wifi(linux): skip nl80211 when no wireless devices are registered

Check /sys/class/ieee80211 before opening netlink sockets. Looking up
the
nl80211 family while cfg80211 is not loaded makes the kernel run
modprobe,
which loads the module on hosts without wireless hardware, or fails
again
on every poll when the module is unavailable.

agent/wifi/README.md

@@ -6,6 +6,7 @@ snapshot keyed by interface (`s` SSID, `r` RSSI in dBm when available);
 interval only; real-time requests reuse the last snapshot.
 
 - Linux: nl80211 via `github.com/mdlayher/wifi`. Docker needs `network_mode: host`.
+  Skipped when `/sys/class/ieee80211` lists no wireless devices.
 - macOS: CoreWLAN via `osascript` (JXA). SSID may be redacted by privacy settings.
 - Windows: native WLAN API, keyed by adapter alias (e.g. `Wi-Fi`), falling back
   to interface GUID.

agent/wifi/wifi_linux.go

@@ -6,6 +6,7 @@ import (
 	"context"
 	"errors"
 	"net"
+	"os"
 	"time"
 
 	"github.com/henrygd/beszel/internal/entities/system"
@@ -80,7 +81,27 @@ func (c *nl80211Client) Close() error {
 	return errors.Join(c.conn.Close(), c.Client.Close())
 }
 
+// wiphyClassDir lists the wireless devices registered with cfg80211.
+const wiphyClassDir = "/sys/class/ieee80211"
+
+// hasWirelessDevice gates nl80211 access. Looking up the nl80211 family while
+// cfg80211 is not loaded makes the kernel run modprobe, which loads the module
+// on hosts without wireless hardware, or fails again on every poll. Not cached,
+// so devices registered after startup are still picked up.
+func hasWirelessDevice(dir string) bool {
+	f, err := os.Open(dir)
+	if err != nil {
+		return false
+	}
+	defer f.Close()
+	names, _ := f.Readdirnames(1)
+	return len(names) > 0
+}
+
 func collect(ctx context.Context) map[string]system.WiFi {
+	if !hasWirelessDevice(wiphyClassDir) {
+		return nil
+	}
 	client, err := newNL80211Client()
 	if err != nil {
 		return nil

agent/wifi/wifi_linux_test.go

@@ -7,6 +7,8 @@ import (
 	"context"
 	"errors"
 	"net"
+	"os"
+	"path/filepath"
 	"testing"
 	"time"
 
@@ -132,6 +134,22 @@ func TestLinuxInvalidSSID(t *testing.T) {
 	}
 }
 
+func TestHasWirelessDevice(t *testing.T) {
+	dir := t.TempDir()
+	if hasWirelessDevice(filepath.Join(dir, "missing")) {
+		t.Fatal("missing class directory")
+	}
+	if hasWirelessDevice(dir) {
+		t.Fatal("empty class directory")
+	}
+	if err := os.Mkdir(filepath.Join(dir, "phy0"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if !hasWirelessDevice(dir) {
+		t.Fatal("registered device not detected")
+	}
+}
+
 func TestLinuxCancelled(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()