Puzzled about the `telnet localhost` and `telnet 0.0.0.0` - networking

Question

I wrote a simple GO program which listens to 0.0.0.0:9999 and 127.0.0.1:9999:
func main() {
go bind("0.0.0.0:9999", "111 ")
go func() {
time.Sleep(2 * time.Second)
bind("127.0.0.1:9999", "222 ")
}()
time.Sleep(time.Hour)
}
func bind(address string, content string) {
fmt.Println("-------------", address, "-----------------")
listener, err := net.Listen("tcp", address)
if err != nil {
panic(err)
return
}
fmt.Println(listener.Addr().String())
conn, _ := listener.Accept()
for {
_, err := conn.Write([]byte(content))
if err != nil {
panic(err)
}
time.Sleep(1 * time.Second)
}
}
The meaning of the code:
It binds two addresses, and gives different responses to the clients of them
binding "0.0.0.0:9999": will send "111 " repeat to client
binding "127.0.0.1:9999": will send "222 " repeat to client
And then I use telnet to try different addresses, and the responses are:
telnet 127.0.0.1 9999: 222 (OK)
telnet localhost 9999: 111 (WHY?!)
telnet 0.0.0.0 9999: 222 (WHY?!)
telnet <my-internal-ip> 9999: 111 (OK)
I'm quite confused about some of them:
telnet localhost 9999: 111 (WHY?!)
localhost should point to 127.0.0.1, so I think it's same to telnet 127.0.0.1 9999 and the response should be 222, but the actual one is 111
telnet 0.0.0.0 9999: 222 (WHY?!)
I think 0.0.0.0 is not same to 127.0.0.1, I expect to get response of 111, but get 222
I also have a demo project: https://github.com/golang-demos/go-bind-0.0.0.0-127.0.0.1-demo
Update: My os is OSX

Answer 1

Both localhost and 0.0.0.0 are resolved to 127.0.0.1 by the OS
$ ping 0.0.0.0
PING 0.0.0.0 (127.0.0.1) 56(84) bytes of data.
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.024 ms
$ping localhost
PING localhost (127.0.0.1) 56(84) bytes of data.
64 bytes from localhost (127.0.0.1): icmp_seq=1 ttl=64 time=0.035 ms`
localhost could resolve to something else depending on /etc/hosts file.
An excellent explanation for Linux ping 0.0.0.0 behavior is here.