CODEWITHSUNDEEP
linuxgoudp
selden
selden

Reputation: 333

golang udp connection refused on every other write

I ran this UDP client program on ubuntu linux 16.04:

package main

import (
    "fmt"
    "net"
    "time"
    "strconv"
)

func CheckError(err error) {
    if err  != nil {
        fmt.Println("Error: " , err)
    }
}

func main() {
    ServerAddr,err := net.ResolveUDPAddr("udp","127.0.0.1:10001")
    CheckError(err)

    LocalAddr, err := net.ResolveUDPAddr("udp", "127.0.0.1:0")
    CheckError(err)

    Conn, err := net.DialUDP("udp", LocalAddr, ServerAddr)
    CheckError(err)

    defer Conn.Close()
    i := 0
    for {
        msg := strconv.Itoa(i)
        i++
        buf := []byte(msg)
        _,err := Conn.Write(buf)
        if err != nil {
            fmt.Println(msg, err)
        }
        time.Sleep(time.Second * 1)
    }
}

It produces this output:

$ go run server.go 
1 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
3 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
5 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused

But I expected this output instead:

$ go run server.go 
1 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
2 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
3 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
4 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused
5 write udp 127.0.0.1:58703->127.0.0.1:10001: write: connection refused

tcpdump says:

15:28:46.453313 IP localhost.47993 > localhost.10001: UDP, length 1
15:28:46.453338 IP localhost > localhost: ICMP localhost udp port 10001 unreachable, length 37
15:28:48.453821 IP localhost.47993 > localhost.10001: UDP, length 1
15:28:48.453852 IP localhost > localhost: ICMP localhost udp port 10001 unreachable, length 37
15:28:50.454242 IP localhost.47993 > localhost.10001: UDP, length 1
15:28:50.454271 IP localhost > localhost: ICMP localhost udp port 10001 unreachable, length 37

Why does this happen every other time conn.Write writes instead of every time? I'm not blaming go, I just want to learn why.

Upvotes: 2

Views: 5303

Answers (2)

Pedro A. Aranda
Pedro A. Aranda

Reputation: 177

Try

LocalAddr, err := net.ResolveUDPAddr("udp", ":0")

or

LocalAddr, err := net.ResolveUDPAddr("udp", "<ip address of outgoing interface>:0")

This should work

Upvotes: -3

Mr_Pink
Mr_Pink

Reputation: 109404

If you look more closely at the packet capture, you'll notice that it is replying to every packet with an ICMP unreachable, and you are only sending every other packet. If you inspect the return value from Write, you'll also see that no data was written on every other packet.

Because UDP has no real connection and there is no ACK for any packets sent, the best a "connected" UDP socket can do to simulate a send failure is to save the ICMP response, and return it as an error on the next write.

So the first packet is sent, an ICMP unreachable message is received, the second send operation fails and returns the error, so no packet is sent, and the cycle repeats.

Upvotes: 11

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