diff --git a/src/bsd/device.c b/src/bsd/device.c
index 8649d89f..419c38fb 100644
--- a/src/bsd/device.c
+++ b/src/bsd/device.c
@@ -245,7 +245,7 @@ bool write_packet(vpn_packet_t *packet)
 
 		case DEVICE_TYPE_TUNIFHEAD: {
 			u_int32_t type;
-			struct iovec vector[2] = {{&type, sizeof(type)}, {packet->data + 14, MTU - 14}};
+			struct iovec vector[2] = {{&type, sizeof(type)}, {packet->data + 14, packet->len - 14}};
 			int af;
 			
 			af = (packet->data[12] << 8) + packet->data[13];