How to detect if two clients/connections are on the same network (nodeJS / express / socket.io)

Question

How would I go about detecting if a connection is on the same network as another connection? This could be from using http, express, socket.io or another library.

This would be used so that a user can quickly join a game hosted by another user in the same house. So other solutions to this would also be appreciated.

What if they're in the same house but on different mobile networks? Or one is on WiFi and the other on mobile? Do they not get to play? :-) — T.J. Crowder, Nov 20 '20 at 15:58

score 1 · Accepted Answer · answered Nov 20 '20 at 23:50

With some research I found a method to do this by looking at the Public IP address. I found a useful free API: http://ipify.org, but I wanted to keep it server side. This question answered how to get the IP from a request, but I decided to use request-ip.

Any thoughts on this approach? Obviously there is a very low chance of two networks having the same IPv4 address.

Here is a working example and below is the code.

index.js

const path = require('path');
const express = require("express");
const socket  = require('socket.io');
const requestIp = require('request-ip');

const app = express();
const server = app.listen(8080, () => console.log(`listening on port ${server.address().port}`));

const io = socket(server);

let networks = {};

app.get('/', (req, res) => {
  res.sendFile( path.join( __dirname, 'index.html') );
});

io.on('connection', function (socket) {
  let clientIP = requestIp.getClientIp(socket.request);
  console.log('New connection from ' + clientIP);
  
  if( networks[clientIP] == undefined ){ networks[clientIP] = []; };
  let network = networks[clientIP];
  
  network.push(socket);
  updateNetwork(network);
  
  socket.on('disconnect', function () {
    console.log('Disconnected ' + clientIP);

    network.splice(network.indexOf(socket), 1);
    updateNetwork(network);
  });
  
});

function updateNetwork(network){
  for( let socket of network ){
    socket.emit( 'network-clients', network.length );
  }
}

index.html

<html>
  <head>
    <meta charset="UTF-8">
    <script src="https://cdnjs.cloudflare.com/ajax/libs/socket.io/1.7.3/socket.io.js"></script>
    
    <script src="script.js" type="text/javascript"></script>
  </head>
  <body>
    <p>
      Clients Connected on the same network: <span id="network-clients">loading...</span>
    </p>
  </body>
  <script>
    let socket = io.connect(window.location.origin);

    socket.on('network-clients', (data) => {
      document.getElementById('network-clients').innerHTML = data;
    });
  </script>
</html>

score 0 · Answer 2 · answered Nov 20 '20 at 15:58

0

You can't because you won't have any way to determine the subnet.

Kind of like having the house number but not the street.

answered Nov 20 '20 at 15:58

Bill Kervaski

542
6
17

Is there not a way to check by comparing IP addresses? I've seen sites before such as voting where accessing it again on same network will not allow you to vote again. – Jared Parker Nov 20 '20 at 17:01
You can certainly check the IP address but no way of knowing the subnet; so no way of knowing the originating network. Assuming 1.2.3.4 is on the same network as 1.2.3.5 is bad practice as 1.2.3.5 could be an entirely different network. – Bill Kervaski Nov 23 '20 at 16:37

score 0 · Answer 3 · answered Nov 20 '20 at 16:01

0

Network configuration is just too complicated, and in some (or even many) cases opaque¹, for you to use the network for this. Moreover, two people can be in the same house but on different networks (different mobile networks; one on WiFi and the other on mobile; even — in rare cases I'd think — different WiFi networks).

Instead, use prior art: User A creates some kind of unique game identifier (or your system does) and provides that information to User B so User B can join the game. (You may want to have User A confirm them before they're allowed in.) There are a million variations on that theme, but that's the basic, time-tested approach.

¹ Opaque from a browser perspective. Remember the browser sends the information Express sees on the server.

answered Nov 20 '20 at 16:01

T.J. Crowder

1,031,962
187
1,923
1,875

Using a unique game identifier was going to be the default way to join, but this idea of checking if they're on the same network would be an ease of use feature. – Jared Parker Nov 20 '20 at 17:01
1

@JaredParker - Ah, that makes sense. Perhaps if you have established links between accounts or something. (But you may have thought of that, too -- still doesn't change the fact it'd be convenient if two people are standing next to each other and don't have a prior connection to just find games "nearby.") Maybe someday [web bluetooth](https://www.w3.org/community/web-bluetooth/) will be a thing. – T.J. Crowder Nov 20 '20 at 17:02
1

We can only hope. Thanks for the help and ideas :) – Jared Parker Nov 20 '20 at 17:25

How to detect if two clients/connections are on the same network (nodeJS / express / socket.io)

3 Answers3