Get IP behind proxy (if available)

gethiddenip.js

var getRealIP = (function() {
    "use strict";

    function tmp(){}

    var RTCPeerConnection = (function getRTC(win, end){
        var RTC = win.RTCPeerConnection || win.mozRTCPeerConnection || win.webkitRTCPeerConnection;
        if (RTC || end) {
            return RTC;
        }

        var iframe = document.createElement('iframe');
        iframe.sandbox = 'allow-same-origin';
        iframe.style.display = 'none';
        document.body.appendChild(iframe);

        return getRTC(iframe, true);
    })(window, false);

    return function getRealIP(callback) {
        var pc, cache = {};

        pc = new RTCPeerConnection({iceServers: [{urls: 'stun:stun.services.mozilla.com'}]}, {
            optional: [{RtpDataChannels: true}]
        });

        pc.onicecandidate = function(ice) {
            if (ice.candidate) {
                var ip = /([0-9]{1,3}(\.[0-9]{1,3}){3})/.exec(ice.candidate.candidate)[1];

                if (!cache[ip] && !ip.match(/^(192\.168\.|169\.254\.|10\.|172\.(1[6-9]|2\d|3[01]))/)) {
                    cache[ip] = true;
                    callback(ip);
                }
            }
        };

        pc.createDataChannel('');
        pc.createOffer(function(result){
            pc.setLocalDescription(result, tmp, tmp);
        }, tmp);
    };
})();

getRealIP(document.write.bind(document));