That second parameter could accept a text IP-address eg "192.168.100.100". Like the address bar of a browser.

So, the dns lookup is still there, it's just optional. You would have to pass a flag to tell it not to try to do the lookup or you would have to do something fancy trying to know if the passed parameter is an IP.

This is already how all the DNS lookup APIs work.

The problem is with asking the kernel to perform dns lookups.

If SystemD had it's way, it would. The local system resolver cache is the default on most systems. DNS is well understood. Why not put it in the Kernel?

If you're doing that, though, you'd better dang well give me secure key storage in the Kernel, too, or full TLS capability. Allow my program to n load it's RSA/ECC key at startup, then drop privileges to access that file, and have a kernel module handle the cryptography.

It extremely doesn't make sense to have a DNS resolver resident in the kernel: DNS resolution is policy-intensive, involves various upgrades to cryptographic transports, and often requires debugging, all weak points of the kernel. Against all that, there's no performance advantage, and doing it admits a new networking API that almost no applications will use --- most new code being written in higher-level languages that already abstract over system calls.

There's a huge performance advantage, if you ask SystemD - Multiple resolutions can be cached. This is incredible bullshit, of course, but that is one of many lies Pottering has gotten away with.

Caching resolutions can be a nice thing, and having in one place can be useful so that the throwing away the cache can be done consistently for all apps when useful (you likely should clear the cache when switching networks, or when the user asks you to), but none of that means it should be a kernel service.

If there's enough volume of requests that a kernel level cache meaningfully improves real world scenarios, something is broken in those scenarios. :p (but I think you agree with me)

How often do you not want to support host lookup when doing a TCP or UDP connection?

Certainly you could implement these functions using the existing BSD socket libraries, but that misses out on having them used everywhere so you get "free" IPv6 transitions and whatnot.