`Socket::bind_device_by_index_{v4,v6}` should be available on linux and android

[ningyuan-wang]

SO_BINDTOIFINDEX has been added to linux since linux v5.1. It behaves similar to SO_BINDTODEVICE but takes the interface index as argument. When a user-space program has the index of a network interface, Calling it directly avoids the possibility where the network device is renamed asynchronously after the program temporarily resolves the interface name from the interface index.

Socket::bind_device_by_index{v4,v6} currently is limited to macOS-like systems and solaris/illumos only. (See also issue #560 )
While current implementation is not valid to linux, another implementation can be defined for linux guarded by cfg macro.

Android also has support for SO_BINDTODEVICE, therefore the support can be extended to android target as well.

[ningyuan-wang]

The initial investigation is incorrect: SO_BINDTOIFINDEX is not defined for libc for android target. (See Android SO definitions in libc 0.2.171). Therefore the target android cannot support this syscall pending an update to it even though Android supports SO_BINDTODEVICE.

[Thomasdezeeuw]

Pr to add support for this would be welcome.

[ningyuan-wang]

For Android and Linux, the syscalls are independent of the V4 or V6 stack used, therefore the function name should be bind_device_by_index, and not having the suffix attached.

PR 573 is created to support this issue.

[Darksonn]

Can you share a bit more info on why this method needs the {v4,v6} naming on some platforms, but not others?

[ningyuan-wang]

The {v4, v6} naming is needed for Darwin-XNU-based operating systems (iOS, Mac OS, and related) since IPv4 and IPv6 are using different socket option parameters than Linux. On XNU IPPROTO_IP and IP_BOUND_IF
is used for IPv4, whereas IPPROTO_IPV6 and IPV6_BOUND_IF are used for IPv6. The syscalls are performed at protocol level (IPv4 or IPv6), and two versions are needed since different parameters are passed to the kernel.

On the other hand, for Linux (and Android), the socket options are at SOL_SOCKET-level and is therefore not related to the IP protocol used. ( See git commit adding SO_BINDTOIFINDEX to linux kernel. )
Quoting the commit message:

This introduces a new generic SOL_SOCKET-level socket option called
SO_BINDTOIFINDEX. It behaves similar to SO_BINDTODEVICE, but takes a
network interface index as argument, rather than the network interface
name.

We note that the existing bind_device, which uses SO_BINDTOIFINDEX also does not have the {v4, v6} naming.