Get client IP address in Go web application

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Get client IP address in Go web application
Get client IP address in Go web application

This post demonstrates how to get client IP address in Go web application, even when the app is behind a proxy or a load balancer.

When the web application is served directly to the Internet, we can get the client IP address like this:

func getClientIpAddr(req *http.Request) string {
	return req.RemoteAddr
}

In reality, most web applications are served behind proxies and load balancers; we can detect client IP address by retrieving the value from X-Forwarded-For header. Combining with fallback to the default client IP address from request, we have this little function that does the job:

func getClientIpAddr(req *http.Request) string {
	clientIp := req.Header.Get("X-FORWARDED-FOR")
	if clientIp != "" {
		return clientIp
	}
	return req.RemoteAddr
}

This is the complete code showing how to get client IP address in Go:

package main

import (
	"fmt"
	"net/http"
)

func getClientIpAddr(req *http.Request) string {
	clientIp := req.Header.Get("X-FORWARDED-FOR")
	if clientIp != "" {
		return clientIp
	}
	return req.RemoteAddr
}

func indexHandler(w http.ResponseWriter, r *http.Request) {
	clientIp := getClientIpAddr(r)

	fmt.Fprintf(w, clientIp+"\n\n")
}

func main() {
	http.HandleFunc("/", indexHandler)
	http.ListenAndServe(":8080", nil)
}

Have fun!