Adding test and some docs

docs/config.md

@@ -555,12 +555,13 @@ Whatever your reasons may be, there are a few things to consider.
 If you are running ntfy behind a proxy, you should set the `behind-proxy` flag. This will instruct the 
 [rate limiting](#rate-limiting) logic to use the `X-Forwarded-For` header as the primary identifier for a visitor, 
 as opposed to the remote IP address. If the `behind-proxy` flag is not set, all visitors will
-be counted as one, because from the perspective of the ntfy server, they all share the proxy's IP address.
+be counted as one, because from the perspective of the ntfy server, they all share the proxy's IP address. If your proxy or CDN provider uses a custom header to securely pass the source IP/Client IP to your application, you can specify that header instead of using the XFF. Using the custom header (unique per provide/cdn/proxy), will disable the use of the XFF header.
 
 === "/etc/ntfy/server.yml"
     ``` yaml
     # Tell ntfy to use "X-Forwarded-For" to identify visitors
     behind-proxy: true
+    proxy-client-ip-header: "X-Client-IP"
     ```
 
 ### TLS/SSL

server/server_test.go

@@ -7,8 +7,6 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"fmt"
-	"golang.org/x/crypto/bcrypt"
-	"heckel.io/ntfy/v2/user"
 	"io"
 	"net/http"
 	"net/http/httptest"
@@ -22,6 +20,9 @@ import (
 	"testing"
 	"time"
 
+	"golang.org/x/crypto/bcrypt"
+	"heckel.io/ntfy/v2/user"
+
 	"github.com/SherClockHolmes/webpush-go"
 	"github.com/stretchr/testify/require"
 	"heckel.io/ntfy/v2/log"
@@ -2181,6 +2182,19 @@ func TestServer_Visitor_XForwardedFor_Multiple(t *testing.T) {
 	require.Equal(t, "234.5.2.1", v.ip.String())
 }
 
+func TestServer_Visitor_Custom_ClientIP_Header(t *testing.T) {
+	c := newTestConfig(t)
+	c.BehindProxy = true
+	c.ProxyClientIPHeader = "X-Client-IP"
+	s := newTestServer(t, c)
+	r, _ := http.NewRequest("GET", "/bla", nil)
+	r.RemoteAddr = "8.9.10.11"
+	r.Header.Set("X-Client-IP", "1.2.3.4")
+	v, err := s.maybeAuthenticate(r)
+	require.Nil(t, err)
+	require.Equal(t, "1.2.3.4", v.ip.String())
+}
+
 func TestServer_PublishWhileUpdatingStatsWithLotsOfMessages(t *testing.T) {
 	t.Parallel()
 	count := 50000