A tiny utility to test how to configure Express in your hosting environment (e.g. Heroku) in order to access Client IP addresses while preventing IP Spoofing.

See Express Docs for information on possible values.

1. Deploy the app to your environment.
2. Set environment variable PORT (listner port number)
3. Set environment variable TRUST_PROXY according to the value you wish to test.
4. Make an HTTP GET request https://your-url-here.com/. Try spoofing your real IP address, see example commands below.
5. The client IP detected by Express will be returned in the response body.

e.g. TRUE or FALSE

If true, the client’s IP address is understood as the left-most entry in the X-Forwarded-* header.

If false, the app is understood as directly facing the Internet and the client’s IP address is derived from req.connection.remoteAddress. This is the default setting.

String OR String containing comma-separated values. e.g. loopback or loopback, 123.123.123.123

Defines an IP address, subnet, or an array of IP addresses, and subnets to trust. Pre-configured subnet names are:

• loopback - 127.0.0.1/8, ::1/128
• linklocal - 169.254.0.0/16, fe80::/10
• uniquelocal - 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, fc00::/7

e.g. 1

Trust the nth hop from the front-facing proxy server as the client.

Here are some example cURL commands that attempt to spoof the client IP address by setting the X-Forwarded-For header:

Spoof as a private IP address:

curl -H "X-Forwarded-For: 192.168.0.1" http://example.com

Spoof as a public IP address:

curl -H "X-Forwarded-For: 138.197.99.92" http://example.com

Spoof as multiple IP addresses:

curl -H "X-Forwarded-For: 203.0.113.195, 70.41.3.18, 138.197.99.92" http://example.com

Spoof the X-Real-IP header instead:

curl -H "X-Real-IP: 124.53.78.9" http://example.com

Set an invalid IP with different formats:

curl -H "X-Forwarded-For: 999.999.999.999" http://example.com
curl -H "X-Forwarded-For: 123.456.789" http://example.com

Following experiments with Heroku, it appears that the following allow the server to correctly get the request's client IP but prevent spoofing using request headers:

app.set("trust proxy", 1);

app.set("trust proxy", "uniquelocal");