net: DialTimeout causes persistent slowdown on windows

[leemojiang]

Go version

go version go1.23.3 windows/amd64

Output of go env in your module/workspace:

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Administrator\AppData\Local\go-build
set GOENV=C:\Users\Administrator\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\Administrator\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\Administrator\go
set GOPRIVATE=
set GOPROXY=https://goproxy.cn,direct
set GOROOT=C:\Program Files\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=C:\Program Files\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.23.3
set GODEBUG=
set GOTELEMETRY=local
set GOTELEMETRYDIR=C:\Users\Administrator\AppData\Roaming\go\telemetry
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=NUL
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=C:\Users\ADMINI~1\AppData\Local\Temp\go-build1513341593=/tmp/go-build -gno-record-gcc-switches

What did you do?

I am practicing with a simple port scanner. At first I used net.Dial and the program completes very fast as the following shows:

However when I switch to net.DialTimeout, the program beomes mysteriously slow (30~40 times slower). The side effect seems to be system level, rollback to net.Dial and recompile can't solve the problem. The side effect is gone after a restart.

I make some test and found out the side effect seems to happen after net.DialTimeout connect a non local address. DialTimeout on 127.0.0.1 is fine, after connecting a nonlocal ip address, the forementioned side effect is triggered. Dialing localhost becomes very slow(both net.Dial and net.DialTimeout ), and the only way to solve it is to restart.

Here is the full code:

package main

import (
	"flag"
	"fmt"
	"net"
	"sync"
	"time"
)

var (
	ip      string
	minport int
	maxport int
	timeout int
)

func init() {
	flag.StringVar(&ip, "ip", "127.0.0.1", "Ip to scan.")
	flag.IntVar(&minport, "p1", 0, "Port to scan from.")
	flag.IntVar(&maxport, "p2", 1024, "Port to scan stop.")
	flag.IntVar(&timeout, "t", 1000, "Time out in microseconds")
	flag.Parse()
}

func main() {
	start := time.Now()

	ports := []int{}
	wg := &sync.WaitGroup{}
	mutex := &sync.Mutex{}

	for p := minport; p <= maxport; p++ {
		wg.Add(1)
		go portScan(ip, p, wg, &ports, mutex)
	}

	wg.Wait()

	elasped := time.Since(start)
	fmt.Println("Scanned from ", minport, " to ", maxport, " in ", elasped)
	fmt.Println(ports)
}
func portScan(ip string, port int, wg *sync.WaitGroup, ports *[]int, mutex *sync.Mutex) {
	defer wg.Done()
	address := fmt.Sprintf("%s:%d", ip, port)
	fmt.Println("Connecting ", address)
	conn, err := net.Dial("tcp", address)
	// conn, err := net.DialTimeout("tcp", address, 500*time.Microsecond)

	if err == nil {
		defer conn.Close()
		// fmt.Println("Connection successful.")
		// localAddr := conn.LocalAddr().String()
		// remoteAddr := conn.RemoteAddr().String()
		// fmt.Println("Local Address:", localAddr)
		// fmt.Println("Remote Address:", remoteAddr)

		mutex.Lock()
		*ports = append(*ports, port)
		mutex.Unlock()

	} else {
		// fmt.Println("Error", err)
	}

}

What did you see happen?

As above.

What did you expect to see?

As above.

[gabyhelp]

Related Issues

• net: Dial is slow on localhost #23366 (closed)
• net: TestDialTimeout never completes on Windows XP #6063 (closed)
• net/http: TestCancelRequestWithChannelBeforeDo_Cancel/h2 failure with `leaked a timing out dial` on Windows #59494
• x/playground,internal/poll: timeout or long run time when using httptest.NewServer #60356
• net: mass tcp dial timeout with the concurrency more than 10000 #32761
• net: TestDialParallel spuriously fails due to hard-coded timeout in dialClosedPort #46884 (closed)
• runtime: unnecessary return in netpoll #49026
• net/http: it's a bug? #14895 (closed)
• net: TestDialParallel leaks worker goroutines and inflight sockets #11878 (closed)

Related Discussions

• Why can net.Dial be very slow on a hostname defined in /etc/hosts?

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

[seankhliao]

cc @golang/windows

note that your timeout is unreasonably short for any sort of non local connection

[qmuntal]

Thansk for reporting. Can you reproduce the same issue with Go 1.22?

[leemojiang]

cc @golang/windows

note that your timeout is unreasonably short for any sort of non local connection

Thanks. I have tried different time (3 seconds) still got the issue. Once dialed a non local address, the slow down will be triggered.

Thansk for reporting. Can you reproduce the same issue with Go 1.22?

Glad I could help. I conduct some tests on go1.22.10 windows/amd64, the issue is also triggered but act a little diferent:The first time dialing non local address, the program finished in about 20s ( time out set to 3 seconds), while the second time it results in 1m38s. After that even localhost is slowed down.