> DNS is a complex topic. Much more complex than people admit it is, and that can definitely not be expressed fully with resolv.conf. I do agree that it is too late to get rid of it (and was not my concern actually), but it is too limited to be of actual use outside of the simple "I have a single DNS server with a single search domain".
resolv.conf is limited, but it's also been highly stable for decades, and it's sufficient if not ideal for controlling how getaddrinfo works (at least for on-the-wire requests), including controlling things like EDNS0, parallel requests, etc. Most if not all libc resolvers support things like parallel querying and other simple knobs which are configurable (if at all--see musl libc) through resolv.conf, demonstrating that it's expressive enough for most if not all common requirements shared among various client-side stub resolvers.
> And for more complex cases, simple integration with the network configuration daemon
But which one? Are you suggesting integration by way of loading it's configuration(s) (which puts us back at square 0), or by a modified query protocol, or by interfacing with the broader but even more diverse native configuration systems? None of the options seem remotely practical from the perspective of most open source projects, unless they're specifically targeting a single environment like Linux/systemd/resolvd. I don't see a viable pathway to make that happen. By contrast, embracing and hopefully improving resolv.conf as an integration point could be done piecemeal, environment by environment. The syntax is already effectively universal across systems, with the options directive providing most of the knobs. We could even make an initial push through POSIX by officially standardizing the syntax, which may even convince musl libc to make its resolver actually configurable.
> In the specific example of resolved, I'd argue it's even less work for applications, because they don't need to query multiple DNS servers at once (it'll handle it for them), don't need to try resolution with and without search domain, etc.
Yes, in most cases it's sufficient for userland applications to just make simple requests to the locally managed resolver service defined in resolv.conf. But the cases and projects needing more control over how they do their requests, using their own resolvers, only grows, especially with the proliferation of DNS schemes-see, e.g., the various recent HTTP-related DNS records which often require multiple queries and can benefit from parallel queries managed internally. A prime example is getaddrinfo itself, some implementations of which do parallel queries for A/AAAA lookups. Which brings us back to my main point: resolv.conf is the only common centralized point across almost all environment (Windows being the major exceptoin) for configuring basic DNS services.
I'm not arguing for improving resolv.conf integration as a way to replace local DNS services or their configuration. Just that for decades the staleness of resolv.conf has been a conspicuous and growing pain point from both a system configuration and userland integration perspective, and a little coordinated love & attention across the ecosystem, if only firmly committing to what's already there (especially for glibc and FreeBSD) as a reliable and more easily leveraged source of truth for code that needs it, would go a long way.