net.Resolver Ignores Custom Dialer

[moloch--]

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

$ go version
go version go1.12.7 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/moloch/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/moloch/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/local/opt/go/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/opt/go/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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/n3/b920qr2s0wj9h9z0hyj678vr0000gn/T/go-build549888698=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Attempting to cross-compile a Windows binary from MacOS that does DNS-over-TCP, to do this I implemented a custom dialer that uses TCP (code excerpt is below). The dialer is ignored and the program still performs DNS resolution over UDP (verified with Wireshark), and does not return any errors. Setting preferGo to true or false does not appear to have any affect. Note that while I'm compiling on my Mac (version/env info above) the code is running on Windows.

func dnsTCPLookup(resolverIP string, domain string) (string, error) {
	tcpResolver := net.Resolver{
		PreferGo: true,
		Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
			log.Printf("CUSTOM DIALER CALLED\n")
			dialer := net.Dialer{}
			return dialer.DialContext(ctx, "tcp", fmt.Sprintf("%s:53", resolverIP))
		},
	}
	log.Printf("[dns] lookup -> %s", domain)
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()
	txts, err := tcpResolver.LookupTXT(ctx, domain)
	if err != nil || len(txts) == 0 {
		log.Printf("[!] failure -> %s", domain)
		return "", err
	}
	return strings.Join(txts, ""), nil
}

CUSTOM DIALER CALLED is not log'd.

What did you expect to see?

Go attempt to establish a connection using the provided TCP Dialer, log.Printf statement, or an error.

What did you see instead?

DNS resolution over UDP, no errors.

[fraenkel]

You might have missed these comments in the net documentation:

 // PreferGo controls whether Go's built-in DNS resolver is preferred
 // on platforms where it's available.

and more importantly

On Windows, the resolver always uses C library functions, such as GetAddrInfo and DnsQuery.

[moloch--]

Ah okay, it's strange because nslookup will fallback to TCP-over-DNS but the C functions that Go attempts to use just fail, which is why I was attempting to use a custom dialer. Is there anyway to do DNS-over-TCP in Go?

This seems to be a duplicate of 29621