How to get local/internal IP with JavaScript

UPDATE

Unfortunately the below answer no longer works as browsers have changed this behavior due to security concerns. See this StackOverflow Q&A for more details.

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where I took code from --> Source

You can find a demo at --> Demo

I have modified the source code, reduced the lines, not making any stun requests since you only want Local IP not the Public IP, the below code works in latest Firefox and Chrome:

    window.RTCPeerConnection = window.RTCPeerConnection || window.mozRTCPeerConnection || window.webkitRTCPeerConnection;   //compatibility for firefox and chrome
    var pc = new RTCPeerConnection({iceServers:[]}), noop = function(){};      
    pc.createDataChannel("");    //create a bogus data channel
    pc.createOffer(pc.setLocalDescription.bind(pc), noop);    // create offer and set local description
    pc.onicecandidate = function(ice){  //listen for candidate events
        if(!ice || !ice.candidate || !ice.candidate.candidate)  return;
        var myIP = /([0-9]{1,3}(\.[0-9]{1,3}){3}|[a-f0-9]{1,4}(:[a-f0-9]{1,4}){7})/.exec(ice.candidate.candidate)[1];
        console.log('my IP: ', myIP);   
        pc.onicecandidate = noop;
    };
    

what is happening here is, we are creating a dummy peer connection, and for the remote peer to contact us, we generally exchange ice candidates with each other. And reading the ice candiates we can tell the ip of the user.

You can use this version on modern browser (with Promises and async / await)

// minified onliner, 219b
const ip = await new Promise((s,f,c=new RTCPeerConnection(),k='candidate')=>(c.createDataChannel(''),c.createOffer(o=>c.setLocalDescription(o),f),c.onicecandidate=i=>i&&i[k]&&i[k][k]&&c.close(s(i[k][k].split(' ')[4]))))

Or, un-minified:

// cleaned, 363b
const ip = await new Promise((resolve, reject) => {
  const conn = new RTCPeerConnection()
  conn.createDataChannel('')
  conn.createOffer(offer => conn.setLocalDescription(offer), reject)
  conn.onicecandidate = ice => {
    if (ice && ice.candidate && ice.candidate.candidate) {
      resolve(ice.candidate.candidate.split(' ')[4])
      conn.close()
    }
  }
})