How to get remote client's IP address in an express based Node JS app running on App-Engine

While trying to get IP, all standard headers values contain incorrect value.

req.ip: ::ffff:172.17.0.5
req.headers['x-forwarded-for']: 169.254.160.2
req.socket.remoteAddress: ::ffff:172.17.0.5

All of these result in wrong ip.

Solutions:

1. Either use let ip = req.headers['x-appengine-user-ip'] or
2. Use Express's trust proxy settings by using app.set('trust proxy', true);

Explanation: https://cloud.google.com/appengine/docs/flexible/nodejs/runtime#https_and_forwarding_proxies

App Engine terminates the HTTPS connection at the load balancer and forwards the request to your application. The user's IP address is available in the standard X-Forwarded-For header as well as the X-Appengine-User-Ip header. Applications that require this information should configure their web framework to trust the proxy.

Complete list of header values for each request:

{
    "host": "<APP_ID>.appspot.com",
    "x-real-ip": "169.254.160.2",
    "x-forwarded-for": "169.254.160.2",
    "x-google-real-ip": "169.254.160.2",
    "x-appengine-user-ip": "2620:0:1002:100a:2c4b:4f6b:7851:a9d9",
    "x-appengine-api-ticket": "aad4544d4e04a1a0",
    "x-appengine-user-email": "",
    "x-appengine-auth-domain": "gmail.com",
    "x-appengine-user-id": "",
    "x-appengine-user-nickname": "",
    "x-appengine-user-organization": "",
    "x-appengine-user-is-admin": "0",
    "x-appengine-request-id-hash": "A104B85B",
    "x-appengine-request-log-id": "5748c500ff00ff09eec9a104b85b0001737e736d6172742d737061726b2d39333632320001323031363035323774313530323538000100",
    "x-appengine-https": "on",
    "x-appengine-datacenter": "us6",
    "x-appengine-default-version-hostname": "smart-spark-93622.appspot.com",
    "cache-control": "max-age=0",
    "accept": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8",
    "upgrade-insecure-requests": "1",
    "save-data": "on",
    "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.63 Safari/537.36",
    "accept-language": "en-US,en;q=0.8",
    "x-appengine-country": "US",
    "x-appengine-region": "ca",
    "x-appengine-city": "san francisco",
    "x-appengine-citylatlong": "37.774929,-122.419416",
    "x-cloud-trace-context": "4a77265a7fa3e11be8cccebce59f7702/9314487514850703289;o=5"
}

As one can see, x-forwarded-for is being set as x-google-real-ip. So user's ip is now available in x-appengine-user-ip.