networkd: warn when ignoring unsupported tuntap options

The interface for creating tuntap devices should be ported to rtnl so it would support the same settings
as other kinds. In the meantime, the best one can do is to drop in a .link file to set the desired options.

src/network/networkd-netdev-tuntap.c

@@ -172,9 +172,35 @@ static void tuntap_done(NetDev *netdev) {
         t->group_name = NULL;
 }
 
+static int tuntap_verify(NetDev *netdev, const char *filename) {
+        TunTap *t = NULL;
+
+        assert(netdev);
+
+        if (netdev->kind == NETDEV_KIND_TUN)
+                t = TUN(netdev);
+        else
+                t = TAP(netdev);
+
+        assert(t);
+
+        if (netdev->mtu) {
+                log_warning_netdev(netdev, "MTU configured for %s, ignoring",
+                                   netdev_kind_to_string(netdev->kind));
+        }
+
+        if (netdev->mac) {
+                log_warning_netdev(netdev, "MAC configured for %s, ignoring",
+                                   netdev_kind_to_string(netdev->kind));
+        }
+
+        return 0;
+}
+
 const NetDevVTable tun_vtable = {
         .object_size = sizeof(TunTap),
         .sections = "Match\0NetDev\0Tun\0",
+        .config_verify = tuntap_verify,
         .done = tuntap_done,
         .create = netdev_create_tuntap,
         .create_type = NETDEV_CREATE_INDEPENDENT,
@@ -183,6 +209,7 @@ const NetDevVTable tun_vtable = {
 const NetDevVTable tap_vtable = {
         .object_size = sizeof(TunTap),
         .sections = "Match\0NetDev\0Tap\0",
+        .config_verify = tuntap_verify,
         .done = tuntap_done,
         .create = netdev_create_tuntap,
         .create_type = NETDEV_CREATE_INDEPENDENT,