gateway: Reject endpoint pointing to local gateway (#4310)

Show an error when the user enters an endpoint url pointing
to the gateway server itself.

cmd/endpoint.go

@@ -127,14 +127,10 @@ func NewEndpoint(arg string) (Endpoint, error) {
 			return Endpoint{}, fmt.Errorf("empty or root path is not supported in URL endpoint")
 		}
 
-		// Get IPv4 address of the host.
-		hostIPs, err := getHostIP4(host)
+		isLocal, err = isLocalHost(host)
 		if err != nil {
 			return Endpoint{}, err
 		}
-
-		// If intersection of two IP sets is not empty, then the host is local host.
-		isLocal = !localIP4.Intersection(hostIPs).IsEmpty()
 	} else {
 		u = &url.URL{Path: path.Clean(arg)}
 		isLocal = true