Added UpstreamHost placeholder to identify host from which request origination

src/Ocelot/Infrastructure/Placeholders.cs

@@ -7,6 +7,7 @@ namespace Ocelot.Infrastructure
     using Ocelot.Responses;
     using System;
     using System.Collections.Generic;
+    using System.Linq;
 
     public class Placeholders : IPlaceholders
     {
@@ -25,7 +26,8 @@ namespace Ocelot.Infrastructure
             {
                 { "{BaseUrl}", GetBaseUrl() },
                 { "{TraceId}", GetTraceId() },
-                { "{RemoteIpAddress}", GetRemoteIpAddress() }
+                { "{RemoteIpAddress}", GetRemoteIpAddress() },
+                { "{UpstreamHost}", GetUpstreamHost() },
             };
 
             _requestPlaceholders = new Dictionary<string, Func<DownstreamRequest, string>>
@@ -130,5 +132,27 @@ namespace Ocelot.Infrastructure
         {
             return () => new OkResponse<string>(_finder.Find());
         }
+
+        private Func<Response<string>> GetUpstreamHost()
+        {
+            return () =>
+            {
+                try
+                {
+                    if (_httpContextAccessor.HttpContext.Request.Headers.TryGetValue("Host", out var upstreamHost))
+                    {
+                        return new OkResponse<string>(upstreamHost.First());
+                    }
+                    else
+                    {
+                        return new ErrorResponse<string>(new CouldNotFindPlaceholderError("{UpstreamHost}"));
+                    }
+                }
+                catch
+                {
+                    return new ErrorResponse<string>(new CouldNotFindPlaceholderError("{UpstreamHost}"));
+                }
+            };
+        }
     }
 }

test/Ocelot.UnitTests/Infrastructure/PlaceholdersTests.cs

@@ -123,5 +123,16 @@ namespace Ocelot.UnitTests.Infrastructure
             result.IsError.ShouldBeTrue();
             result.Errors[0].Message.ShouldBe("Unable to remove placeholder: {Test}, placeholder does not exists");
         }
+
+        [Fact]
+        public void should_return_upstreamHost()
+        {
+            var upstreamHost = "UpstreamHostA";
+            var httpContext = new DefaultHttpContext();
+            httpContext.Request.Headers.Add("Host", upstreamHost);
+            _accessor.Setup(x => x.HttpContext).Returns(httpContext);
+            var result = _placeholders.Get("{UpstreamHost}");
+            result.Data.ShouldBe(upstreamHost);
+        }
     }
 }