Backend plugins: Prepare and clean request headers before resource calls (#22321)

Moves common request proxy utilities to proxyutil package with
support for removing X-Forwarded-Host, X-Forwarded-Port,
X-Forwarded-Proto headers, setting X-Forwarded-For header
and cleaning Cookie header.
Using the proxyutil package to prepare and clean request
headers before resource calls.

Closes #21512

pkg/util/proxyutil/proxyutil.go

@@ -0,0 +1,44 @@
+package proxyutil
+
+import (
+	"net"
+	"net/http"
+)
+
+// PrepareProxyRequest prepares a request for being proxied.
+// Removes X-Forwarded-Host, X-Forwarded-Port, X-Forwarded-Proto headers.
+// Set X-Forwarded-For headers.
+func PrepareProxyRequest(req *http.Request) {
+	req.Header.Del("X-Forwarded-Host")
+	req.Header.Del("X-Forwarded-Port")
+	req.Header.Del("X-Forwarded-Proto")
+
+	if req.RemoteAddr != "" {
+		remoteAddr, _, err := net.SplitHostPort(req.RemoteAddr)
+		if err != nil {
+			remoteAddr = req.RemoteAddr
+		}
+		if req.Header.Get("X-Forwarded-For") != "" {
+			req.Header.Set("X-Forwarded-For", req.Header.Get("X-Forwarded-For")+", "+remoteAddr)
+		} else {
+			req.Header.Set("X-Forwarded-For", remoteAddr)
+		}
+	}
+}
+
+// ClearCookieHeader clear cookie header, except for cookies specified to be kept.
+func ClearCookieHeader(req *http.Request, keepCookiesNames []string) {
+	var keepCookies []*http.Cookie
+	for _, c := range req.Cookies() {
+		for _, v := range keepCookiesNames {
+			if c.Name == v {
+				keepCookies = append(keepCookies, c)
+			}
+		}
+	}
+
+	req.Header.Del("Cookie")
+	for _, c := range keepCookies {
+		req.AddCookie(c)
+	}
+}