feat(wireguard): WIREGUARD_GSO option to disable TUN vnet hdr batching (#3424)

internal/amneziawg/offload_linux.go

@@ -0,0 +1,27 @@
+package amneziawg
+
+import (
+	"fmt"
+
+	amneziatun "github.com/amnezia-vpn/amneziawg-go/tun"
+	"github.com/qdm12/gluetun/internal/wireguard"
+)
+
+// createTUN creates a TUN device. When gso is false, IFF_VNET_HDR is
+// omitted so amneziawg-go's initFromFlags sees no vnet header support and
+// keeps tun.vnetHdr=false, falling back to simple single-packet writes instead
+// of the GRO/GSO batch path that causes EINVAL on some vendor kernels.
+func createTUN(name string, mtu int, gso bool) (amneziatun.Device, error) { //nolint:ireturn
+	if gso {
+		return amneziatun.CreateTUN(name, mtu)
+	}
+	tunFile, err := wireguard.OpenTUNFile(name)
+	if err != nil {
+		return nil, fmt.Errorf("creating tun fd file: %w", err)
+	}
+	tunDevice, err := amneziatun.CreateTUNFromFile(tunFile, mtu)
+	if err != nil {
+		return nil, fmt.Errorf("creating TUN device from file: %w", err)
+	}
+	return tunDevice, nil
+}