net: resolver error messages indicate wrong dns server when using custom resolver

[harshathulasi]

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

go1.14.13

Does this issue reproduce with the latest release?

Yes. At least the latest on go playgound

What operating system and processor architecture are you using (go env)?

go env Output

louper$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/hthulasi/Library/Caches/go-build"
GOENV="/Users/hthulasi/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GONOPROXY="git.soma.salesforce.com"
GONOSUMDB="git.soma.salesforce.com"
GOOS="darwin"
GOPATH="/Users/hthulasi/go"
GOPRIVATE="git.soma.salesforce.com"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/hthulasi/go/src/git.soma.salesforce.com/GovCloud-SecDev/louper/go.mod"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/17/vmjmghsj28d4lzrx7mqshr9r0000gp/T/go-build117441851=/tmp/go-build -gno-record-gcc-switches -fno-common"
louper$ 

What did you do?

If I update/set a the DNS resolver to a custom address the error when host can't connect or couldn't resolve is using the incorrect DNS server address in the response. Example: play.golang.org link: https://play.golang.org/p/LmTM-eOy3q-

What did you expect to see?

I see the error message indicating that it failed with the provisioned DNS server. I am assuming this is just invalid error message but not an issue with golang not using the correct DNS server.

What did you see instead?

It is using the default DNS server (likely configured through DSCP) in the error message instead of 8.8.8.8.

from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
addr: []
err: lookup google.com on 169.254.169.254:53: dial udp 8.8.8.8:53: connect: no route to host
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
from default resolver
addr: []
err: lookup gafwefjawioefjoawjfoaewif.com on 169.254.169.254:53: dial udp 8.8.8.8:53: connect: no route to host

from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
addr: []
err: lookup google.com on 169.254.169.254:53: dial udp 8.8.8.8:53: connect: no route to host
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
from custom resolver
addr: []
err: lookup gafwefjawioefjoawjfoaewif.com on 169.254.169.254:53: dial udp 8.8.8.8:53: connect: no route to host

[seankhliao]

I guess the calling code could check the returned conn and try to extract the actual remote destination if it's valid?

[harshathulasi]

Not sure about the implementation. Its very misleading on which IP the net resolver is using. The above example is in go playground where it can't connect externally but on my local machine its showing the incorrect server. Little troubling when I have to share application logs to others and they question why the application is using the wrong server.

[dmitshur]

CC @bradfitz, @ianlancetaylor per owners.