added real-client-ip faq (kubernetes#11664)

Co-authored-by: longwuyuan <longwuyuan@gmail.com>

docs/faq.md

@@ -1,7 +1,9 @@
 
 # FAQ
 
-## How can I easily install multiple instances of the ingress-nginx controller in the same cluster?
+## Multiple controller in one cluster
+
+Question - How can I easily install multiple instances of the ingress-nginx controller in the same cluster?
 
 You can install them in different namespaces.
 
@@ -61,7 +63,80 @@ helm install ingress-nginx-2 ingress-nginx/ingress-nginx  \
 
 ## Retaining Client IPAddress
 
-Please read [Retain Client IPAddress Guide here](./user-guide/retaining-client-ipaddress.md).
+Question - How to obtain the real-client-ipaddress ?
+
+The goto solution for retaining the real-client IPaddress is to enable PROXY protocol.
+
+Enabling PROXY protocol has to be done on both, the Ingress NGINX controller, as well as the L4 load balancer, in front of the controller.
+
+The real-client IP address is lost by default, when traffic is forwarded over the network. But enabling PROXY protocol ensures that the connection details are retained and hence the real-client IP address doesn't get lost.
+
+Enabling proxy-protocol on the controller is documented [here](https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/configmap/#use-proxy-protocol) .
+
+For enabling proxy-protocol on the LoadBalancer, please refer to the documentation of your infrastructure provider because that is where the LB is provisioned.
+
+Some more info available [here](https://kubernetes.github.io/ingress-nginx/user-guide/miscellaneous/#source-ip-address)
+
+Some more info on proxy-protocol is [here](https://kubernetes.github.io/ingress-nginx/user-guide/miscellaneous/#proxy-protocol)
+
+### client-ipaddress on single-node cluster
+
+Single node clusters are created for dev & test uses with tools like "kind" or "minikube". A trick to simulate a real use network with these clusters (kind or minikube) is to install Metallb and configure the ipaddress of the kind container or the minikube vm/container, as the starting and ending of the pool for Metallb in L2 mode. Then the host ip becomes a real client ipaddress, for curl requests sent from the host.
+
+After installing ingress-nginx controller on a kind or a minikube cluster with helm, you can configure it for real-client-ip with a simple change to the service that ingress-nginx controller creates. The service object of --type LoadBalancer has a field service.spec.externalTrafficPolicy. If you set the value of this field to "Local" then the real-ipaddress of a client is visible to the controller.
+
+```
+% kubectl explain service.spec.externalTrafficPolicy
+KIND:       Service
+VERSION:    v1
+
+FIELD: externalTrafficPolicy <string>
+
+DESCRIPTION:
+    externalTrafficPolicy describes how nodes distribute service traffic they
+    receive on one of the Service's "externally-facing" addresses (NodePorts,
+    ExternalIPs, and LoadBalancer IPs). If set to "Local", the proxy will
+    configure the service in a way that assumes that external load balancers
+    will take care of balancing the service traffic between nodes, and so each
+    node will deliver traffic only to the node-local endpoints of the service,
+    without masquerading the client source IP. (Traffic mistakenly sent to a
+    node with no endpoints will be dropped.) The default value, "Cluster", uses
+    the standard behavior of routing to all endpoints evenly (possibly modified
+    by topology and other features). Note that traffic sent to an External IP or
+    LoadBalancer IP from within the cluster will always get "Cluster" semantics,
+    but clients sending to a NodePort from within the cluster may need to take
+    traffic policy into account when picking a node.
+    
+    Possible enum values:
+     - `"Cluster"` routes traffic to all endpoints.
+     - `"Local"` preserves the source IP of the traffic by routing only to
+    endpoints on the same node as the traffic was received on (dropping the
+    traffic if there are no local endpoints).
+```
+
+### client-ipaddress L7
+
+The solution is to get the real client IPaddress from the ["X-Forward-For" HTTP header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-For)
+
+Example : If your application pod behind Ingress NGINX controller, uses the NGINX webserver and the reverseproxy inside it, then you can do the following to preserve the remote client IP.
+
+- First you need to make sure that the X-Forwarded-For header reaches the backend pod. This is done by using a Ingress NGINX conftroller ConfigMap key. Its documented [here](https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/configmap/#use-forwarded-headers)
+
+- Next, edit `nginx.conf` file inside your app pod, to contain the directives shown below:
+
+```
+set_real_ip_from 0.0.0.0/0; # Trust all IPs (use your VPC CIDR block in production)
+real_ip_header X-Forwarded-For;
+real_ip_recursive on;
+
+log_format main '$remote_addr - $remote_user [$time_local] "$request" '
+                '$status $body_bytes_sent "$http_referer" '
+                '"$http_user_agent" '
+                'host=$host x-forwarded-for=$http_x_forwarded_for';
+
+access_log /var/log/nginx/access.log main;
+
+```
 
 ## Kubernetes v1.22 Migration
 

docs/user-guide/miscellaneous.md

@@ -17,7 +17,7 @@ By default NGINX path type is Prefix to not break existing definitions
 
 ## Proxy Protocol
 
-If you are using a L4 proxy to forward the traffic to the NGINX pods and terminate HTTP/HTTPS there, you will lose the remote endpoint's IP address. To prevent this you could use the [Proxy Protocol](http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt) for forwarding traffic, this will send the connection details before forwarding the actual TCP connection itself.
+If you are using a L4 proxy to forward the traffic to the Ingress NGINX pods and terminate HTTP/HTTPS there, you will lose the remote endpoint's IP address. To prevent this you could use the [PROXY Protocol](http://www.haproxy.org/download/1.5/doc/proxy-protocol.txt) for forwarding traffic, this will send the connection details before forwarding the actual TCP connection itself.
 
 Amongst others [ELBs in AWS](http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/enable-proxy-protocol.html) and [HAProxy](http://www.haproxy.org/) support Proxy Protocol.