nixos.nix

        systemd.services.eth0-mac-change = 
          let iname = "eth0";
              macaddress = "aa:aa:aa:aa:aa:aa";
              subsystemdevice = interface:
                "sys-subsystem-net-devices-${lib.escapesystemdpath interface}.device";
              devicedependency = if config.boot.iscontainer
                then []
                else [ (subsystemdevice i.name) ];
          in {
            description = "mac configuration of ${iname}";
            wantedby = [ "network-interfaces.target" ];
            before = [ "network-interfaces.target" ];
            after = [ "network-link-${iname}" ];
            path = [ pkgs.iproute pkgs.nettools ];
            serviceconfig = {
              type = "oneshot";
              remainafterexit = true;
            };
            script =
              ''
                echo "configuring link..."
                echo "setting mac address to ${macaddress}..."
                ifconfig down ${iname}
                ip link set "${iname}" address "${macaddress}"
                ifconfig up ${iname}
              '';
          };