Hi John,
The oob data used in these system calls are not for communicating with the
peer. UDP doesn't support OOB data on the wire, I think the name here is a
bit unfortunate, it should rather be called "control" or somesuch. This
data is processed/created locally by the kernel and its structure is
probably OS specific. On Linux see the ip(7) man page for information about
how it can be used (search for control).
Regards,
--
Per Johansson
On Saturday, April 6, 2019 at 11:24:04 PM UTC+2, John Dreystadt wrote:
>
> I wrote some sample code (at the end of this message) to see how UDP
> worked in the Go environment. And I wound up with some issues because
> things did not seem to work as I expected. I am running go1.11.2 on both a
> Windows box and a Ubuntu box.
>
> Issue 1, when I call WriteMsgUDP with both a regular buffer and an out of
> band buffer, I get back the length as I expected. But I never see any out
> of band data on the read side. Is this a known error or am I just missing
> something?
>
> Issue 2, when I send a UDP message longer than the buffer at the receiving
> end I get an error on the Windows side along with a bit set in flags. The
> Ubuntu side does not report an error but does set a bit (but a different
> one). Even more odd, the Windows side does not return the address of the
> sending machine when this error occurs. While having one report an error
> while not the other is not unreasonable, losing the address of the sending
> machine seems really bad unless it just is not there on Windows.
>
> Issue 3, the documentation for the flags return from ReadMsgUDP just says
> "the flags set on the message" which is pretty short and does not even
> indicate who set the flags. Maybe something like "the flags are set on the
> message by the network stack and are operating system dependent".
>
> I used two copies of the following program running to see the issues. To
> replicate, first run one without any flags and then run the second with
> -mode=wr .
>
> package main
>
> import (
> "errors"
> "flag"
> "fmt"
> "net"
> )
>
> var rfserver = flag.String("RFS", "127.0.0.1:6000", "Read First Server
> Name:Port Number")
> var wfserver = flag.String("WFS", "127.0.0.1:6001", "Write First Server
> Name:Port Number")
>
> type modeValue string
>
> func (mode *modeValue) String() string {
> return string(*mode)
> }
>
> func (mode *modeValue) Set(s string) error {
> switch s {
> case "rw":
> *mode = modeValue(s)
> return nil
> case "wr":
> *mode = modeValue(s)
> return nil
> default:
> return errors.New("Mode must be rw or wr")
> }
> }
>
> var mode modeValue
>
> func main() {
> mode = modeValue("rw")
> flag.Var(&mode, "mode", "rw for read then write and wr for the reverse")
> flag.Parse()
> fmt.Println("Parameters", *rfserver, *wfserver, mode)
>
> rfudpaddr, err := net.ResolveUDPAddr("udp", *rfserver)
> if err != nil {
> panic(err)
> }
> wfudpaddr, err := net.ResolveUDPAddr("udp", *wfserver)
> if err != nil {
> panic(err)
> }
> var rudpaddr, wudpaddr *net.UDPAddr
> if mode == "rw" {
> rudpaddr, wudpaddr = rfudpaddr, wfudpaddr
> } else {
> wudpaddr, rudpaddr = rfudpaddr, wfudpaddr
> }
> pc, err := net.ListenUDP("udp", rudpaddr)
> if err != nil {
> panic(err)
> }
>
> if mode == "rw" {
> buffer := make([]byte, 5)
> oobbuffer := make([]byte, 5)
> n, oobn, flags, addr, err := pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> n, oobn, flags, addr, err = pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> n, oobn, flags, addr, err = pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> }
>
> n, oobn, err := pc.WriteMsgUDP([]byte("1234"), []byte("1"), wudpaddr)
> fmt.Println("n, oobn, err", n, oobn, err)
> n, oobn, err = pc.WriteMsgUDP([]byte("12345"), nil, wudpaddr)
> fmt.Println("n, err", n, oobn, err)
> n, oobn, err = pc.WriteMsgUDP([]byte("123456"), nil, wudpaddr)
> fmt.Println("n, err", n, oobn, err)
>
> if mode == "wr" {
> buffer := make([]byte, 5)
> oobbuffer := make([]byte, 5)
> n, oobn, flags, addr, err := pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> n, oobn, flags, addr, err = pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> n, oobn, flags, addr, err = pc.ReadMsgUDP(buffer, oobbuffer)
> fmt.Println("n,oobn,flags,addr,err", n, oobn, flags, addr, err)
> }
>
> }
>
>
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