cmd/ctrld: workaround setting DNS issue on Linux

On some Ubuntu systems, we experiment with DNS is not set even though
systemd-resolved log indicates that it set them. To ensure the DNS will
be set, after setting them, double check the current DNS for interface
is actually the value was set, if not, attempting to set again.

While at it, also make sure the DNS is set when ctrld start on Linux.

cmd/ctrld/cli.go

@@ -169,6 +169,8 @@ func initCLI() {
 	runCmd.Flags().StringVarP(&cdUID, "cd", "", "", "Control D resolver uid")
 	runCmd.Flags().StringVarP(&homedir, "homedir", "", "", "")
 	_ = runCmd.Flags().MarkHidden("homedir")
+	runCmd.Flags().StringVarP(&iface, "iface", "", "", `Update DNS setting for iface, "auto" means the default interface gateway`)
+	_ = runCmd.Flags().MarkHidden("iface")
 
 	rootCmd.AddCommand(runCmd)
 
@@ -226,7 +228,7 @@ func initCLI() {
 			}
 		},
 	}
-	// Keep these flags in sync with runCmd above, except for "-d", "--iface".
+	// Keep these flags in sync with runCmd above, except for "-d".
 	startCmd.Flags().StringVarP(&configPath, "config", "c", "", "Path to config file")
 	startCmd.Flags().StringVarP(&configBase64, "base64_config", "", "", "Base64 encoded config")
 	startCmd.Flags().StringVarP(&listenAddress, "listen", "", "", "Listener address and port, in format: address:port")
@@ -321,7 +323,6 @@ func initCLI() {
 			}
 			initLogging()
 			if doTasks(tasks) {
-				prog.resetDNS()
 				mainLog.Info().Msg("Service uninstalled")
 				return
 			}
@@ -393,6 +394,9 @@ func initCLI() {
 		Use:    "start",
 		Short:  "Quick start service and configure DNS on interface",
 		Run: func(cmd *cobra.Command, args []string) {
+			if !cmd.Flags().Changed("iface") {
+				os.Args = append(os.Args, "--iface="+ifaceStartStop)
+			}
 			iface = ifaceStartStop
 			startCmd.Run(cmd, args)
 		},
@@ -405,6 +409,9 @@ func initCLI() {
 		Use:    "stop",
 		Short:  "Quick stop service and remove DNS from interface",
 		Run: func(cmd *cobra.Command, args []string) {
+			if !cmd.Flags().Changed("iface") {
+				os.Args = append(os.Args, "--iface="+ifaceStartStop)
+			}
 			iface = ifaceStartStop
 			stopCmd.Run(cmd, args)
 		},

cmd/ctrld/os_linux.go

@@ -8,6 +8,7 @@ import (
 	"net"
 	"net/netip"
 	"os/exec"
+	"reflect"
 	"strings"
 	"syscall"
 	"time"
@@ -41,6 +42,8 @@ func deAllocateIP(ip string) error {
 	return nil
 }
 
+const maxSetDNSAttempts = 5
+
 // set the dns server for the provided network interface
 func setDNS(iface *net.Interface, nameservers []string) error {
 	logf := func(format string, args ...any) {
@@ -57,10 +60,22 @@ func setDNS(iface *net.Interface, nameservers []string) error {
 	for _, nameserver := range nameservers {
 		ns = append(ns, netip.MustParseAddr(nameserver))
 	}
-	return r.SetDNS(dns.OSConfig{
+
+	osConfig := dns.OSConfig{
 		Nameservers:   ns,
 		SearchDomains: []dnsname.FQDN{},
-	})
+	}
+
+	for i := 0; i < maxSetDNSAttempts; i++ {
+		if err := r.SetDNS(osConfig); err != nil {
+			return err
+		}
+		currentNS := currentDNS(iface)
+		if reflect.DeepEqual(currentNS, nameservers) {
+			break
+		}
+	}
+	return nil
 }
 
 func resetDNS(iface *net.Interface) error {

cmd/ctrld/prog.go

@@ -35,6 +35,7 @@ func (p *prog) Start(s service.Service) error {
 }
 
 func (p *prog) run() {
+	p.preRun()
 	if p.cfg.Service.CacheEnable {
 		cacher, err := dnscache.NewLRUCache(p.cfg.Service.CacheSize)
 		if err != nil {
@@ -173,6 +174,11 @@ func (p *prog) Stop(s service.Service) error {
 	return nil
 }
 
+func (p *prog) Uninstall(s service.Service) error {
+	p.resetDNS()
+	return nil
+}
+
 func (p *prog) allocateIP(ip string) error {
 	if !p.cfg.Service.AllocateIP {
 		return nil

cmd/ctrld/prog_linux.go

@@ -0,0 +1,9 @@
+package main
+
+import "github.com/kardianos/service"
+
+func (p *prog) preRun() {
+	if !service.Interactive() {
+		p.setDNS()
+	}
+}

cmd/ctrld/prog_others.go

@@ -0,0 +1,6 @@
+//go:build !linux
+// +build !linux
+
+package main
+
+func (p *prog) preRun() {}