Some EtcPal APIs make use of an index value to identify network interfaces. Almost all underlying network stacks have the notion of a network interface index, which is standardized for IPv6 by RFC 3493.

Interface indexes may also be used for specifying the zone index or scope ID of an IPv6 address (see RFC 4007). However, depending on the address scope, this doesn't always apply. It also seems to currently be a lightly-specified area of the IPv6 protocol, so it should not be treated as a guarantee.

In our protocols, we use the interface index mostly for multicasting purposes. When used this way, it is used to specify an interface on which to send or receive multicast traffic for a given socket. The socket (Network Socket Interface) API uses them to specify network interfaces for the ETCPAL_MCAST_JOIN_GROUP, ETCPAL_MCAST_LEAVE_GROUP, and ETCPAL_IP_MULTICAST_IF socket options.

The interface index is of type unsigned int; 0 is an invalid value for specifying a specific interface, but can be used as a wildcard (signaling the kernel to pick the "best" or most appropriate interface). Interface indexes for each connected interface can be obtained from the netint (Network Interfaces) API (in the EtcPalNetintInfo structure returned from etcpal_netint calls), or from the relevant OS APIs on OSes that support them (many conform to the standard interface specified by RFC 3493).

Some embedded stacks have limited or nonstandard ways to obtain a network interface index. For clarity, the exact way that interface indexes are obtained or derived in each stack is listed below.

OS/Stack	Method
macOS	Standard / RFC 3493, can also be inferred from data in `ifconfig` and `netstat -rn`
Microsoft Windows	Indicated in IP_ADAPTER_ADDRESSES structure, also visible via `route print` at command line
Linux	Standard / RFC 3493, visible via `ip link` on newer/systemd systems, or can be inferred from data in `ifconfig`
lwIP	Standard / RFC 3493 as of lwIP 2.1.0. Also available using the lwIP API function netif_get_index().
MQX RTOS	The index is derived from the RTCS device identifier (an index starting from 0 into a linked list of network interfaces) plus 1. This is translated internally for relevant socket calls.