IIS application missing Content-Encoding - gzip in Response Header

Question

In Firebug the request header has the following entry:
Accept-Encoding: gzip, deflate

But there\'s no:
Content-Encoding: gzip

Answer 1

I am using IIS10 and my web.config has

When I do test requests from a browser (Firefox, IE11, Edge, Google Chrome) to a simple MVC application.

The requests all have Accept-Encoding: gzip, deflate and the responses return Content-Encoding:gzip.

I Even tested it with Fiddler. Composing the request manually

GET http://localhost/MyWebApplication HTTP/1.1
User-Agent: Fiddler
Host: localhost
Accept-Encoding: gzip, deflate

and get the same result

HTTP/1.1 200 OK
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Encoding: gzip
Vary: Accept-Encoding
Server: Microsoft-IIS/10.0
X-AspNetMvc-Version: 5.2
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET
Date: Mon, 18 Jul 2016 15:26:06 GMT
Content-Length: 3826

...

Css, Js and all other text based files are being compressed.

You may need to re-check your configuration to make sure you have the compression properly configured in IIS and your web.config.

UPDATE:

I did notice that images were not being compressed

Request

GET http://localhost/MyWebApplication/Images/Logo_small.png HTTP/1.1
User-Agent: Fiddler
Host: localhost
Accept-Encoding: gzip, deflate

Response

HTTP/1.1 200 OK
Cache-Control: max-age=604800
Content-Type: image/png
Last-Modified: Fri, 27 Nov 2015 03:15:22 GMT
Accept-Ranges: bytes
ETag: "c9d1fdd9c128d11:0"
Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET
Date: Mon, 18 Jul 2016 15:33:02 GMT
Content-Length: 2970

...

And after some google-fu found out that the images are usually already compressed so gzip was not applied.

FULL system.webServer from web.config