tailscale/net/dns/resolved.go

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// 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"
"github.com/godbus/dbus/v5"
"golang.org/x/sys/unix"
"inet.af/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")
type resolvedLinkNameserver struct {
Family int32
Address []byte
}
type resolvedLinkDomain struct {
Domain string
RoutingOnly bool
}
// isResolvedActive determines if resolved is currently managing system DNS settings.
func isResolvedActive() bool {
ctx, cancel := context.WithTimeout(context.Background(), reconfigTimeout)
defer cancel()
conn, err := dbus.SystemBus()
if err != nil {
// Probably no DBus on the system, or we're not allowed to use
// it. Cannot control resolved.
return false
}
rd := conn.Object("org.freedesktop.resolve1", dbus.ObjectPath("/org/freedesktop/resolve1"))
call := rd.CallWithContext(ctx, "org.freedesktop.DBus.Peer.Ping", 0)
if call.Err != nil {
// Can't talk to resolved.
return false
}
config, err := newDirectManager(logger.Discard).readResolvFile(resolvConf)
if err != nil {
return false
}
// The sole nameserver must be the systemd-resolved stub.
if len(config.Nameservers) == 1 && config.Nameservers[0] == resolvedListenAddr {
return true
}
return false
}
// resolvedManager is an OSConfigurator which uses the systemd-resolved DBus API.
type resolvedManager struct {
logf logger.Logf
ifidx int
resolved dbus.BusObject
}
func newResolvedManager(logf logger.Logf, interfaceName string) (*resolvedManager, error) {
conn, err := dbus.SystemBus()
if err != nil {
return nil, err
}
iface, err := net.InterfaceByName(interfaceName)
if err != nil {
return nil, err
}
return &resolvedManager{
logf: logf,
ifidx: iface.Index,
resolved: conn.Object("org.freedesktop.resolve1", dbus.ObjectPath("/org/freedesktop/resolve1")),
}, nil
}
func (m *resolvedManager) SetDNS(config OSConfig) error {
ctx, cancel := context.WithTimeout(context.Background(), 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 := m.resolved.CallWithContext(
ctx, "org.freedesktop.resolve1.Manager.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 = m.resolved.CallWithContext(
ctx, "org.freedesktop.resolve1.Manager.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 = m.resolved.CallWithContext(
ctx, "org.freedesktop.resolve1.Manager.SetLinkDomains", 0,
m.ifidx, linkDomainsWithoutReverseDNS(linkDomains),
).Store()
}
if err != nil {
return fmt.Errorf("setLinkDomains: %w", err)
}
if call := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.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 := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.SetLinkLLMNR", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable LLMNR: %v", call.Err)
}
// Disable mdns.
if call := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.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 := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.SetLinkDNSSEC", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable DNSSEC: %v", call.Err)
}
if call := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.SetLinkDNSOverTLS", 0, m.ifidx, "no"); call.Err != nil {
m.logf("[v1] failed to disable DoT: %v", call.Err)
}
if call := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.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 {
ctx, cancel := context.WithTimeout(context.Background(), reconfigTimeout)
defer cancel()
if call := m.resolved.CallWithContext(ctx, "org.freedesktop.resolve1.Manager.RevertLink", 0, m.ifidx); call.Err != nil {
return fmt.Errorf("RevertLink: %w", call.Err)
}
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
}