API to open a socket on a specific interface

I'm working on iOS.

OK then, the System Configuration framework APIs you’re referencing are only available on macOS.

This issue breaks down as follows:

• Binding a connection to an interface

• Selecting an interface.

I’ll discuss each in turn.

If you’re working with low-level APIs — those that let you get at the socket (in the BSD Sockets sense) for the connection — you can force the connection to run over a specific interface in two ways:

• By using

  bind

  to bind the source address of the socket to the interface’s IP address

• Using the

  IP_BOUND_IF

  socket option

This works well for BSD Sockets and things tightly tied to it (like CFSocket and GCD). It does not work at all for higher-level APIs, like NSURLSession. You can bodgy it into working for CFSocketStream, but that has some serious caveats.

With regards identifying an interface, iOS doesn’t have any really good options on that front. You can call

getifaddrs

AF_LINK

struct if_data

• The constants needed to test

  ifi_type

  are not part of the iOS SDK, and hence not officially supported

• Even when you identify the interface type, you’ll find that there are often multiple active interfaces of each type and there’s no supported way to pick between them

What’s your high-level goal here? Specific subsystems within iOS have supported ways to run connections over a specific interface, so there might be a subsystem-specific mechanism that can help you out. For example, captive network apps can force the connection to run over the captive Wi-Fi interface by calling

-bindToHotspotHelperCommand:

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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