You have to retrieve it from the HOST header.
const 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.
const origin = req.get('origin');
Note that some cross-origin requests require validation through a "preflight" request:
req.options('/route', function (req, res) {
const origin = req.get('origin');
// ...
});
If you're looking for the client's IP, you can retrieve that with:
const 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.
