Check if a port is open

Question

I can\'t seem to find anything that tells me if a port in my router is open or not. Is this even possible?

The code I have right now doesn\'t really seem to work...<

Answer 1

If you're connecting to the loopback adapter — localhost or 127.0.0.1 (there's no place like 127.0.0.1!), you're unlikely to ever go out to the router. The OS is smart enough to recognize that it's a special address. Dunno if that holds true as well if you actually specify your machine's "real" IP address.

See also this question: What is the purpose of the Microsoft Loopback Adapter?

Also note that running traceroute localhost (tracert localhost in Windows) shows that the only network node involved is your own machine. The router is never involved.

Answer 2

If it is Router the simplest way to check it through online services like

• Port Checker
• Port Forwarding Test

You can also try using telenet to chek wether port is accessible or not

telenet [ip-address] [port] 

Answer 3

A port forward on the router cannot be tested from inside the LAN, you need to connect from the WAN (internet) side to see if a port forward is working or not.

Several internet sites offer services to check if a port is open:

What's My IP Port Scanner

GRC | ShieldsUP!

If you want to check with your own code, then you need to make sure the TCP/IP connection is rerouted via an external proxy or setup a tunnel. This has nothing to do with your code, it's basic networking 101.

Answer 4

A better solution where you can even specify a timeout:

using System;
using System.Net.Sockets;

// ...

bool IsPortOpen(string host, int port, TimeSpan timeout)
{
    try
    {
        using(var client = new TcpClient())
        {
            var result = client.BeginConnect(host, port, null, null);
            var success = result.AsyncWaitHandle.WaitOne(timeout);
            client.EndConnect(result);
            return success;
        }
    }
    catch
    {
        return false;
    }
}

And, in F#:

open System
open System.Net.Sockets

let isPortOpen (host: string) (port: int) (timeout: TimeSpan): bool =
    try
        use client = new TcpClient()
        let result = client.BeginConnect(host, port, null, null)
        let success = result.AsyncWaitHandle.WaitOne timeout
        client.EndConnect result
        success
    with
    | _ -> false

let available = isPortOpen "stackoverflow.com" 80 (TimeSpan.FromSeconds 10.)
printf "Is stackoverflow available? %b" available