net: Support the /etc/resolver DNS resolution configuration hierarchy on OS X when cgo is disabled

[Rotonen]

https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man5/resolver.5.html

OS X allows you to add TLD specific resolver configurations. Quite popular ones are /etc/resolver/vm for local virtual machines and /etc/resolver/dev for local development purposes.

https://golang.org/src/net/dnsclient_unix.go#L231

Go seems to be hardcoded to only take /etc/resolv.conf into account on Unix platforms.

[nodirt]

I don't think Go-native DNS resolving mechanism is used on Mac.
https://golang.org/src/net/dnsclient_unix.go#L231 is not executed if I run

addrs, err := net.LookupHost("google.com")

on my Mac.

If I enable debugging (GODEBUG=netdns=2 go run test.go), the following is printed:

go package net: using cgo DNS resolver
go package net: hostLookupOrder(google.com) = cgo

which means that OS-native DNS resolving is used.

Can you supply an exact configuration file, Go code, actual and expected output?

[titanous]

@nodirt This is for a binary with cgo off.

[davecheney]

If cgo is disabled then the pure go DNS resolver will be used. If you want
to use the Mac DNS resolver, plese build with cgo.

On Mon, 7 Sep 2015 07:47 Jonathan Rudenberg notifications@github.com
wrote:

@nodirt https://github.com/nodirt This is for a binary with cgo off.

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#12524 (comment).

[nodirt]

Shouldn't be a problem since this is needed only on a dev machine.

On Sun, Sep 6, 2015 at 4:06 PM Dave Cheney notifications@github.com wrote:

If cgo is disabled then the pure go DNS resolver will be used. If you want
to use the Mac DNS resolver, plese build with cgo.

On Mon, 7 Sep 2015 07:47 Jonathan Rudenberg notifications@github.com
wrote:

@nodirt https://github.com/nodirt This is for a binary with cgo off.

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Reply to this email directly or view it on GitHub
#12524 (comment).

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#12524 (comment).

[titanous]

In this specific case, @Rotonen was using the Flynn binary that we distribute as a compiled artifact, it is compiled without cgo to ease cross-compilation. Just because the user is a developer doesn't mean that they are a Go developer or want to compile the binary for themselves. The only question here is if this feature is out of scope for the pure-Go resolver.

[minux]

cross compilation with cgo-enabled net package is not that hard. You can reuse the package contained in binary distribution and force internal linking.

[ianlancetaylor]

I don't see anything wrong with supporting the OS X /etc/resolver directory. That said, my understanding is that the Go DNS resolver does not work well on most OS X machines. That is why it is disabled by default.

[mterron]

This would be great in all platforms anyway. Is there any disadvantage from supporting this behaviour? It seems that it'd neatly resolve the need to install and configure dnsmasq to provide the simple function of having different resolvers for different TLDs.

[bradfitz]

Any updates would be posted here. No updates have been posted here.

[bitglue]

See resolver(5). Just reading the files out of /etc/resolver/* will miss out on other mechanisms for configuring the same thing, for example configuration profiles or IKE attributes.

[flyinprogrammer]

Just stumbled upon this today while attempting to use coredns as a dns proxy for local development. It's a real bummer to discover how naive our support for os x is.

[bradfitz]

We've generally assumed people use cgo on Darwin, so this bug has never been a priority.

I do admit that practically means that Darwin binaries need to be built on Darwin, which is difficult for people wanting to cross-compile for a dozen platforms as part of their release process.

Perhaps on Darwin without cgo we could just shell out to a program to do DNS resolution (e.g. host, dig, nslookup?). At least nslookup has an interactive mode that would permit re-using a child process for multiple lookups, if that proves necessary for performance.

[ianlancetaylor]

Probably best to discuss further on a patch submission. Thanks.

[gopherbot]

Change https://go.dev/cl/448020 mentions this issue: net: adjust build tags for darwin libc calls

[AaronFriel]

@bradfitz I may reach out, thank you! I'm not sure if we're ready to take on maintaining a fork, but you've piqued my interest.