var linkNameservers = make([]resolvedLinkNameserver, len(config.Nameservers))
for i, server := range config.Nameservers {
ip := server.As16()
if server.Is4() {
linkNameservers[i] = resolvedLinkNameserver{
Family: unix.AF_INET,
Address: ip[12:],
}
} else {
linkNameservers[i] = resolvedLinkNameserver{
Family: unix.AF_INET6,
Address: ip[:],
}
}
}
err := rManager.CallWithContext(
ctx, dbusResolvedInterface+".SetLinkDNS", 0,
m.ifidx, linkNameservers,
).Store()
if err != nil {
return fmt.Errorf("setLinkDNS: %w", err)
}
linkDomains := make([]resolvedLinkDomain, 0, len(config.SearchDomains)+len(config.MatchDomains))
seenDomains := map[dnsname.FQDN]bool{}
for _, domain := range config.SearchDomains {
if seenDomains[domain] {
continue
}
seenDomains[domain] = true
linkDomains = append(linkDomains, resolvedLinkDomain{
Domain: domain.WithTrailingDot(),
RoutingOnly: false,
})
}
for _, domain := range config.MatchDomains {
if seenDomains[domain] {
// Search domains act as both search and match in
// resolved, so it's correct to skip.
continue
}
seenDomains[domain] = true
linkDomains = append(linkDomains, resolvedLinkDomain{
Domain: domain.WithTrailingDot(),
RoutingOnly: true,
})
}
if len(config.MatchDomains) == 0 && len(config.Nameservers) > 0 {
// Caller requested full DNS interception, install a
// routing-only root domain.
linkDomains = append(linkDomains, resolvedLinkDomain{
Domain: ".",
RoutingOnly: true,
})
}
err = rManager.CallWithContext(
ctx, dbusResolvedInterface+".SetLinkDomains", 0,
m.ifidx, linkDomains,
).Store()
if err != nil && err.Error() == "Argument list too long" { // TODO: better error match
// Issue 3188: older systemd-resolved had argument length limits.
// Trim out the *.arpa. entries and try again.
err = rManager.CallWithContext(
ctx, dbusResolvedInterface+".SetLinkDomains", 0,
m.ifidx, linkDomainsWithoutReverseDNS(linkDomains),
).Store()
}
if err != nil {
return fmt.Errorf("setLinkDomains: %w", err)
}
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".SetLinkDefaultRoute", 0, m.ifidx, len(config.MatchDomains) == 0); call.Err != nil {
if dbusErr, ok := call.Err.(dbus.Error); ok && dbusErr.Name == dbus.ErrMsgUnknownMethod.Name {
// on some older systems like Kubuntu 18.04.6 with systemd 237 method SetLinkDefaultRoute is absent,
// but otherwise it's working good
m.logf("[v1] failed to set SetLinkDefaultRoute: %v", call.Err)
} else {
return fmt.Errorf("setLinkDefaultRoute: %w", call.Err)
}
}
// Some best-effort setting of things, but resolved should do the
// right thing if these fail (e.g. a really old resolved version
// or something).
// Disable LLMNR, we don't do multicast.
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".SetLinkLLMNR", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable LLMNR: %v", call.Err)
}
// Disable mdns.
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".SetLinkMulticastDNS", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable mdns: %v", call.Err)
}
// We don't support dnssec consistently right now, force it off to
// avoid partial failures when we split DNS internally.
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".SetLinkDNSSEC", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable DNSSEC: %v", call.Err)
}
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".SetLinkDNSOverTLS", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable DoT: %v", call.Err)
}
if call := rManager.CallWithContext(ctx, dbusResolvedInterface+".FlushCaches", 0); call.Err != nil {
m.logf("failed to flush resolved DNS cache: %v", call.Err)
}
return nil
}
func (m *resolvedManager) SupportsSplitDNS() bool {
return true
}
func (m *resolvedManager) GetBaseConfig() (OSConfig, error) {
return OSConfig{}, ErrGetBaseConfigNotSupported
}
func (m *resolvedManager) Close() error {
m.cancel() // stops the 'run' method goroutine
return nil
}
// linkDomainsWithoutReverseDNS returns a copy of v without
// *.arpa. entries.
func linkDomainsWithoutReverseDNS(v []resolvedLinkDomain) (ret []resolvedLinkDomain) {
for _, d := range v {
if strings.HasSuffix(d.Domain, ".arpa.") {
// Oh well. At least the rest will work.
continue
}
ret = append(ret, d)
}
return ret
}