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net: GODEBUG=netdns=cgo doesn't override netgo build tag #57203

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roidelapluie opened this issue Dec 9, 2022 · 3 comments
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net: GODEBUG=netdns=cgo doesn't override netgo build tag #57203

roidelapluie opened this issue Dec 9, 2022 · 3 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.

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@roidelapluie
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roidelapluie commented Dec 9, 2022 •
edited

What version of Go are you using (go version)?

$ go version
go 1.19.3

(this is Prometheus, cross compiled from linux)

Does this issue reproduce with the latest release?

yes

What did you do?

Used GODEBUG=netdns=cgo

https://gist.github.com/roidelapluie/8a87428055a77bfc3a08dc427e595599

What did you expect to see?

Using C for DNS, because the C library routines are always available on Windows.

What did you see instead?

It is using the cgo resolved, which fails to resolve e.g. localhost.
@roidelapluie roidelapluie changed the title net: Can't force cgo resolution on Windonws net: Can't force cgo resolution on Windows Dec 9, 2022
@seankhliao seankhliao changed the title net: Can't force cgo resolution on Windows net: GODEBUG=netdns=cgo doesn't override netgo build tag Dec 9, 2022
@seankhliao seankhliao added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Dec 9, 2022
@mateusz834
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mateusz834 commented Dec 9, 2022 •
edited

I feel like that is the way it is supposed to work.
If you compile with netgo flag, then you cannot change the resolver to a cgo one.

Edit:
But I think that you can easily cross-compile from linux to windows without -tags netgo, it should use then the "cgo" version. This should work on gotip, probably related to: #53490

@roidelapluie
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Yes but I would expect to be able to make the choice at runtime

@rsc
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rsc commented Dec 9, 2022

The netgo build tag means do not compile cgo into the program at all.
If you want cgo linked into your program, don't use -tags netgo.
If you want a program linked with cgo to ignore using cgo by default, then you could put this at the start of func main:

os.Setenv("GODEBUG", "netdns=go,"+os.Getenv("GODEBUG"))

If #56986 is accepted, you could write that as //go:debug netdns=go instead.

@rsc rsc closed this as completed Dec 9, 2022
@thaJeztah thaJeztah mentioned this issue May 24, 2023
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@golang golang locked and limited conversation to collaborators Dec 9, 2023
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@rsc @roidelapluie @gopherbot @seankhliao @mateusz834