I was able in an afternoon to implement a pretty decent completely async Swift DNS resolver client for my app. DNS clients are simple enough to build that rolling your own async is not a big deal anymore.
Yes, there is separate work to discern what DNS server the system is currently using: on macOS this requires a call to an undocumented function in libSystem - that both Chromium and Tailscale use!
A lot of folks think this, but did you also implement EDNS0?
The golang team also thought DNS clients were simple, and it led to almost ten years of difficult to debug panics in Docker, Mesos, Terraform, Mesos, Consul, Heroku, Weave and countless other services and CLI tools written in Go. (Search "cannot unmarshal DNS message" and marvel at the thousands of forum threads and GitHub issues that all bottom out at Go implementing the original DNS spec and not following later updates.)
Even once you use the private `dns_config*()` APIs on macOS, you need to put in heavy lifting to correctly handle scoped, service-specific providers, supplemental matching rules, etc -- none of which is documented, and can change in the future.
Since you're not using the system resolver, you won't benefit from mDNSResponder's built-in DNS caching and mDNS resolution/caching/service registration, so you're going to need to reimplement all of of that, too. And don't forget about nsswitch on BSD/Linux/Solaris/etc -- there's no generic API that let's you plug into that cleanly, so for a complete implementation there, you need to:
- Reimplement built-in modules like `hosts` (for `/etc/hosts`), `cache` (query a local `nscd` cache, etc), and more.
- Parse the nsswitch.conf configuration file, including the rule syntax for defining whether to continue/return on different status codes.
- Reimplement rule-based dispatch to both the built-in modules and custom, dynamically loaded modules (like `nss_mdns` for mDNS resolution).
Each OS has its own set of built-ins, private, incompatible interfaces for interacting with things like the `nscd` cache daemon, and the nsswitch APIs and config files themselves differ across operating systems. And we haven't even discussed Windows yet.
Re-implementing all of this correctly, thoroughly, and keeping it working across OS changes is extremely non-trivial.
The simplest and most correct solution is to just:
- Use OS-specific async APIs when available; e.g. `CFHostStartInfoResolution()` on macOS, `DnsQueryEx()` on Windows, `getaddrinfo_a()` on glibc (although that spawns a thread, too), etc.
- If you have a special use-case where you need absolutely need better performance, and do not need to support all the system resolver functionality above (i.e. server-side, controlled deployment environment), use an event-based async resolver library.
- Otherwise, issue a blocking call to `getaddrinfo()` on a new thread. If you're very worried about unbounded resource consumption, use a size-limited thread pool.
I am always amused when folks rediscover the bad idea that is `pthread_cancel()` — it’s amazing that it was ever part of the standard.
We knew it was a bad idea at the time it was standardized in the 1990s, but politics — and the inevitable allure of a very convenient sounding (but very bad) idea — meant that the bad idea won.
Funny enough, while Java has deprecated their version of thread cancellation for the same reasons, Haskell still has theirs. When you’re writing code in IO, you have to be prepared for async cancellation anywhere, at any time.
This leads to common bugs in the standard library that you really wouldn’t expect from a language like Haskell; e.g. https://github.com/haskell/process/issues/183 (withCreateProcess async exception safety)
What's crazy is that it's almost good. All they had to do was make the next syscall return ECANCELED (already a defined error code!) rather than terminating the thread.
Musl has an undocumented extension that does exactly this: PTHREAD_CANCEL_MASKED passed to pthread_setcancelstate.
You can sort of emulate that with pthread_kill and EINTR but you need to control all code that can call interruptable sys calls to correctly return without retry (or longjmp/throw from the signal handler, but then we are back in phtread_cancel territory)
That would have been fantastic. My worry is if we standardized it now, a lot of library code would be unexpectedly dealing with ECANCELED from APIs that previously were guaranteed to never fail outside of programmer error, e.g. `pthread_mutex_lock()`.
Looking at some of my shipping code, there's a fair bit that triggers a runtime `assert()` if `pthread_mutex_lock()` fails, as that should never occur outside of a locking bug of my own making.
It’s extremely easy to write application code in Haskell that handles async cancellation correctly without even thinking about it. The async library provides high level abstractions. However your point is still valid as I do think if you write library code at a low level of abstraction (the standard library must) it is just as error prone as in Java or C.
For those using it in Python, Gevent provides a pluggable set of DNS resolvers that monkey-patch the standard library's functions for async/cooperative use, including one built on c-ares: https://www.gevent.org/dns.html
Still alive and kicking in production for us! For situations where many requests are bound by external HTTP requests to third-party suppliers, it's an amazing way to allow for practically unlimited concurrency with limited cores.
It's weird to me that event-based DNS using epoll or similar doesn't have a battle-tested implementation. I know it's harder to do in C than in Rust but I'm pretty sure that's what Hickory does internally.
it’s a weird problem, in that (1) DNS is hard, and (2) you really need the upstream vendor to solve the problem, because correct applications want to use the system resolver.
If you don’t use the system resolver, you have to glue into the system’s configuration mechanism for resolvers somehow … which isn’t simple — for example, there’s a lot of complex logic on macOS around handling which resolver to use based on what connections, VPNs, etc, are present.
And the there’s nsswitch and other plugin systems that are meant to allow globally configured hooks plug into the name resolution path.
It falls into the category that most people think they understand DNS, the same as JavaScript, or e.g. elections, but the devil is in the detail. And I can tell you, at least for DNS (and Dutch Elections), it's kind of tricky, see fun cases like https://github.com/internetstandards/Internet.nl/issues/1370 and I thought the same before I had my current job which involves quite some tricky DNS stuff (and regarding this we also sometimes encounter bugs in unbound https://github.com/internetstandards/Internet.nl/issues/1803 )
Yes, there is separate work to discern what DNS server the system is currently using: on macOS this requires a call to an undocumented function in libSystem - that both Chromium and Tailscale use!
The golang team also thought DNS clients were simple, and it led to almost ten years of difficult to debug panics in Docker, Mesos, Terraform, Mesos, Consul, Heroku, Weave and countless other services and CLI tools written in Go. (Search "cannot unmarshal DNS message" and marvel at the thousands of forum threads and GitHub issues that all bottom out at Go implementing the original DNS spec and not following later updates.)
Since you're not using the system resolver, you won't benefit from mDNSResponder's built-in DNS caching and mDNS resolution/caching/service registration, so you're going to need to reimplement all of of that, too. And don't forget about nsswitch on BSD/Linux/Solaris/etc -- there's no generic API that let's you plug into that cleanly, so for a complete implementation there, you need to:
- Reimplement built-in modules like `hosts` (for `/etc/hosts`), `cache` (query a local `nscd` cache, etc), and more.
- Parse the nsswitch.conf configuration file, including the rule syntax for defining whether to continue/return on different status codes.
- Reimplement rule-based dispatch to both the built-in modules and custom, dynamically loaded modules (like `nss_mdns` for mDNS resolution).
Each OS has its own set of built-ins, private, incompatible interfaces for interacting with things like the `nscd` cache daemon, and the nsswitch APIs and config files themselves differ across operating systems. And we haven't even discussed Windows yet.
Re-implementing all of this correctly, thoroughly, and keeping it working across OS changes is extremely non-trivial.
The simplest and most correct solution is to just:
- Use OS-specific async APIs when available; e.g. `CFHostStartInfoResolution()` on macOS, `DnsQueryEx()` on Windows, `getaddrinfo_a()` on glibc (although that spawns a thread, too), etc.
- If you have a special use-case where you need absolutely need better performance, and do not need to support all the system resolver functionality above (i.e. server-side, controlled deployment environment), use an event-based async resolver library.
- Otherwise, issue a blocking call to `getaddrinfo()` on a new thread. If you're very worried about unbounded resource consumption, use a size-limited thread pool.
In that discussion, most of the same points as in this article were already discussed, specifically some async DNS alternatives.
See also here the discussion: https://github.com/crystal-lang/crystal/issues/13619
We knew it was a bad idea at the time it was standardized in the 1990s, but politics — and the inevitable allure of a very convenient sounding (but very bad) idea — meant that the bad idea won.
Funny enough, while Java has deprecated their version of thread cancellation for the same reasons, Haskell still has theirs. When you’re writing code in IO, you have to be prepared for async cancellation anywhere, at any time.
This leads to common bugs in the standard library that you really wouldn’t expect from a language like Haskell; e.g. https://github.com/haskell/process/issues/183 (withCreateProcess async exception safety)
Musl has an undocumented extension that does exactly this: PTHREAD_CANCEL_MASKED passed to pthread_setcancelstate.
It's great and it should be standardized.
Looking at some of my shipping code, there's a fair bit that triggers a runtime `assert()` if `pthread_mutex_lock()` fails, as that should never occur outside of a locking bug of my own making.
POSIX can specify a new version of DNS resolution.
libcs can add extensions, allowing applications to detect when they are targeting those systems and use them.
Applications on Linux and Windows can bypass libc.
If you don’t use the system resolver, you have to glue into the system’s configuration mechanism for resolvers somehow … which isn’t simple — for example, there’s a lot of complex logic on macOS around handling which resolver to use based on what connections, VPNs, etc, are present.
And the there’s nsswitch and other plugin systems that are meant to allow globally configured hooks plug into the name resolution path.
It's really not.
Just because some systems took something fundamentally simple and wrapped a bunch of unnecessary complexity around it does not make it hard.
At its core, it's an elegant, minimal protocol.
But maybe DNSSEC is the 'unnecessary complexity' for you (I think it's kind of fundamental to secure DNS). Also without DNSSEC they needed RFC's like https://datatracker.ietf.org/doc/html/rfc8020 to clarify fundamentals (same goes for https://datatracker.ietf.org/doc/html/rfc8482 to fix stuff).