APM server behind a proxy

I think this would be a common usecase, where the APM server is behind a proxy (in my case, Cloudflare). In which case, user IP is unavailable and we have to rely on custom headers; is there any provision of setting a custom header to be considered as user IP at the APM server or make the RUM agent set a custom header? In case of Cloudflare, they set the real user IP in the header ‘True-Client-IP’.

Edit: It’d be nice if we could it set it during client init as well. e.g.

initApm({
	environment: window.location.hostname,
	serviceName: 'my-app',
	serverUrl: 'https://apm.example.com',
	serviceVersion: getAppVersion() || 'v0.0.0',
	distributedTracing: true,
	distributedTracingOrigins: [
		'https://api.example.com'
	],
	XFFHeader: {
		key: 'X-Forwarded-For',
		value: '127.0.0.1'
	}
})

ref: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For

In this case, if the value is provided, then it shall be set as value in request header. The APM server should be configurable or should understand that it should pickup user IP from this header.

[axw]

@bnkamalesh OK, thanks for the additional context.

Given that we already support Forwarded, X-Forwarded-For, and X-Real-IP, I think folks are pretty well covered at the moment.

Unless there is a clear need, I would prefer not to add another option. If it turns out there is a need to handle True-Client-IP (or other headers) instead of one of the other more common headers mentioned above, I’d be happy to learn more and consider adding an option.

[axw]

Ok this might sound silly, but if all 3 are set. I presume it’d pick 1 based on some precedence, right?

Not at all silly!

Indeed, the server will pick the first of Forwarded, X-Real-IP, or X-Forwarded-For in that order. See also: https://github.com/elastic/apm-server/blob/b3759fcc5cbdfb316634eeb81e43274c5fc9354e/utility/remoteaddr.go#L32-L45