Can't connect to tcp server more than 140 connections? - macos

Question

I am building a tcp socket server in golang. It works well with less than ~140 client connection. But if i try to set max connection number to 500, after 140th clients do not connect to the server.
I increased file descriptor count to 1048576 but it does not still work.
$ulimit -n
1048576
I think the problem comes from operation system.(server and clients work on same machine) So OS information:
Mac OS 10.12 Sierra 64 bit.
Does anyone have any idea why i can't increase tcp connection number?
github

Answer 1

I am on the same operating system as you, but I could not reproduce your issue. I am using Go version 1.7.4. I have not tested on Go 1.8, but it was released earlier today.
I created two files, server.go and client.go (reproduced below). When I run them, I get way more than 140 connections. Before running, I switched to root and setup the ulimit like so:
$ sudo -s
$ ulimit -n 10000
The client outputs:
Established 1 connections
...
Established 2971 connections
panic: dial tcp 127.0.0.1:1337: socket: too many open files in system
The server outputs something very similar.
Here is client.go:
package main
import (
"fmt"
"net"
)
func main() {
var numConns int
for {
_, err := net.Dial("tcp", "127.0.0.1:1337")
if err != nil {
panic(err)
}
numConns++
fmt.Println("Established", numConns, "connections")
}
}
And server.go:
package main
import (
"fmt"
"net"
)
func main() {
listener, err := net.Listen("tcp", ":1337")
if err != nil {
panic(err)
}
var numConns int
for {
_, err := listener.Accept()
if err != nil {
panic(err)
}
numConns++
fmt.Println("Got", numConns, "connections")
}
}

Answer 2

This is not a problem with the operating system, but the hardware. It is likely a problem with your router. Commercial routers do tend to fail when you get into that range. My guess is that Alex is testing at a large company or university where they have commercial grade routers, but you are testing at home.