enum_netdev_priv_flags (9)

enum netdev_priv_flags {
  IFF_802_1Q_VLAN,
  IFF_EBRIDGE,
  IFF_SLAVE_INACTIVE,
  IFF_MASTER_8023AD,
  IFF_MASTER_ALB,
  IFF_BONDING,
  IFF_SLAVE_NEEDARP,
  IFF_ISATAP,
  IFF_MASTER_ARPMON,
  IFF_WAN_HDLC,
  IFF_XMIT_DST_RELEASE,
  IFF_DONT_BRIDGE,
  IFF_DISABLE_NETPOLL,
  IFF_MACVLAN_PORT,
  IFF_BRIDGE_PORT,
  IFF_OVS_DATAPATH,
  IFF_TX_SKB_SHARING,
  IFF_UNICAST_FLT,
  IFF_TEAM_PORT,
  IFF_SUPP_NOFCS,
  IFF_LIVE_ADDR_CHANGE,
  IFF_MACVLAN
};

IFF_802_1Q_VLAN

802.1Q VLAN device

IFF_EBRIDGE

Ethernet bridging device

IFF_SLAVE_INACTIVE

bonding slave not the curr. active

IFF_MASTER_8023AD

bonding master, 802.3ad

IFF_MASTER_ALB

bonding master, balance-alb

IFF_BONDING

bonding master or slave

IFF_SLAVE_NEEDARP

need ARPs for validation

IFF_ISATAP

ISATAP interface (RFC4214)

IFF_MASTER_ARPMON

bonding master, ARP mon in use

IFF_WAN_HDLC

WAN HDLC device

IFF_XMIT_DST_RELEASE

dev_hard_start_xmit is allowed to release skb->dst

IFF_DONT_BRIDGE

disallow bridging this ether dev

IFF_DISABLE_NETPOLL

disable netpoll at run-time

IFF_MACVLAN_PORT

device used as macvlan port

IFF_BRIDGE_PORT

device used as bridge port

IFF_OVS_DATAPATH

device used as Open vSwitch datapath port

IFF_TX_SKB_SHARING

The interface supports sharing skbs on transmit

IFF_UNICAST_FLT

Supports unicast filtering

IFF_TEAM_PORT

device used as team port

IFF_SUPP_NOFCS

device supports sending custom FCS

IFF_LIVE_ADDR_CHANGE

device supports hardware address change when it's running

IFF_MACVLAN

Macvlan device

These are the struct net_device, they are only set internally by drivers and used in the kernel. These flags are invisible to userspace, this means that the order of these flags can change during any kernel release.

You should have a pretty good reason to be extending these flags.