changens.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"path"
	"strconv"
	"syscall"
)

const (
	cont_id = "3d92b3b1bdb2"
	pid     = 7492
)

func getNS(cont_id string, pid int) string {
	// try /var/run/netns -> for novadocker containers
	filepath := path.Join("/var/run/netns/", cont_id)
	if _, err := os.Stat(filepath); err == nil {
		return filepath
	}

	// docker containers
	filepath = path.Join("/proc/", strconv.Itoa(pid), "/ns/net")
	if _, err := os.Stat(filepath); err == nil {
		return filepath
	}

	return ""
}

func main() {
	// change namespace
	ns := getNS(cont_id, pid)
	if "" == ns {
		fmt.Println("[ERROR] unable to figure out ns for container")
		panic("ns not found")
	}

	fd, err := syscall.Open(ns, syscall.O_RDONLY, 0666)
	if err != nil {
		fmt.Println("[ERROR] unable to open ns file for container")
		panic(err)
	}
	defer syscall.Close(fd)
	if _, _, err := syscall.RawSyscall(308, uintptr(fd), 0, 0); err != 0 {
		fmt.Println("[ERROR] unable to change namespace for container")
		panic(err)
	}

	out, err := ioutil.ReadFile("/proc/net/udp")
	fmt.Println(string(out))
}