router/dns: set all domains on Windows (#672)

Signed-off-by: Dmytro Shynkevych <dmytro@tailscale.com>

wgengine/router/dns/manager_windows.go

@@ -5,6 +5,7 @@
 package dns
 
 import (
+	"errors"
 	"fmt"
 	"strings"
 
@@ -13,6 +14,12 @@ import (
 	"tailscale.com/types/logger"
 )
 
+const (
+	ipv4RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip\Parameters`
+	ipv6RegBase = `SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters`
+	tsRegBase   = `SOFTWARE\Tailscale IPN`
+)
+
 type windowsManager struct {
 	logf logger.Logf
 	guid string
@@ -25,29 +32,88 @@ func newManager(mconfig ManagerConfig) managerImpl {
 	}
 }
 
-func setRegistry(path, nameservers, domains string) error {
+func setRegistryString(path, name, value string) error {
+	key, err := registry.OpenKey(registry.LOCAL_MACHINE, path, registry.SET_VALUE)
+	if err != nil {
+		return fmt.Errorf("opening %s: %w", path, err)
+	}
+	defer key.Close()
+
+	err = key.SetStringValue(name, value)
+	if err != nil {
+		return fmt.Errorf("setting %s[%s]: %w", path, name, err)
+	}
+	return nil
+}
+
+func getRegistryString(path, name string) (string, error) {
+	key, err := registry.OpenKey(registry.LOCAL_MACHINE, path, registry.READ)
+	if err != nil {
+		return "", fmt.Errorf("opening %s: %w", path, err)
+	}
+	defer key.Close()
+
+	value, _, err := key.GetStringValue(name)
+	if err != nil {
+		return "", fmt.Errorf("getting %s[%s]: %w", path, name, err)
+	}
+	return value, nil
+}
+
+func (m windowsManager) setNameservers(basePath string, nameservers []string) error {
+	path := fmt.Sprintf(`%s\Interfaces\%s`, basePath, m.guid)
+	value := strings.Join(nameservers, ",")
+	return setRegistryString(path, "NameServer", value)
+}
+
+func (m windowsManager) setDomains(path string, oldDomains, newDomains []string) error {
+	// We reimplement setRegistryString to ensure that we hold the key for the whole operation.
 	key, err := registry.OpenKey(registry.LOCAL_MACHINE, path, registry.READ|registry.SET_VALUE)
 	if err != nil {
 		return fmt.Errorf("opening %s: %w", path, err)
 	}
 	defer key.Close()
 
-	err = key.SetStringValue("NameServer", nameservers)
-	if err != nil {
-		return fmt.Errorf("setting %s/NameServer: %w", path, err)
+	searchList, _, err := key.GetStringValue("SearchList")
+	if err != nil && err != registry.ErrNotExist {
+		return fmt.Errorf("getting %s[SearchList]: %w", path, err)
+	}
+	currentDomains := strings.Split(searchList, ",")
+
+	var domainsToSet []string
+	for _, domain := range currentDomains {
+		inOld, inNew := false, false
+
+		// The number of domains should be small,
+		// so this is probaly faster than constructing a map.
+		for _, oldDomain := range oldDomains {
+			if domain == oldDomain {
+				inOld = true
+			}
+		}
+		for _, newDomain := range newDomains {
+			if domain == newDomain {
+				inNew = true
+			}
 		}
 
-	err = key.SetStringValue("Domain", domains)
-	if err != nil {
-		return fmt.Errorf("setting %s/Domain: %w", path, err)
+		if !inNew && !inOld {
+			domainsToSet = append(domainsToSet, domain)
 		}
+	}
+	domainsToSet = append(domainsToSet, newDomains...)
 
+	searchList = strings.Join(domainsToSet, ",")
+	if err := key.SetStringValue("SearchList", searchList); err != nil {
+		return fmt.Errorf("setting %s[SearchList]: %w", path, err)
+	}
 	return nil
 }
 
 func (m windowsManager) Up(config Config) error {
 	var ipsv4 []string
 	var ipsv6 []string
+
 	for _, ip := range config.Nameservers {
 		if ip.Is4() {
 			ipsv4 = append(ipsv4, ip.String())
@@ -55,23 +121,29 @@ func (m windowsManager) Up(config Config) error {
 			ipsv6 = append(ipsv6, ip.String())
 		}
 	}
-	nsv4 := strings.Join(ipsv4, ",")
-	nsv6 := strings.Join(ipsv6, ",")
 
-	var domains string
-	if len(config.Domains) > 0 {
-		if len(config.Domains) > 1 {
-			m.logf("only a single search domain is supported")
-		}
-		domains = config.Domains[0]
-	}
-
-	v4Path := `SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\` + m.guid
-	if err := setRegistry(v4Path, nsv4, domains); err != nil {
+	lastSearchList, err := getRegistryString(tsRegBase, "SearchList")
+	if err != nil && !errors.Is(err, registry.ErrNotExist) {
 		return err
 	}
-	v6Path := `SYSTEM\CurrentControlSet\Services\Tcpip6\Parameters\Interfaces\` + m.guid
-	if err := setRegistry(v6Path, nsv6, domains); err != nil {
+	lastDomains := strings.Split(lastSearchList, ",")
+
+	if err := m.setNameservers(ipv4RegBase, ipsv4); err != nil {
+		return err
+	}
+	if err := m.setDomains(ipv4RegBase, lastDomains, config.Domains); err != nil {
+		return err
+	}
+
+	if err := m.setNameservers(ipv6RegBase, ipsv6); err != nil {
+		return err
+	}
+	if err := m.setDomains(ipv6RegBase, lastDomains, config.Domains); err != nil {
+		return err
+	}
+
+	newSearchList := strings.Join(config.Domains, ",")
+	if err := setRegistryString(tsRegBase, "SearchList", newSearchList); err != nil {
 		return err
 	}