middleware/real_ip.go

package middleware

import (
	"context"
	"net/http"
	"strings"
)

var (
	RealIPCtxKey = &CtxKey{Pkg: "go.pkg.cx/middleware", Name: "RealIP"}
)

// RealIP is a middleware that sets a http.Request's RemoteAddr to the results
// of parsing either the X-Forwarded-For header or the X-Real-IP header (in that
// order).
//
// This middleware should be inserted fairly early in the middleware stack to
// ensure that subsequent layers (e.g., request loggers) which examine the
// RemoteAddr will see the intended value.
//
// You should only use this middleware if you can trust the headers passed to
// you (in particular, the two headers this middleware uses), for example
// because you have placed a reverse proxy like HAProxy or nginx in front of
// chi. If your reverse proxies are configured to pass along arbitrary header
// values from the client, or if you use this middleware without a reverse
// proxy, malicious clients will be able to make you very sad (or, depending on
// how you're using RemoteAddr, vulnerable to an attack of some sort).
func RealIP(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if rip := realIP(r); rip != "" {
			r.RemoteAddr = rip
		}

		ctx := context.WithValue(r.Context(), RealIPCtxKey, strings.Split(r.RemoteAddr, ":")[0])
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func realIP(r *http.Request) string {
	if xrip := r.Header.Get("X-Real-IP"); xrip != "" {
		return xrip
	}

	if xff := r.Header.Get("X-Forwarded-For"); xff != "" {
		i := strings.Index(xff, ", ")
		if i == -1 {
			i = len(xff)
		}

		return xff[:i]
	}

	return ""
}