test(vault01/APs): advertise low MTU on DHCP

machines/nixos/vault01/networking.nix

@@ -59,6 +59,12 @@ let
             LinkLocalAddressing = "no";
             DHCPServer = "yes";
           };
+          dhcpServerConfig = {
+            SendOption = [
+              # FIXME: should be removed, it's used only for tests
+              "26:uint16:1378" # send MTU to users
+            ];
+          };
           linkConfig = {
             Promiscuous = true;
             MTUBytes = 1500;
@@ -109,6 +115,7 @@ let
     vlan-admin = {
       Id = 3000;
       address = [ "fd26:baf9:d250:8000::1/64" ];
+      networkConfig.linkConfig.MTUBytes = 1500;
     };
 
     vlan-admin-ap = {
@@ -123,6 +130,7 @@ let
           IPv6SendRA = true;
           DHCPServer = "yes";
         };
+        linkConfig.MTUBytes = 1500;
         ipv6Prefixes = [
           {
             AddressAutoconfiguration = false;