Support for proxies and load balancers

StackExchange.Profiling/IpAddressProvider.cs

@@ -1,4 +1,5 @@
-using System.Web;
+using System;
+using System.Web;
 
 namespace StackExchange.Profiling
 {
@@ -9,10 +10,22 @@ public class IpAddressIdentity : IUserProvider
     {
         /// <summary>
         /// Returns the paramter HttpRequest's client ip address.
+        /// We combine both the REMOTE_ADDR header (which is the connecting device's IP address),
+        /// plus the HTTP_X_FORWARDED_FOR header if present (which is set by some proxy
+        /// servers and load balancers). This allows us to have a unique per-user view, even
+        /// when behind a proxy or load balancer.
         /// </summary>
         public string GetUser(HttpRequest request)
         {
-            return request.ServerVariables["REMOTE_ADDR"] ?? "";
+            // If there's no X_FORWARDED_FOR header, just return REMOTE_ADDR
+            if (String.IsNullOrWhiteSpace(request.ServerVariables["HTTP_X_FORWARDED_FOR"]))
+            {
+                return request.ServerVariables["REMOTE_ADDR"] ?? "";
+            }
+
+            // Otherwise return the concatenation of the REMOTE_ADDR and the X_FORWARDED_FOR header
+            return string.Format("{0} - {1}", request.ServerVariables["REMOTE_ADDR"] ?? "",
+                                              request.ServerVariables["HTTP_X_FORWARDED_FOR"]);
         }
     }
 }