My APP needs to send HTTP request via Cellular while WiFi is connected. I can use setsockopt and IP_BOUND_IF to bind low level socket that sends request data with Cellular interface. But I have no idea how to bind DNS resolving to Cellular. If I use getaddrinfo, DNS resolving will go through WiFi by default, and the result is not what I want.

The network operator companies of WWAN and WiFi are different. As far as I know, the server end's access point(such as an AWS Gateway) IPs maybe different in different operators' network. For example, if I resolve DNS under Cellular, it will return the IPs of access point in Cellular operator's network. In order to get a shortest path and good network performance, it is not recommended to connect WiFi operator's access point when the APP is using Cellular to send request. Emmmm, and another situation is considered: When the WiFi network is poor, I need my APP to send request via Cellular, as well as DNS resolving request.

Yes, my APP's network stack is implemented on BSD Sockets. The network stack is cross-platform, also works on Android, and I have done a lot of works to optimize it. iOS Network framework is convenient, but I have no choice in this circumstance. Does it seem that there is no low-level API on iOS to handle the DNS resolving work mentioned above? If there is a high-level API that only deal with the DNS on multi-network, it's also great.

we now keep the WWAN interface up where possible It's a good decision. I don't need to bring WWAN up when WiFi is connected. But I have to do this on Android :-( Even on non-Apple platforms, a best-of-class Happy Eyeballs implementation is really frikkin’ hard. Agree. Low-level APIs are not that friendly, and it's a bit challenging to develop on them :-) Thanks buddy! You helped me a lot.