preflight-linux: Add logic to check if /etc/resolv.conf managed by sy…

…stemd-resolved

Current logic checks if systemd-resolved service is running and add the
dispatcher file to network manager config which uses `systemd-resolve`
to update the domain and dns for `crc` interface. But it is observed
that the dispatcher is called when the NM interface changes state,
but then the resolvectl call it contains has no effect on DNS
resolution as systemd-resolved does not manage /etc/resolv.conf.

This PR add logic to check if `/etc/resolv.conf` actually managed by
systemd-resolved and if not then use `dnsmasq` configuration which
works as expected.

pkg/crc/preflight/preflight_checks_network_linux.go

@@ -8,6 +8,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"slices"
 
 	"github.com/crc-org/crc/v2/pkg/crc/logging"
 	"github.com/crc-org/crc/v2/pkg/crc/systemd"
@@ -275,6 +276,20 @@ func checkNetworkManagerIsRunning() error {
 }
 
 func checkSystemdResolvedIsRunning() error {
+	resolvFilePath := "/etc/resolv.conf"
+	// Check /ETC/RESOLV.CONF section in `man systemd-resolved`
+	systemdResolvedManageResolveFilePath := []string{"/run/systemd/resolve/stub-resolv.conf",
+		"/usr/lib/systemd/resolv.conf",
+		"/run/systemd/resolve/resolv.conf"}
+	// Check if /etc/resolv.conf is managed by systemd-resolved
+	rFilePath, err := filepath.EvalSymlinks(resolvFilePath)
+	logging.Debugf("resolve conf file path: %s", rFilePath)
+	if err != nil {
+		return err
+	}
+	if !slices.Contains(systemdResolvedManageResolveFilePath, rFilePath) {
+		return fmt.Errorf("%s not managed by systemd-resolved", resolvFilePath)
+	}
 	return checkSystemdServiceRunning("systemd-resolved.service")
 }