Specify local IP address in net.DialTCP throwing error bind: address already in use

[Chandan Pal]

Written one TCP client which is used to create 10 TCP concurrent connections(holding for logger time) and enable only 10 ephemeral ports to use(49001 - 49010 ip_local_port_range file).

func createConnection(c int, desAddr, desPort string) (brokerCon net.Conn, err error) {

    localips := GetLocalIP()

    maxRetry := len(localips)

    retry := 0

    for retry < maxRetry {

        lIPPort := fmt.Sprintf("%s:0", strings.Split(localips[retry].String(), "/")[0])

        fmt.Println("\n---", lIPPort)

        laddr, lerr := net.ResolveTCPAddr("tcp4", lIPPort)

        if lerr != nil {

            fmt.Println("Getting Error ResolveTCPAddr for local ", lerr)

        }

        raddr, rerr := net.ResolveTCPAddr("tcp4", desAddr+desPort)

        if rerr != nil {

            fmt.Println("Getting Error ResolveTCPAddr for local ", rerr)

        }

        brokerCon, err = net.DialTCP("tcp", laddr, raddr)

        if err != nil {

            fmt.Printf("Failed to connect connetion %d , %d retrying with seconday IP, err:\n",

                c, retry, err)

            retry = retry + 1

            time.Sleep(1 * time.Second)

            continue

        } else {

            //fmt.Println("successfull ")

            break

        }

    }

    return

}

Scenario 1:

It's able to create 10 connections if the laddr value is nil on the DialTCP method. DialTCP is able to use 49009 and 49007 port which is already used by some different destination(as because A TCP connection is determined by a 5-tuple: [local IP, local port, remote IP, remote port, protocol] )

sudo netstat -anpl --tcp --udp | grep 490 

tcp        0      0 10.50.1.245:49005       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49010       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49008       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49009       XXX.XXX.XXX.X44:443     ESTABLISHED 3250/XXXX          

tcp        0      0 10.50.1.245:49006       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49002       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49004       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49001       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49007       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49003       10.50.1.41:9999         ESTABLISHED 23408/client        

tcp        0      0 10.50.1.245:49007       XXX.XXX.XXX.X29:443        ESTABLISHED 2806/XXXX 

tcp        0      0 10.50.1.245:49009       10.50.1.41:9999         ESTABLISHED 23408/client  

Scenario 2:

If I Pass local IP details(10.50.1.245) with 0 port, it's able to create only 8 connections and throwing bind: address already in use for the remaining 2 connections. if the local address is not nil why dialTCP is not able to use 49005 and 49007 port which is already used by some different destination.

sudo netstat -anpl --tcp --udp | grep 490 

tcp        0      0 10.50.1.245:49010       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49005       XXX.XXX.XXX.X66:443        ESTABLISHED 2510/XXX 

tcp        0      0 10.50.1.245:49008       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49005       XXX.XXX.XXX.X44:443     ESTABLISHED 3250/XXX         

tcp        0      0 10.50.1.245:49006       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49002       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49004       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49001       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49003       10.50.1.41:9999         ESTABLISHED 25841/client        

tcp        0      0 10.50.1.245:49007       XXX.XXX.XXX.X29:443        ESTABLISHED 2806/XXX 

tcp        0      0 10.50.1.245:49009       10.50.1.41:9999         ESTABLISHED 25841/client 

[Brian Candler]

I'm afraid the question isn't related to the Go programming language at all.  It's related to the operating system you're running on - which as far as I can see, you haven't mentioned.  It's whatever the underlying system calls and TCP stack do.