I think it very well could be.

However, I would imagine (not having looked at the problem in depth) that one would want to be able to use things like Vec<T> or CString from the Rust standard library when implementing a DNS resolver. Since the Rust standard library depends on libc, I'm not sure how one would properly allow the Rust symbols to replace the linked-in libc symbols while still depending on the rest of libc. Maybe it's not an issue? Maybe there's some magic which could be done?

Nonetheless, I mostly started doing this to see if it could be done, not to prove any point (although I'd love it if a point is proven along the way), so a bottom-up approach with #![no_std] was the easiest way to prevent any sort of cyclic dependencies and/or linker issues.