Golang cmd/go: godebug default=go1.20 does not set same netedns0 as GOPATH mode

Go version

go version go1.24.3 darwin/amd64

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE='auto'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/tmp/.gocache'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/rittneje/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
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-build2339782732=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='amd64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD=''
GOMODCACHE='/Users/rittneje/go/pkg/mod'
GONOPROXY='[REDACTED]'
GONOSUMDB='[REDACTED]'
GOOS='darwin'
GOPATH='/Users/rittneje/go'
GOPRIVATE='[REDACTED]'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/rittneje/go1.24.3'
GOSUMDB='sum.golang.org'
GOTELEMETRY='off'
GOTELEMETRYDIR='/Users/rittneje/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/Users/rittneje/go1.24.3/pkg/tool/darwin_amd64'
GOVCS='[REDACTED]'
GOVERSION='go1.24.3'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Consider the following program.

main.go

package main

import (
    "fmt"
    "runtime/debug"
)

func main() {
    bi, ok := debug.ReadBuildInfo()
    if !ok {
        panic("could not read build info")
    }
    for _, s := range bi.Settings {
        if s.Key == "DefaultGODEBUG" {
            fmt.Println(s.Value)
        }
    }
}

go.mod

module godebugtest

godebug default=go1.20

go 1.24.3

What did you see happen?

If I build this application in GOPATH mode, it prints:

asynctimerchan=1,gotestjsonbuildtext=1,gotypesalias=0,httplaxcontentlength=1,httpmuxgo121=1,httpservecontentkeepheaders=1,multipathtcp=0,netedns0=0,panicnil=1,randseednop=0,rsa1024min=0,tls10server=1,tls3des=1,tlsmlkem=0,tlsrsakex=1,tlsunsafeekm=1,winreadlinkvolume=0,winsymlink=0,x509keypairleaf=0,x509negativeserial=1,x509rsacrt=0,x509usepolicies=0

However, if I build this application in go mod mode, it prints:

asynctimerchan=1,gotestjsonbuildtext=1,gotypesalias=0,httplaxcontentlength=1,httpmuxgo121=1,httpservecontentkeepheaders=1,multipathtcp=0,panicnil=1,randseednop=0,rsa1024min=0,tls10server=1,tls3des=1,tlsmlkem=0,tlsrsakex=1,tlsunsafeekm=1,winreadlinkvolume=0,winsymlink=0,x509keypairleaf=0,x509negativeserial=1,x509rsacrt=0,x509usepolicies=0

As you can see, they differ only in the value/presence of netedns0.

What did you expect to see?

From what I can tell, it is not documented anywhere what GOPATH mode is supposed to do for DefaultGODEBUG.

My guess is that GOPATH mode is supposed to be the same as godebug default=go1.20 given that they report almost identical values for DefaultGODEBUG, in which they should agree on the value of netedns0. And the fact that GOPATH mode means godebug default=go1.20 should be mentioned on https://go.dev/doc/godebug.