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net/dns: only restart systemd-resolved if we changed /etc/resolv.conf.

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Reported on IRC: in an edge case, you can end up with a directManager DNS
manager and --accept-dns=false, in which case we should do nothing, but
actually end up restarting resolved whenever the netmap changes, even though
the user told us to not manage DNS.

Signed-off-by: David Anderson <danderson@tailscale.com>
This commit is contained in:
David Anderson 2021-09-04 23:46:15 -07:00 committed by Dave Anderson
parent 10547d989d
commit b3b1c06b3a
1 changed files with 24 additions and 11 deletions
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35
net/dns/direct.go
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@ -204,21 +204,21 @@ func (m directManager) backupConfig() error {
return m.fs.Rename(resolvConf, backupConf)
}
func (m directManager) restoreBackup() error {
func (m directManager) restoreBackup() (restored bool, err error) {
if _, err := m.fs.Stat(backupConf); err != nil {
if os.IsNotExist(err) {
// No backup, nothing we can do.
return nil
return false, nil
}
return err
return false, err
}
owned, err := m.ownedByTailscale()
if err != nil {
return err
return false, err
}
_, err = m.fs.Stat(resolvConf)
if err != nil && !os.IsNotExist(err) {
return err
return false, err
}
resolvConfExists := !os.IsNotExist(err)
@ -226,23 +226,26 @@ func (m directManager) restoreBackup() error {
// There's already a non-tailscale config in place, get rid of
// our backup.
m.fs.Remove(backupConf)
return nil
return false, nil
}
// We own resolv.conf, and a backup exists.
if err := m.fs.Rename(backupConf, resolvConf); err != nil {
return err
return false, err
}
return nil
return true, nil
}
func (m directManager) SetDNS(config OSConfig) error {
func (m directManager) SetDNS(config OSConfig) (err error) {
var changed bool
if config.IsZero() {
if err := m.restoreBackup(); err != nil {
changed, err = m.restoreBackup()
if err != nil {
return err
}
} else {
changed = true
if err := m.backupConfig(); err != nil {
return err
}
@ -260,7 +263,17 @@ func (m directManager) SetDNS(config OSConfig) error {
// try to manage DNS through resolved when it's around, but as a
// best-effort fallback if we messed up the detection, try to
// restart resolved to make the system configuration consistent.
if isResolvedRunning() && !runningAsGUIDesktopUser() {
//
// We take care to only kick systemd-resolved if we've made some
// change to the system's DNS configuration, because this codepath
// can end up running in cases where the user has manually
// configured /etc/resolv.conf to point to systemd-resolved (but
// it's not managed explicitly by systemd-resolved), *and* has
// --accept-dns=false, meaning we pass an empty configuration to
// the running DNS manager. In that very edge-case scenario, we
// cause a disruptive DNS outage each time we reset an empty
// OS configuration.
if changed && isResolvedRunning() && !runningAsGUIDesktopUser() {
exec.Command("systemctl", "restart", "systemd-resolved.service").Run()
}