You have to retrieve it from the HOST header.
var host = req.get('host');
It is optional with HTTP 1.0, but required by 1.1. And, the app can always impose a requirement of its own.
If this is for supporting cross-origin requests, you would instead use the Origin header.
var origin = req.get('origin');
Note that some cross-origin requests require validation through a "preflight" request:
req.options('/route', function (req, res) {
var origin = req.get('origin');
// ...
});
If you're looking for the client's IP, you can retrieve that with:
var userIP = req.socket.remoteAddress;
Note that, if your server is behind a proxy, this will likely give you the proxy's IP. Whether you can get the user's IP depends on what info the proxy passes along. But, it'll typically be in the headers as well.