by chris@chrisduffin.org:

The following code generates a "socket is already connected" error in OS X.
Error does not occur in Linux.

package main

import (
    "fmt"
    "net"
)

func main() {
    dest := "192.168.2.253"
    host := "192.168.2.56"
    sendpkt := []byte{'r', 'o', 'a', 'd'}
    remote, err := net.ResolveIPAddr("ip4", dest)
    local, err := net.ResolveIPAddr("ip4", host)
    if err != nil {
        fmt.Printf("Could not parse IP address", 1)
    }
    ipconn, err := net.DialIP("ip4:icmp", local, remote)
    if err != nil {
        fmt.Printf("Could not create connection to %s", remote)
    }

    n, err := ipconn.WriteToIP(sendpkt, remote)
    if err != nil {
        fmt.Printf(`net.WriteToIP(..., %v) = %v, %v`, remote, n, err)
    }

}


Prints:

sudo ./ping                                                                             
                                                                                        
                                                                                 ⏎
net.WriteToIP(..., 192.168.2.253) = 0, write ip4 192.168.2.253: socket is already
connected%