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net/dns: ensure the Windows configurator does not touch the hosts file unless the configuration actually changed

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We build up maps of both the existing MagicDNS configuration in hosts
and the desired MagicDNS configuration, compare the two, and only
write out a new one if there are changes. The comparison doesn't need
to be perfect, as the occasional false-positive is fine, but this
should greatly reduce rewrites of the hosts file.

I also changed the hosts updating code to remove the CRLF/LF conversion
stuff, and use Fprintf instead of Frintln to let us write those inline.

Updates #14428

Signed-off-by: Aaron Klotz <aaron@tailscale.com>
This commit is contained in:
Aaron Klotz 2025-01-13 13:47:56 -07:00
parent 27477983e3
commit d818a58a77
2 changed files with 107 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

74
net/dns/manager_windows.go
View File

@ -8,10 +8,12 @@ import (
"bytes"
"errors"
"fmt"
"maps"
"net/netip"
"os"
"os/exec"
"path/filepath"
"slices"
"sort"
"strings"
"sync"
@ -140,9 +142,8 @@ func (m *windowsManager) setSplitDNS(resolvers []netip.Addr, domains []dnsname.F
return m.nrptDB.WriteSplitDNSConfig(servers, domains)
}
func setTailscaleHosts(prevHostsFile []byte, hosts []*HostEntry) ([]byte, error) {
b := bytes.ReplaceAll(prevHostsFile, []byte("\r\n"), []byte("\n"))
sc := bufio.NewScanner(bytes.NewReader(b))
func setTailscaleHosts(logf logger.Logf, prevHostsFile []byte, hosts []*HostEntry) ([]byte, error) {
sc := bufio.NewScanner(bytes.NewReader(prevHostsFile))
const (
header = "# TailscaleHostsSectionStart"
footer = "# TailscaleHostsSectionEnd"
@ -151,6 +152,32 @@ func setTailscaleHosts(prevHostsFile []byte, hosts []*HostEntry) ([]byte, error)
"# This section contains MagicDNS entries for Tailscale.",
"# Do not edit this section manually.",
}
prevEntries := make(map[netip.Addr][]string)
addPrevEntry := func(line string) {
if line == "" || line[0] == '#' {
return
}
parts := strings.Split(line, " ")
if len(parts) < 1 {
return
}
addr, err := netip.ParseAddr(parts[0])
if err != nil {
logf("Parsing address from hosts: %v", err)
return
}
prevEntries[addr] = parts[1:]
}
nextEntries := make(map[netip.Addr][]string, len(hosts))
for _, he := range hosts {
nextEntries[he.Addr] = he.Hosts
}
var out bytes.Buffer
var inSection bool
for sc.Scan() {
@ -164,26 +191,34 @@ func setTailscaleHosts(prevHostsFile []byte, hosts []*HostEntry) ([]byte, error)
continue
}
if inSection {
addPrevEntry(line)
continue
}
fmt.Fprintln(&out, line)
fmt.Fprintf(&out, "%s\r\n", line)
}
if err := sc.Err(); err != nil {
return nil, err
}
if len(hosts) > 0 {
fmt.Fprintln(&out, header)
for _, c := range comments {
fmt.Fprintln(&out, c)
}
fmt.Fprintln(&out)
for _, he := range hosts {
fmt.Fprintf(&out, "%s %s\n", he.Addr, strings.Join(he.Hosts, " "))
}
fmt.Fprintln(&out)
fmt.Fprintln(&out, footer)
unchanged := maps.EqualFunc(prevEntries, nextEntries, func(a, b []string) bool {
return slices.Equal(a, b)
})
if unchanged {
return nil, nil
}
return bytes.ReplaceAll(out.Bytes(), []byte("\n"), []byte("\r\n")), nil
if len(hosts) > 0 {
fmt.Fprintf(&out, "%s\r\n", header)
for _, c := range comments {
fmt.Fprintf(&out, "%s\r\n", c)
}
fmt.Fprintf(&out, "\r\n")
for _, he := range hosts {
fmt.Fprintf(&out, "%s %s\r\n", he.Addr, strings.Join(he.Hosts, " "))
}
fmt.Fprintf(&out, "\r\n%s\r\n", footer)
}
return out.Bytes(), nil
}
// setHosts sets the hosts file to contain the given host entries.
@ -197,10 +232,15 @@ func (m *windowsManager) setHosts(hosts []*HostEntry) error {
if err != nil {
return err
}
outB, err := setTailscaleHosts(b, hosts)
outB, err := setTailscaleHosts(m.logf, b, hosts)
if err != nil {
return err
}
if outB == nil {
// No change to hosts file, therefore no write necessary.
return nil
}
const fileMode = 0 // ignored on windows.
// This can fail spuriously with an access denied error, so retry it a

52
net/dns/manager_windows_test.go
View File

@ -15,6 +15,7 @@ import (
"golang.org/x/sys/windows"
"golang.org/x/sys/windows/registry"
"tailscale.com/types/logger"
"tailscale.com/util/dnsname"
"tailscale.com/util/winutil"
"tailscale.com/util/winutil/gp"
@ -24,9 +25,56 @@ const testGPRuleID = "{7B1B6151-84E6-41A3-8967-62F7F7B45687}"
func TestHostFileNewLines(t *testing.T) {
in := []byte("#foo\r\n#bar\n#baz\n")
want := []byte("#foo\r\n#bar\r\n#baz\r\n")
want := []byte("#foo\r\n#bar\r\n#baz\r\n# TailscaleHostsSectionStart\r\n# This section contains MagicDNS entries for Tailscale.\r\n# Do not edit this section manually.\r\n\r\n192.168.1.1 aaron\r\n\r\n# TailscaleHostsSectionEnd\r\n")
got, err := setTailscaleHosts(in, nil)
he := []*HostEntry{
&HostEntry{
Addr: netip.MustParseAddr("192.168.1.1"),
Hosts: []string{"aaron"},
},
}
got, err := setTailscaleHosts(logger.Discard, in, he)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(got, want) {
t.Errorf("got %q, want %q\n", got, want)
}
}
func TestHostFileUnchanged(t *testing.T) {
in := []byte("#foo\r\n#bar\r\n#baz\r\n# TailscaleHostsSectionStart\r\n# This section contains MagicDNS entries for Tailscale.\r\n# Do not edit this section manually.\r\n\r\n192.168.1.1 aaron\r\n\r\n# TailscaleHostsSectionEnd\r\n")
he := []*HostEntry{
&HostEntry{
Addr: netip.MustParseAddr("192.168.1.1"),
Hosts: []string{"aaron"},
},
}
got, err := setTailscaleHosts(logger.Discard, in, he)
if err != nil {
t.Fatal(err)
}
if got != nil {
t.Errorf("got %q, want nil\n", got)
}
}
func TestHostFileChanged(t *testing.T) {
in := []byte("#foo\r\n#bar\r\n#baz\r\n# TailscaleHostsSectionStart\r\n# This section contains MagicDNS entries for Tailscale.\r\n# Do not edit this section manually.\r\n\r\n192.168.1.1 aaron1\r\n\r\n# TailscaleHostsSectionEnd\r\n")
want := []byte("#foo\r\n#bar\r\n#baz\r\n# TailscaleHostsSectionStart\r\n# This section contains MagicDNS entries for Tailscale.\r\n# Do not edit this section manually.\r\n\r\n192.168.1.1 aaron1\r\n192.168.1.2 aaron2\r\n\r\n# TailscaleHostsSectionEnd\r\n")
he := []*HostEntry{
&HostEntry{
Addr: netip.MustParseAddr("192.168.1.1"),
Hosts: []string{"aaron1"},
},
&HostEntry{
Addr: netip.MustParseAddr("192.168.1.2"),
Hosts: []string{"aaron2"},
},
}
got, err := setTailscaleHosts(logger.Discard, in, he)
if err != nil {
t.Fatal(err)
}