No way to differentiate between layer 3 device and loopback device

[weskoerber]

This is sort of a follow-up from #9.

Layer 3 network devices aren't assigned a MAC address since they're not at the physical data link layer. This can cause confusion between a MAC address that is the loopback and a MAC address that's a layer 3 device.

Consider the following:

❯ ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host noprefixroute 
       valid_lft forever preferred_lft forever
2: tailscale0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1280 qdisc fq_codel state UNKNOWN group default qlen 500
    link/none 
    inet 100.107.149.105/32 scope global tailscale0
       valid_lft forever preferred_lft forever
    inet6 fd7a:115c:a1e0:ab12:4843:cd96:626b:9569/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::4c60:e0c2:4fc:cab1/64 scope link stable-privacy proto kernel_ll 
       valid_lft forever preferred_lft forever
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default 
    link/ether 02:42:37:65:32:3d brd ff:ff:ff:ff:ff:ff
    inet 172.17.0.1/16 brd 172.17.255.255 scope global docker0
       valid_lft forever preferred_lft forever

In this example, the first non-loopback device is tailscale0 whose MAC address is 00:00:00:00:00:00. How is the user supposed to know the difference between this and the loopback?

I think the data field in the MacAddress struct should be optional, and set to null if the device doesn't have a MAC address. This would disambiguate the loopback device from a layer 3 device, while also being more true to form.

However, this library is called "mac_address". If layer 3 devices do not have MAC addresses, should we even consider them at all?

[weskoerber]

The interface flag IFF_NOARP may identify layer 3 devices:

Via man netdevice:

              IFF_NOARP         No arp protocol, L2 destination address not set.

Also in net/if.h:

/* Standard interface flags. */
enum {
// -- snip --
    IFF_NOARP = 0x80,		/* No address resolution protocol.  */
// -- snip --
};

[weskoerber]

The linux kernel explicitly zeroes out the memory for devices with IFF_LOOPBACK and IFF_NOARP flags:

https://github.com/torvalds/linux/blob/77f587896757708780a7e8792efe62939f25a5ab/net/ethernet/eth.c#L107-L110:

	if (dev->flags & (IFF_LOOPBACK | IFF_NOARP)) {
		eth_zero_addr(eth->h_dest);
		return ETH_HLEN;
	}

From Wikipedia on ARP:

The Address Resolution Protocol (ARP) is a communication protocol used for discovering the link layer address, such as a MAC address, associated with a given internet layer address, typically an IPv4 address.

So from all this I conclude that we can exclude devices with the IFF_NOARP flag.