net/dns: implement a DNS override workaround for legacy resolvconf.

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -216,6 +216,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         debug/elf                                                    from rsc.io/goversion/version
         debug/macho                                                  from rsc.io/goversion/version
         debug/pe                                                     from rsc.io/goversion/version
+        embed                                                        from tailscale.com/net/dns
         encoding                                                     from encoding/json+
         encoding/asn1                                                from crypto/x509+
         encoding/base64                                              from encoding/json+

net/dns/direct.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux freebsd openbsd
-
 package dns
 
 import (

net/dns/resolvconf-workaround.sh

@@ -0,0 +1,55 @@
+#!/bin/sh
+# 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.
+#
+# This script is a workaround for a vpn-unfriendly behavior of the
+# original resolvconf by Thomas Hood. Unlike the `openresolv`
+# implementation (whose binary is also called resolvconf,
+# confusingly), the original resolvconf lacks a way to specify
+# "exclusive mode" for a provider configuration. In practice, this
+# means that if Tailscale wants to install a DNS configuration, that
+# config will get "blended" with the configs from other sources,
+# rather than override those other sources.
+#
+# This script gets installed at /etc/resolvconf/update-libc.d, which
+# is a directory of hook scripts that get run after resolvconf's libc
+# helper has finished rewriting /etc/resolv.conf. It's meant to notify
+# consumers of resolv.conf of a new configuration.
+#
+# Instead, we use that hook mechanism to reach into resolvconf's
+# stuff, and rewrite the libc-generated resolv.conf to exclusively
+# contain Tailscale's configuration - effectively implementing
+# exclusive mode ourselves in post-production.
+
+set -e
+
+if [ -n "$TAILSCALE_RESOLVCONF_HOOK_LOOP" ]; then
+	# Hook script being invoked by itself, skip.
+	exit 0
+fi
+
+if [ ! -f tun-tailscale.inet ]; then
+	# Tailscale isn't trying to manage DNS, do nothing.
+	exit 0
+fi
+
+if ! grep resolvconf /etc/resolv.conf >/dev/null; then
+	# resolvconf isn't managing /etc/resolv.conf, do nothing.
+	exit 0
+fi
+
+# Write out a modified /etc/resolv.conf containing just our config.
+(
+	if [ -f /etc/resolvconf/resolv.conf.d/head ]; then
+		cat /etc/resolvconf/resolv.conf.d/head
+	fi
+	echo "# Tailscale workaround applied to set exclusive DNS configuration."
+	cat tun-tailscale.inet
+) >/etc/resolv.conf
+
+if [ -d /etc/resolvconf/update-libc.d ] ; then
+	# Re-notify libc watchers that we've changed resolv.conf again.
+	export TAILSCALE_RESOLVCONF_HOOK_LOOP=1
+	exec run-parts /etc/resolvconf/update-libc.d
+fi

net/dns/resolvconf.go

@@ -2,20 +2,47 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build linux freebsd
-
 package dns
 
 import (
 	"bufio"
 	"bytes"
+	_ "embed"
 	"fmt"
 	"os"
 	"os/exec"
+	"path/filepath"
 
+	"tailscale.com/atomicfile"
 	"tailscale.com/types/logger"
 )
 
+//go:embed resolvconf-workaround.sh
+var legacyResolvconfScript []byte
+
+// resolvconfConfigName is the name of the config submitted to
+// resolvconf.
+// The name starts with 'tun' in order to match the hardcoded
+// interface order in debian resolvconf, which will place this
+// configuration ahead of regular network links. In theory, this
+// doesn't matter because we then fix things up to ensure our config
+// is the only one in use, but in case that fails, this will make our
+// configuration slightly preferred.
+// The 'inet' suffix has no specific meaning, but conventionally
+// resolvconf implementations encourage adding a suffix roughly
+// indicating where the config came from, and "inet" is the "none of
+// the above" value (rather than, say, "ppp" or "dhcp").
+const resolvconfConfigName = "tun-tailscale.inet"
+
+// resolvconfLibcHookPath is the directory containing libc update
+// scripts, which are run by Debian resolvconf when /etc/resolv.conf
+// has been updated.
+const resolvconfLibcHookPath = "/etc/resolvconf/update-libc.d"
+
+// resolvconfHookPath is the name of the libc hook script we install
+// to force Tailscale's DNS config to take effect.
+var resolvconfHookPath = filepath.Join(resolvconfLibcHookPath, "tailscale")
+
 // isResolvconfActive indicates whether the system appears to be using resolvconf.
 // If this is true, then directManager should be avoided:
 // resolvconf has exclusive ownership of /etc/resolv.conf.
@@ -98,24 +125,33 @@ func getResolvconfImpl() resolvconfImpl {
 }
 
 type resolvconfManager struct {
-	impl resolvconfImpl
+	logf              logger.Logf
+	impl              resolvconfImpl
+	workaroundApplied bool // libc update script has been installed.
 }
 
-func newResolvconfManager(logf logger.Logf) resolvconfManager {
+func newResolvconfManager(logf logger.Logf) *resolvconfManager {
 	impl := getResolvconfImpl()
 	logf("resolvconf implementation is %s", impl)
 
-	return resolvconfManager{
+	return &resolvconfManager{
+		logf: logf,
 		impl: impl,
 	}
 }
 
-// resolvconfConfigName is the name of the config submitted to resolvconf.
+func (m *resolvconfManager) SetDNS(config OSConfig) error {
-// It has this form to match the "tun*" rule in interface-order
+	if m.impl == resolvconfLegacy && !m.workaroundApplied {
-// when running resolvconfLegacy, hopefully placing our config first.
+		m.logf("injecting resolvconf workaround script")
-const resolvconfConfigName = "tun-tailscale.inet"
+		if err := os.MkdirAll(resolvconfLibcHookPath, 0755); err != nil {
+			return err
+		}
+		if err := atomicfile.WriteFile(resolvconfHookPath, legacyResolvconfScript, 0755); err != nil {
+			return err
+		}
+		m.workaroundApplied = true
+	}
 
-func (m resolvconfManager) SetDNS(config OSConfig) error {
 	stdin := new(bytes.Buffer)
 	writeResolvConf(stdin, config.Nameservers, config.SearchDomains) // dns_direct.go
 
@@ -138,15 +174,15 @@ func (m resolvconfManager) SetDNS(config OSConfig) error {
 	return nil
 }
 
-func (m resolvconfManager) SupportsSplitDNS() bool {
+func (m *resolvconfManager) SupportsSplitDNS() bool {
 	return false
 }
 
-func (m resolvconfManager) GetBaseConfig() (OSConfig, error) {
+func (m *resolvconfManager) GetBaseConfig() (OSConfig, error) {
 	return OSConfig{}, ErrGetBaseConfigNotSupported
 }
 
-func (m resolvconfManager) Close() error {
+func (m *resolvconfManager) Close() error {
 	var cmd *exec.Cmd
 	switch m.impl {
 	case resolvconfOpenresolv:
@@ -161,5 +197,10 @@ func (m resolvconfManager) Close() error {
 		return fmt.Errorf("running %s: %s", cmd, out)
 	}
 
+	if m.workaroundApplied {
+		m.logf("removing resolvconf workaround script")
+		os.Remove(resolvconfHookPath) // Best-effort
+	}
+
 	return nil
 }