Add default DnsMode function for non-linux platforms

Signed-off-by: Ghvstcode <impoissants@gmail.com>

net/dns/manager_linux.go

@@ -13,6 +13,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/godbus/dbus/v5"
 	"tailscale.com/health"
 	"tailscale.com/net/netaddr"
 	"tailscale.com/types/logger"
@@ -31,8 +32,8 @@ func (kv kv) String() string {
 var publishOnce sync.Once
 
 var (
-	dnsMode      string
-	dnsModeMutex sync.RWMutex
+	globalDnsMode string
+	dnsModeMutex  sync.RWMutex
 )
 
 func NewOSConfigurator(logf logger.Logf, interfaceName string) (ret OSConfigurator, err error) {
@@ -437,12 +438,12 @@ func dbusReadString(name, objectPath, iface, member string) (string, error) {
 func setGlobalDnsMode(mode string) {
 	dnsModeMutex.Lock()
 	defer dnsModeMutex.Unlock()
-	dnsMode = mode
+	globalDnsMode = mode
 }
 
 // GetGlobalDnsMode safely returns the global DNS mode variable
 func GetGlobalDnsMode() string {
 	dnsModeMutex.RLock()
 	defer dnsModeMutex.RUnlock()
-	return dnsMode
+	return globalDnsMode
 }

net/dns/manager_notlinux.go

@@ -0,0 +1,9 @@
+//go:build !linux
+
+package dns
+
+// GetGlobalDnsMode exists to make the build happy on non-linux platforms.
+// The actual implementation is in manager_linux.go & in there it safely returns the current resolv.conf mode
+func GetGlobalDnsMode() string {
+	return ""
+}