cmd/ctrld: optimizing set/reset DNS

Currently, when reset DNS, ctrld always find the net.Interface by
interface name. This may produce unexpected error because the interface
table may be cleared at the time ctrld is being stopped.

Instead, we can get the net interface only once, and use that interface
for restoring the DNS before shutting down.

While at it, also making logging message clearer.

cmd/ctrld/cli.go

@@ -421,32 +421,28 @@ func initCLI() {
 	}
 }
 
-func writeConfigFile() {
+func writeConfigFile() error {
 	if cfu := v.ConfigFileUsed(); cfu != "" {
 		defaultConfigFile = cfu
 	}
 	f, err := os.OpenFile(defaultConfigFile, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, os.FileMode(0o644))
 	if err != nil {
-		log.Printf("failed to open config file: %v\n", err)
+		return err
-		os.Exit(1)
 	}
 	defer f.Close()
 	if cdUID != "" {
 		if _, err := f.WriteString("# AUTO-GENERATED VIA CD FLAG - DO NOT MODIFY\n\n"); err != nil {
-			log.Printf("failed to write header to config file: %v\n", err)
+			return err
-			os.Exit(1)
 		}
 	}
 	enc := toml.NewEncoder(f).SetIndentTables(true)
 	if err := enc.Encode(v.AllSettings()); err != nil {
-		log.Printf("failed to encode config file: %v\n", err)
+		return err
-		os.Exit(1)
 	}
 	if err := f.Close(); err != nil {
-		log.Printf("failed to write config file: %v\n", err)
+		return err
-		os.Exit(1)
 	}
-	fmt.Println("writing config file to:", defaultConfigFile)
+	return nil
 }
 
 func readConfigFile(writeDefaultConfig bool) bool {
@@ -464,7 +460,11 @@ func readConfigFile(writeDefaultConfig bool) bool {
 
 	// If error is viper.ConfigFileNotFoundError, write default config.
 	if _, ok := err.(viper.ConfigFileNotFoundError); ok {
-		writeConfigFile()
+		if err := writeConfigFile(); err != nil {
+			log.Fatalf("failed to write default config file: %v", err)
+		} else {
+			fmt.Println("writing default config file to: " + defaultConfigFile)
+		}
 		defaultConfigWritten = true
 		return false
 	}
@@ -526,6 +526,7 @@ func processCDFlags() {
 		iface = "auto"
 	}
 	logger := mainLog.With().Str("mode", "cd").Logger()
+	logger.Info().Msg("fetching Controld-D configuration")
 	resolverConfig, err := controld.FetchResolverConfig(cdUID)
 	if uer, ok := err.(*controld.UtilityErrorResponse); ok && uer.ErrorField.Code == controld.InvalidConfigCode {
 		s, err := service.New(&prog{}, svcConfig)
@@ -533,10 +534,8 @@ func processCDFlags() {
 			logger.Warn().Err(err).Msg("failed to create new service")
 			return
 		}
-		if iface == "auto" {
+
-			iface = defaultIfaceName()
+		if netIface, _ := netInterface(iface); netIface != nil {
-		}
-		if netIface, _ := netIfaceFromName(iface); netIface != nil {
 			if err := resetDNS(netIface); err != nil {
 				logger.Warn().Err(err).Msg("something went wrong while restoring DNS")
 			}
@@ -552,6 +551,7 @@ func processCDFlags() {
 		return
 	}
 
+	logger.Info().Msg("generating ctrld config from Controld-D configuration")
 	cfg = ctrld.Config{}
 	cfg.Network = make(map[string]*ctrld.NetworkConfig)
 	cfg.Network["0"] = &ctrld.NetworkConfig{
@@ -583,7 +583,11 @@ func processCDFlags() {
 	v.Set("upstream", cfg.Upstream)
 	v.Set("listener", cfg.Listener)
 	processLogAndCacheFlags()
-	writeConfigFile()
+	if err := writeConfigFile(); err != nil {
+		logger.Fatal().Err(err).Msg("failed to write config file")
+	} else {
+		logger.Info().Msg("writing config file to: " + defaultConfigFile)
+	}
 }
 
 func processListenFlag() {
@@ -620,7 +624,10 @@ func processLogAndCacheFlags() {
 	v.Set("service", cfg.Service)
 }
 
-func netIfaceFromName(ifaceName string) (*net.Interface, error) {
+func netInterface(ifaceName string) (*net.Interface, error) {
+	if ifaceName == "auto" {
+		ifaceName = defaultIfaceName()
+	}
 	var iface *net.Interface
 	err := interfaces.ForeachInterface(func(i interfaces.Interface, prefixes []netip.Prefix) {
 		if i.Name == ifaceName {

cmd/ctrld/main.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"io"
+	"net"
 	"os"
 	"path/filepath"
 	"time"
@@ -35,6 +36,7 @@ var (
 
 	cdUID          string
 	iface          string
+	netIface       *net.Interface
 	ifaceStartStop string
 )
 

cmd/ctrld/os_linux.go

@@ -81,7 +81,7 @@ func resetDNS(iface *net.Interface) error {
 		c := client6.NewClient()
 		conversation, err := c.Exchange(iface.Name)
 		if err != nil {
-			mainLog.Warn().Err(err).Msg("failed to exchange DHCPv6")
+			mainLog.Warn().Err(err).Msg("could not exchange DHCPv6")
 		}
 		for _, packet := range conversation {
 			if packet.Type() == dhcpv6.MessageTypeReply {

cmd/ctrld/prog.go

@@ -34,17 +34,7 @@ func (p *prog) Start(s service.Service) error {
 }
 
 func (p *prog) run() {
-	if iface != "" {
+	p.setDNS()
-		netIface, err := netIfaceFromName(iface)
-		if err != nil {
-			mainLog.Error().Err(err).Msg("could not get interface")
-		} else {
-			if err := setDNS(netIface, []string{cfg.Listener["0"].IP}); err != nil {
-				mainLog.Error().Err(err).Str("iface", iface).Msgf("could not set DNS for interface")
-			}
-		}
-	}
-
 	if p.cfg.Service.CacheEnable {
 		cacher, err := dnscache.NewLRUCache(p.cfg.Service.CacheSize)
 		if err != nil {
@@ -150,7 +140,11 @@ func (p *prog) run() {
 							Port: port,
 						},
 					})
-					writeConfigFile()
+					if err := writeConfigFile(); err != nil {
+						proxyLog.Fatal().Err(err).Msg("failed to write config file")
+					} else {
+						mainLog.Info().Msg("writing config file to: " + defaultConfigFile)
+					}
 					mainLog.Info().Msgf("Starting DNS server on listener.%s: %s", listenerNum, pc.LocalAddr())
 					// There can be a race between closing the listener and start our own UDP server, but it's
 					// rare, and we only do this once, so let conservative here.
@@ -175,15 +169,7 @@ func (p *prog) Stop(s service.Service) error {
 		mainLog.Error().Err(err).Msg("de-allocate ip failed")
 		return err
 	}
-	if iface != "" {
+	p.resetDNS()
-		if netIface, err := netIfaceFromName(iface); err == nil {
-			if err := resetDNS(netIface); err != nil {
-				mainLog.Error().Err(err).Str("iface", iface).Msgf("could not reset DNS")
-			}
-		} else {
-			mainLog.Error().Err(err).Msg("could not get interface")
-		}
-	}
 	return nil
 }
 
@@ -205,3 +191,35 @@ func (p *prog) deAllocateIP() error {
 	}
 	return nil
 }
+
+func (p *prog) setDNS() {
+	if iface == "" {
+		return
+	}
+	logger := mainLog.With().Str("iface", iface).Logger()
+	var err error
+	netIface, err = netInterface(iface)
+	if err != nil {
+		logger.Error().Err(err).Msg("could not get interface")
+		return
+	}
+	logger.Debug().Msg("setting DNS for interface")
+	if err := setDNS(netIface, []string{p.cfg.Listener["0"].IP}); err != nil {
+		logger.Error().Err(err).Msgf("could not set DNS for interface")
+		return
+	}
+	logger.Debug().Msg("setting DNS successfully")
+}
+
+func (p *prog) resetDNS() {
+	if netIface == nil {
+		return
+	}
+	logger := mainLog.With().Str("iface", iface).Logger()
+	logger.Debug().Msg("Restoring DNS for interface")
+	if err := resetDNS(netIface); err != nil {
+		logger.Error().Err(err).Msgf("could not reset DNS")
+		return
+	}
+	logger.Debug().Msg("Restoring DNS successfully")
+}