Refactor macOS system DNS configuration (#2284)

On macOS use the recommended settings for providing split DNS. As per
the docs an empty string will force the configuration to be the default.
In order to to support split DNS an additional service config is added
for the local server and search domain settings.

see: https://developer.apple.com/documentation/devicemanagement/vpn/dns

client/internal/dns/host.go

@@ -15,6 +15,12 @@ type hostManager interface {
 	restoreUncleanShutdownDNS(storedDNSAddress *netip.Addr) error
 }
 
+type SystemDNSSettings struct {
+	Domains    []string
+	ServerIP   string
+	ServerPort int
+}
+
 type HostDNSConfig struct {
 	Domains    []DomainConfig `json:"domains"`
 	RouteAll   bool           `json:"routeAll"`

client/internal/dns/host_darwin.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"net"
 	"net/netip"
 	"os/exec"
 	"strconv"
@@ -18,7 +19,7 @@ import (
 const (
 	netbirdDNSStateKeyFormat            = "State:/Network/Service/NetBird-%s/DNS"
 	globalIPv4State                     = "State:/Network/Global/IPv4"
-	primaryServiceSetupKeyFormat        = "Setup:/Network/Service/%s/DNS"
+	primaryServiceStateKeyFormat        = "State:/Network/Service/%s/DNS"
 	keySupplementalMatchDomains         = "SupplementalMatchDomains"
 	keySupplementalMatchDomainsNoSearch = "SupplementalMatchDomainsNoSearch"
 	keyServerAddresses                  = "ServerAddresses"
@@ -28,12 +29,12 @@ const (
 	scutilPath                          = "/usr/sbin/scutil"
 	searchSuffix                        = "Search"
 	matchSuffix                         = "Match"
+	localSuffix                         = "Local"
 )
 
 type systemConfigurator struct {
-	// primaryServiceID primary interface in the system. AKA the interface with the default route
-	primaryServiceID string
-	createdKeys      map[string]struct{}
+	createdKeys       map[string]struct{}
+	systemDNSSettings SystemDNSSettings
 }
 
 func newHostManager() (hostManager, error) {
@@ -49,20 +50,6 @@ func (s *systemConfigurator) supportCustomPort() bool {
 func (s *systemConfigurator) applyDNSConfig(config HostDNSConfig) error {
 	var err error
 
-	if config.RouteAll {
-		err = s.addDNSSetupForAll(config.ServerIP, config.ServerPort)
-		if err != nil {
-			return fmt.Errorf("add dns setup for all: %w", err)
-		}
-	} else if s.primaryServiceID != "" {
-		err = s.removeKeyFromSystemConfig(getKeyWithInput(primaryServiceSetupKeyFormat, s.primaryServiceID))
-		if err != nil {
-			return fmt.Errorf("remote key from system config: %w", err)
-		}
-		s.primaryServiceID = ""
-		log.Infof("removed %s:%d as main DNS resolver for this peer", config.ServerIP, config.ServerPort)
-	}
-
 	// create a file for unclean shutdown detection
 	if err := createUncleanShutdownIndicator(); err != nil {
 		log.Errorf("failed to create unclean shutdown file: %s", err)
@@ -73,6 +60,19 @@ func (s *systemConfigurator) applyDNSConfig(config HostDNSConfig) error {
 		matchDomains  []string
 	)
 
+	err = s.recordSystemDNSSettings(true)
+	if err != nil {
+		log.Errorf("unable to update record of System's DNS config: %s", err.Error())
+	}
+
+	if config.RouteAll {
+		searchDomains = append(searchDomains, "\"\"")
+		err = s.addLocalDNS()
+		if err != nil {
+			log.Infof("failed to enable split DNS")
+		}
+	}
+
 	for _, dConf := range config.Domains {
 		if dConf.Disabled {
 			continue
@@ -119,10 +119,6 @@ func (s *systemConfigurator) restoreHostDNS() error {
 		}
 		log.Infof("removing %s domains from system", keyType)
 	}
-	if s.primaryServiceID != "" {
-		lines += buildRemoveKeyOperation(getKeyWithInput(primaryServiceSetupKeyFormat, s.primaryServiceID))
-		log.Infof("restoring DNS resolver configuration for system")
-	}
 	_, err := runSystemConfigCommand(wrapCommand(lines))
 	if err != nil {
 		log.Errorf("got an error while cleaning the system configuration: %s", err)
@@ -148,6 +144,97 @@ func (s *systemConfigurator) removeKeyFromSystemConfig(key string) error {
 	return nil
 }
 
+func (s *systemConfigurator) addLocalDNS() error {
+	if s.systemDNSSettings.ServerIP == "" || len(s.systemDNSSettings.Domains) == 0 {
+		err := s.recordSystemDNSSettings(true)
+		log.Errorf("Unable to get system DNS configuration")
+		return err
+	}
+	localKey := getKeyWithInput(netbirdDNSStateKeyFormat, localSuffix)
+	if s.systemDNSSettings.ServerIP != "" && len(s.systemDNSSettings.Domains) != 0 {
+		err := s.addSearchDomains(localKey, strings.Join(s.systemDNSSettings.Domains, " "), s.systemDNSSettings.ServerIP, s.systemDNSSettings.ServerPort)
+		if err != nil {
+			return fmt.Errorf("couldn't add local network DNS conf: %w", err)
+		}
+	} else {
+		log.Info("Not enabling local DNS server")
+	}
+
+	return nil
+}
+
+func (s *systemConfigurator) recordSystemDNSSettings(force bool) error {
+	if s.systemDNSSettings.ServerIP != "" && len(s.systemDNSSettings.Domains) != 0 && !force {
+		return nil
+	}
+
+	systemDNSSettings, err := s.getSystemDNSSettings()
+	if err != nil {
+		return fmt.Errorf("couldn't get current DNS config: %w", err)
+	}
+	s.systemDNSSettings = systemDNSSettings
+
+	return nil
+}
+
+func (s *systemConfigurator) getSystemDNSSettings() (SystemDNSSettings, error) {
+	primaryServiceKey, _, err := s.getPrimaryService()
+	if err != nil || primaryServiceKey == "" {
+		return SystemDNSSettings{}, fmt.Errorf("couldn't find the primary service key: %w", err)
+	}
+	dnsServiceKey := getKeyWithInput(primaryServiceStateKeyFormat, primaryServiceKey)
+	line := buildCommandLine("show", dnsServiceKey, "")
+	stdinCommands := wrapCommand(line)
+
+	b, err := runSystemConfigCommand(stdinCommands)
+	if err != nil {
+		return SystemDNSSettings{}, fmt.Errorf("sending the command: %w", err)
+	}
+
+	var dnsSettings SystemDNSSettings
+	inSearchDomainsArray := false
+	inServerAddressesArray := false
+
+	scanner := bufio.NewScanner(bytes.NewReader(b))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		switch {
+		case strings.HasPrefix(line, "DomainName :"):
+			domainName := strings.TrimSpace(strings.Split(line, ":")[1])
+			dnsSettings.Domains = append(dnsSettings.Domains, domainName)
+		case line == "SearchDomains : <array> {":
+			inSearchDomainsArray = true
+			continue
+		case line == "ServerAddresses : <array> {":
+			inServerAddressesArray = true
+			continue
+		case line == "}":
+			inSearchDomainsArray = false
+			inServerAddressesArray = false
+		}
+
+		if inSearchDomainsArray {
+			searchDomains := strings.Split(line, " : ")
+			dnsSettings.Domains = append(dnsSettings.Domains, searchDomains...)
+		} else if inServerAddressesArray {
+			address := strings.Split(line, " : ")[1]
+			if ip := net.ParseIP(address); ip != nil && ip.To4() != nil {
+				dnsSettings.ServerIP = address
+				inServerAddressesArray = false // Stop reading after finding the first IPv4 address
+			}
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return dnsSettings, err
+	}
+
+	// default to 53 port
+	dnsSettings.ServerPort = 53
+
+	return dnsSettings, nil
+}
+
 func (s *systemConfigurator) addSearchDomains(key, domains string, ip string, port int) error {
 	err := s.addDNSState(key, domains, ip, port, true)
 	if err != nil {
@@ -194,23 +281,6 @@ func (s *systemConfigurator) addDNSState(state, domains, dnsServer string, port
 	return nil
 }
 
-func (s *systemConfigurator) addDNSSetupForAll(dnsServer string, port int) error {
-	primaryServiceKey, existingNameserver, err := s.getPrimaryService()
-	if err != nil || primaryServiceKey == "" {
-		return fmt.Errorf("couldn't find the primary service key: %w", err)
-	}
-
-	err = s.addDNSSetup(getKeyWithInput(primaryServiceSetupKeyFormat, primaryServiceKey), dnsServer, port, existingNameserver)
-	if err != nil {
-		return fmt.Errorf("add dns setup: %w", err)
-	}
-
-	log.Infof("configured %s:%d as main DNS resolver for this peer", dnsServer, port)
-	s.primaryServiceID = primaryServiceKey
-
-	return nil
-}
-
 func (s *systemConfigurator) getPrimaryService() (string, string, error) {
 	line := buildCommandLine("show", globalIPv4State, "")
 	stdinCommands := wrapCommand(line)
@@ -239,19 +309,6 @@ func (s *systemConfigurator) getPrimaryService() (string, string, error) {
 	return primaryService, router, nil
 }
 
-func (s *systemConfigurator) addDNSSetup(setupKey, dnsServer string, port int, existingDNSServer string) error {
-	lines := buildAddCommandLine(keySupplementalMatchDomainsNoSearch, digitSymbol+strconv.Itoa(0))
-	lines += buildAddCommandLine(keyServerAddresses, arraySymbol+dnsServer+" "+existingDNSServer)
-	lines += buildAddCommandLine(keyServerPort, digitSymbol+strconv.Itoa(port))
-	addDomainCommand := buildCreateStateWithOperation(setupKey, lines)
-	stdinCommands := wrapCommand(addDomainCommand)
-	_, err := runSystemConfigCommand(stdinCommands)
-	if err != nil {
-		return fmt.Errorf("applying dns setup, error: %w", err)
-	}
-	return nil
-}
-
 func (s *systemConfigurator) restoreUncleanShutdownDNS(*netip.Addr) error {
 	if err := s.restoreHostDNS(); err != nil {
 		return fmt.Errorf("restoring dns via scutil: %w", err)