x/net/ipv4: SetTOS should return errNotImplemented on Windows

[Eelco]

Go version

go version go1.22.3 darwin/arm64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='arm64'
GOBIN='/Users/gopher/go/bin'
GOCACHE='/Users/gopher/Library/Caches/go-build'
GOENV='/Users/gopher/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMODCACHE='/Users/gopher/go/pkg/mod'
GOOS='darwin'
GOPATH='/Users/gopher/go'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/Cellar/go/1.22.3/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.22.3/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.3'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/Users/gopher/reproduce/go.mod'
GOWORK=''
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 arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/68/ch3mzzs55bz8x9mzk1dsd35c0000gn/T/go-build1659040423=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

I cross build on MacOS for Windows so the version and env above is all output for MacOS but this issue is created for the Windows implementation of SetTOS.

See the following code:

package main

import (
	"flag"
	"log"
	"net"

	"golang.org/x/net/ipv4"
)

func main() {
	name := flag.String("i", "Ethernet", "interface name")
	flag.Parse()

	iface, err := net.InterfaceByName(*name)
	if err != nil {
		log.Fatalf("%v", err)
	}

	group := &net.UDPAddr{IP: net.ParseIP("224.0.0.1"), Port: 1234}
	conn, err := net.ListenPacket("udp4", "224.0.0.1:1234")
	if err != nil {
		log.Fatalf("%v", err)
	}

	p := ipv4.NewPacketConn(conn)
	p.SetMulticastInterface(iface)

	if err := p.JoinGroup(iface, &net.UDPAddr{IP: group.IP}); err != nil {
		log.Fatalf("%v", err)
	}

	err = p.SetTOS(0x40)
	if err != nil {
		log.Fatalf("%v", err)
	}
	_, err = p.WriteTo([]byte("test1"), nil, group)
	if err != nil {
		log.Fatalf("%v", err)
	}
}

What did you see happen?

The code runs successful on Windows but does not set the TOS value on the outgoing UDP packet.

What did you expect to see?

According to the document that is referenced by the source code the IP_TOS is described as Do not use as the TOS cannot be set programmatically.

I would think it would be better to remove this line from the code and make calling SetTOS on Windows fail with an error because currently the call seems to succeed without any effect to the actual packet.

[Dmitri Shuralyov]

CC @golang/windows, @ianlancetaylor.

[bjorndm]

It seems the TOS field itself is obsolete: https://en.m.wikipedia.org/wiki/Differentiated_services

It should be replaced by a combination of the DS field and ECN field which winsock does support : https://learn.microsoft.com/en-us/windows/win32/winsock/winsock-ecn

[Eelco]

Fair point. Actually what I am trying to achieve is to set a value on the DS field part of the DS/ECN byte.

The TOS field has been absolute well before Go development was even started ;-). TOS and DS/ECN refer to the same byte in the IP header. In the MS document you refer is mentioned how to set the ECN field using winsock but it does not mention setting the DS field of the byte?

[bjorndm]

It seems to set the TOS bits directly since windows2000 you need admin privileges. To set the diffserve quality of service there is a different API https://learn.microsoft.com/en-us/windows/win32/api/qosobjs/ns-qosobjs-qos_diffserv

[Eelco]

I tried running the code with admin privileges on windows 10 but the TOS bits were not set.