Do you want to build a website clone similar to whatismyipaddress.com? It is actually easy to develop. I decided to create this tool because I was working on an automated DNS client that will check my public IP address. Perhaps, somebody might need this in the future. I already made the completed demo available online at https://ip.johnpili.com

How it works

It works by reading the HTTP header request which contains information such as IP Address, User-Agent, Scheme, Accept-Encoding, etc. If you are using a reverse proxy like Cloudflare, you can extract IP information from header keys Cf-Connecting-Ip or X-Real-Ip

Sample HTTP header

map[
    Accept:[text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8] 
    Accept-Encoding:[gzip] 
    Accept-Language:[en-US,en;q=0.5] 
    Cache-Control:[no-cache] 
    Cdn-Loop:[cloudflare] 
    Cf-Connecting-Ip:[193.169.145.66] 
    Cf-Ipcountry:[T1] 
    Cf-Visitor:[{"scheme":"https"}] 
    Connection:[upgrade] 
    Dnt:[1] 
    Pragma:[no-cache] 
    Upgrade-Insecure-Requests:[1] 
    User-Agent:[Mozilla/5.0 (Windows NT 10.0; rv:68.0) Gecko/20100101 Firefox/68.0] 
    X-Forwarded-For:[193.169.145.66, 193.169.145.66] 
    X-Forwarded-Proto:[https] 
    X-Real-Ip:[193.169.145.66]
]

Snippet extracting IP address from header

func getIPDetails(r *http.Request) models.IPInfo {
	ip := ""
	if len(configuration.Extraction.HeaderKey) > 0 {
		ip = r.Header.Get(configuration.Extraction.HeaderKey) // Extract IP from header because we are using reverse proxy example X-Real-Ip
	}

	if len(ip) == 0 { // Fallback
		ip = extractIPAddress(r.RemoteAddr)
	}

	ipInfo := models.IPInfo{
		IP:        ip,
		UserAgent: r.Header.Get("User-Agent"),
		IPCountry: r.Header.Get("CF-IPCountry"),
	}

	if configuration.Extraction.DebugHeader {
		log.Print(r.Header)
	}
	return ipInfo
}