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net/dns: retrample resolve.conf when another process has trampled it (#18069)

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When using the resolve.conf file for setting DNS, it is possible that
some other services will trample the file and overwrite our set DNS
server. Experiments has shown this to be a racy error depending on how
quickly processes start.

Make an attempt to trample back the file a limited number of times if
the file is changed.

Updates #16635

Signed-off-by: Claus Lensbøl <claus@tailscale.com>
This commit is contained in:
Claus Lensbøl
2025-12-09 14:55:26 -05:00
committed by GitHub
parent a9b37c510c
commit 1dfdee8521
17 changed files with 261 additions and 45 deletions
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11
net/dns/manager_openbsd.go
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@@ -11,6 +11,7 @@ import (
"tailscale.com/control/controlknobs"
"tailscale.com/health"
"tailscale.com/types/logger"
"tailscale.com/util/eventbus"
"tailscale.com/util/syspolicy/policyclient"
)
@@ -25,8 +26,8 @@ func (kv kv) String() string {
// NewOSConfigurator created a new OS configurator.
//
// The health tracker may be nil; the knobs may be nil and are ignored on this platform.
func NewOSConfigurator(logf logger.Logf, health *health.Tracker, _ policyclient.Client, _ *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
return newOSConfigurator(logf, health, interfaceName,
func NewOSConfigurator(logf logger.Logf, health *health.Tracker, bus *eventbus.Bus, _ policyclient.Client, _ *controlknobs.Knobs, interfaceName string) (OSConfigurator, error) {
return newOSConfigurator(logf, health, bus, interfaceName,
newOSConfigEnv{
rcIsResolvd: rcIsResolvd,
fs: directFS{},
@@ -39,7 +40,7 @@ type newOSConfigEnv struct {
rcIsResolvd func(resolvConfContents []byte) bool
}
func newOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName string, env newOSConfigEnv) (ret OSConfigurator, err error) {
func newOSConfigurator(logf logger.Logf, health *health.Tracker, bus *eventbus.Bus, interfaceName string, env newOSConfigEnv) (ret OSConfigurator, err error) {
var debug []kv
dbg := func(k, v string) {
debug = append(debug, kv{k, v})
@@ -54,7 +55,7 @@ func newOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName s
bs, err := env.fs.ReadFile(resolvConf)
if os.IsNotExist(err) {
dbg("rc", "missing")
return newDirectManager(logf, health), nil
return newDirectManager(logf, health, bus), nil
}
if err != nil {
return nil, fmt.Errorf("reading /etc/resolv.conf: %w", err)
@@ -66,7 +67,7 @@ func newOSConfigurator(logf logger.Logf, health *health.Tracker, interfaceName s
}
dbg("resolvd", "missing")
return newDirectManager(logf, health), nil
return newDirectManager(logf, health, bus), nil
}
func rcIsResolvd(resolvConfContents []byte) bool {