AspNet Request-Ip with support for CheckForwardedForHeaderOffset

src/Shared/LayoutRenderers/AspNetRequestIpLayoutRenderer.cs

@@ -1,7 +1,5 @@
-using System;
-using System.ComponentModel;
+using System.ComponentModel;
 using System.Text;
-using NLog.Config;
 using NLog.LayoutRenderers;
 using NLog.Layouts;
 using NLog.Web.Internal;
@@ -17,7 +15,11 @@ namespace NLog.Web.LayoutRenderers
     /// ASP.NET Request IP address of the remote client
     /// </summary>
     /// <remarks>
-    /// <code>${aspnet-request-ip}</code>
+    /// <code>${aspnet-request-ip}</code> to return the Remote IP
+    /// <code>${aspnet-request-ip:CheckForwardedForHeader=true}</code> to return first element in the X-Forwarded-For header
+    /// <code>${aspnet-request-ip:CheckForwardedForHeader=true:Index=1}</code> to return second element in the X-Forwarded-For header
+    /// <code>${aspnet-request-ip:CheckForwardedForHeader=true:Index=-1}</code> to return last element in the X-Forwarded-For header
+    /// <code>${aspnet-request-ip:CheckForwardedForHeader=true:Index=1:ForwardedForHeader=myHeader}</code> to return second element in the myHeader header
     /// </remarks>
     /// <seealso href="https://github.com/NLog/NLog/wiki/AspNet-Request-IP-Layout-Renderer">Documentation on NLog Wiki</seealso>
     [LayoutRenderer("aspnet-request-ip")]
@@ -35,6 +37,15 @@ public class AspNetRequestIpLayoutRenderer : AspNetLayoutRendererBase
         /// <docgen category='Rendering Options' order='10' />
         public bool CheckForwardedForHeader { get; set; }
 
+        /// <summary>
+        /// Gets or sets the array index of the X-Forwarded-For header to use, if the desired client IP is not at
+        /// the zeroth index.  Defaults to zero.  If the index is too large the last array element is returned instead.
+        /// If a negative index is used, this is used as the position from the end of the array.
+        /// Minus one will indicate the last element in the array.  If the negative index is too large the first index
+        /// of the array is returned instead.
+        /// </summary>
+        public int Index { get; set; } = 0;
+
         /// <inheritdoc/>
         protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
         {
@@ -60,6 +71,25 @@ protected override void DoAppend(StringBuilder builder, LogEventInfo logEvent)
             builder.Append(ip);
         }
 
+        private int CalculatePosition(string[] headerContents)
+        {
+            var position = Index;
+
+            if (position < 0)
+            {
+                position = headerContents.Length + position;
+            }
+            if (position < 0)
+            {
+                position = 0;
+            }
+            if (position >= headerContents.Length)
+            {
+                position = headerContents.Length - 1;
+            }
+            return position;
+        }
+
 #if !ASP_NET_CORE
         string TryLookupForwardHeader(HttpRequestBase httpRequest, LogEventInfo logEvent)
         {
@@ -71,7 +101,8 @@ string TryLookupForwardHeader(HttpRequestBase httpRequest, LogEventInfo logEvent
                 var addresses = forwardedHeader.Split(',');
                 if (addresses.Length > 0)
                 {
-                    return addresses[0];
+                    var position = CalculatePosition(addresses);
+                    return addresses[position]?.Trim();
                 }
             }
 
@@ -86,7 +117,8 @@ private string TryLookupForwardHeader(HttpRequest httpRequest, LogEventInfo logE
                 var forwardedHeaders = httpRequest.Headers.GetCommaSeparatedValues(headerName);
                 if (forwardedHeaders.Length > 0)
                 {
-                    return forwardedHeaders[0];
+                    var position = CalculatePosition(forwardedHeaders);
+                    return forwardedHeaders[position]?.Trim();
                 }
             }
 

tests/Shared/LayoutRenderers/AspNetRequestIpLayoutRendererTests.cs

@@ -116,5 +116,121 @@ public void ForwardedForHeaderPresentWithCustomRenderForwardedValue()
             // Assert
             Assert.Equal("127.0.0.1", result);
         }
+
+        [Fact]
+        public void ForwardedForHeaderContainsMultipleEntriesRendersIndexValue()
+        {
+            // Arrange
+            var (renderer, httpContext) = CreateWithHttpContext();
+
+#if !ASP_NET_CORE
+            httpContext.Request.ServerVariables.Returns(new NameValueCollection {{"REMOTE_ADDR", "192.0.0.0"}});
+            httpContext.Request.Headers.Returns(
+                new NameValueCollection {{ForwardedForHeader, "192.168.1.1, 127.0.0.1"}});
+#else
+            var headers = new HeaderDict();
+            headers.Add(ForwardedForHeader, new StringValues("192.168.1.1, 127.0.0.1"));
+            httpContext.Request.Headers.Returns(callinfo => headers);
+#endif
+            renderer.CheckForwardedForHeader = true;
+            renderer.Index = 1;
+
+            // Act
+            string result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+        }
+
+        [Fact]
+        public void ForwardedForHeaderContainsMultipleEntriesRendersLastValue()
+        {
+            // Arrange
+            var (renderer, httpContext) = CreateWithHttpContext();
+
+#if !ASP_NET_CORE
+            httpContext.Request.ServerVariables.Returns(new NameValueCollection {{"REMOTE_ADDR", "192.0.0.0"}});
+            httpContext.Request.Headers.Returns(
+                new NameValueCollection {{ForwardedForHeader, "192.168.1.1, 127.0.0.1"}});
+#else
+            var headers = new HeaderDict();
+            headers.Add(ForwardedForHeader, new StringValues("192.168.1.1, 127.0.0.1"));
+            httpContext.Request.Headers.Returns(callinfo => headers);
+#endif
+            renderer.CheckForwardedForHeader = true;
+            renderer.Index = -1;
+
+            // Act
+            string result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+        }
+
+        [Fact]
+        public void ForwardedForHeaderContainsMultipleEntriesExcessiveIndexRendersLastValue()
+        {
+            // Arrange
+            var (renderer, httpContext) = CreateWithHttpContext();
+
+#if !ASP_NET_CORE
+            httpContext.Request.ServerVariables.Returns(new NameValueCollection {{"REMOTE_ADDR", "192.0.0.0"}});
+            httpContext.Request.Headers.Returns(
+                new NameValueCollection {{ForwardedForHeader, "192.168.1.1, 127.0.0.1"}});
+#else
+            var headers = new HeaderDict();
+            headers.Add(ForwardedForHeader, new StringValues("192.168.1.1, 127.0.0.1"));
+            httpContext.Request.Headers.Returns(callinfo => headers);
+#endif
+            renderer.CheckForwardedForHeader = true;
+            renderer.Index = 2;
+
+            // Act
+            string result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+
+            renderer.Index = 3;
+
+            // Act
+            result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+        }
+
+        [Fact]
+        public void ForwardedForHeaderContainsMultipleEntriesExcessiveNegativeIndexRendersFirstValue()
+        {
+            // Arrange
+            var (renderer, httpContext) = CreateWithHttpContext();
+
+#if !ASP_NET_CORE
+            httpContext.Request.ServerVariables.Returns(new NameValueCollection {{"REMOTE_ADDR", "192.0.0.0"}});
+            httpContext.Request.Headers.Returns(
+                new NameValueCollection {{ForwardedForHeader, "127.0.0.1, 192.168.1.1"}});
+#else
+            var headers = new HeaderDict();
+            headers.Add(ForwardedForHeader, new StringValues("127.0.0.1, 192.168.1.1"));
+            httpContext.Request.Headers.Returns(callinfo => headers);
+#endif
+            renderer.CheckForwardedForHeader = true;
+            renderer.Index = -3;
+
+            // Act
+            string result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+
+            renderer.Index = -4;
+
+            // Act
+            result = renderer.Render(new LogEventInfo());
+
+            // Assert
+            Assert.Equal("127.0.0.1", result);
+        }
     }
 }