net: GODEBUG=netdns=cgo does not take priority over netgo build tag

[rittneje]

Go version

go version go1.22.7 darwin/amd64

Output of go env in your module/workspace:

GO111MODULE='auto'
GOARCH='amd64'
GOBIN=''
GOCACHE='/tmp/.gocache'
GOENV='/Users/rittneje/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/rittneje/test/pkg/mod'
GONOPROXY='[REDACTED]'
GONOSUMDB='[REDACTED]'
GOOS='darwin'
GOPATH='/Users/rittneje/test'
GOPRIVATE='[REDACTED]'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/rittneje/go1.22.7'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/Users/rittneje/go1.22.7/pkg/tool/darwin_amd64'
GOVCS='[REDACTED]'
GOVERSION='go1.22.7'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/Users/rittneje/test/src/dialtest/go.mod'
GOWORK='/Users/rittneje/test/go.work'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/kf/kr7_s3xx0l12zbj3jrn082hmzy5gvy/T/go-build3261540356=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I ran a program as follows:

GODEBUG=netdns=cgo+1 go run -tags=netgo main.go

What did you see happen?

go package net: built with netgo build tag; using Go's DNS resolver

What did you expect to see?

go package net: GODEBUG setting forcing use of cgo's resolver

---

It should be noted that GODEBUG takes priority in the reverse situation.

GODEBUG=netdns=go+1 go run -tags=netcgo main.go

go package net: GODEBUG setting forcing use of Go's resolver

[gabyhelp]

Related Issues and Documentation

• net: cannot enforce usage of netgo for DNS resolution #33904 (closed)
• net: export GODEBUG=netdns=cgo doesn't force to use cgo DNS resolver #11816 (closed)
• net: respect GODEBUG=netdns=go on macOS #27774 (closed)
• net: GODEBUG=netdns=cgo doesn't override netgo build tag #57203 (closed)
• net: go cannot resolve localhost on darwin when net.DefaultResolver.PreferGo = true #24393 (closed)
• net: DNS Lookups Very Slow (netgo) #49253 (closed)
• net: force cgo for myhostname and mdns nss modules for LookupAddr on unix #59921 (closed)
• x/net: MacOS lookupIP resolves domains different if CGO_ENABLED flag used. #51753 (closed)
• net: LookupCNAME returns empty string if entry from /etc/hosts is used #44741 (closed)
• net: cgo dns resolver doesn't work as expected. #66114 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[mateusz834]

It works as expected -tags netgo causes the net package to be build entirely without cgo, whereas -tags netcgo sets a preference on the cgo resolver, but the go is still in the binary, so we can run it.

[rittneje]

@mateusz834 I don't think the docs currently mention that. https://pkg.go.dev/net#hdr-Name_Resolution

That also seems like something the debug log should directly state, instead of seemingly ignoring the GODEBUG setting entirely.

[mateusz834]

@rittneje I agree, will send a CL to mention that in the docs.

[mateusz834]

That also seems like something the debug log should directly state, instead of seemingly ignoring the GODEBUG setting entirely.

@ianlancetaylor any thoughts on this? We can mention that in the GODEBUG=netdns=1 mode, but not sure whether this is really that useful. Panic would be better, but i am not sure whether we can do this at this point.

[gopherbot]

Change https://go.dev/cl/616262 mentions this issue: net: improve documentation of netgo/netcgo build tags

[ianlancetaylor]

For the case of GODEBUG=netdns=cgo go run -tags=netgo I don't think we should panic. I don't think that serves anybody. For GODEBUG=netdns=cgo+1 I think it would be OK to log something like "ignoring GODEBUG=netdns=cgo as -tags=netgo forces netdns=go".

[mateusz834]

Also the same issue exists for CGO_ENABLED=0.

[gopherbot]

Change https://go.dev/cl/616263 mentions this issue: net: improve GODEBUG=netdns=1 debug messages