`ClientBuilder::interface` could use `IP_BOUND_IF` on macOS-ish and Solarish systems

[hawkw]

reqwest's ClientBuilder provides an interface method that will configure the client to bind all sockets on a particular interface. Currently, this method is only available on Linux/Android, as it uses the Linux-specific socket option SO_BINDTODEVICE.

On BSD-like systems, including macOS, illumos, etc, similar functionality exists for IP sockets in the form of the IP_BOUND_IF socket option. Per the manual page ip(4p):

IP_BOUND_IF
Limit reception and transmission of packets to this interface. Takes an integer as an argument. The integer is the selected interface index.

It seems like reqwest could probably provide an implementation of the ClientBuilder::interface method on BSD-ish systems using this socket option. I'd be happy to contribute such an implementation if maintainers are open to it.

[seanmonstar]

Sounds fine to me!

[hawkw]

It looks like the interface setting in reqwest's client builder just forwards to the HttpConnector config in hyper-util, so this is (primarily) a hyper-util change, and reqwest will just have to change the cfg attributes once that's in.

[hawkw]

Further research indicates that this socket option actually doesn't exist on FreeBSD, OpenBSD, and NetBSD; it is only on SunOS-like systems (illumos, Solaris, etc) and macOS-like systems (macOS, iOS, and ${APPLE_PRODUCT}OS).

It looks like socket2 presently has methods (Socket::bind_device_by_index_{v4,v6 and Socket::device_index_{v4,v6}) that call getsockopt and setsockopt with IP_BOUND_IF, but they're currently only enabled on ${APPLE_PRODUCT}OS. I've opened rust-lang/socket2#561 to also enable them on illumos and Solaris.