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IIS stopped serving static content over a certain (small) size

Super User Asked by K. Meke on January 17, 2021

I’m really hoping somebody can help apply some sanity to this IIS issue that has been driving me crazy. I seem to be experiencing the same problem as the poor soul in this thread: https://forums.iis.net/t/1240175.aspx.

OS:
Windows 10 Pro 64-bit 2004 19041.508

Problem:
Basically, IIS will not serve static content over a certain arbitrary size – static files over that size will just timeout eventually. Non-static content, for example asp files do not have this limit and load as expected regardless of size. This started happening seemingly out of nowhere with no directly related IIS/system/network/permission changes.

Things I’ve Tried:

  • Yes, static content is turned on!
  • Same thing, every browser
  • Completely virus/malware free
  • Disabled ad blocker/anti-virus/firewall/etc.
  • No proxies (except when Fiddler is running)
  • Completely removed IIS (disabled feature, moved inetpub and windowssystem32inetsrv) and re-enabled feature which recreated them)
  • Checked logs, enabled tracing
  • Checked permissions, toggled app user between IUSR and domain user

Details:

About a 1 or 1.5 months ago my local IIS stopped working, out of the blue. By out of the blue I mean the the previous day it was working fine as always and the next it stopped – without having done any system changes whatsoever. By stopped working I mean (eventually realized) that it was only static files that were not serving correctly. Unfortunately/fortunately, I noticed the problem because I had created the obligatory personal COVID tracker. It was html/js running against a .NET API back end. The pages stopped loading and I spent time wondering what I had borked in the code (which I also hadn’t touched).
Eventually I realized no static content – images, css, js, html, txt etc. – was loading properly. I ran some tests and I found that there was a cutoff point in size of content that would load, but it is somewhat arbitrary. For example, the iisstart.png image in the default IIS website does not load – it’s 97K. I replaced it with an 8k file that loads fine. I could get about 1500 bytes served in an html file – 1501 bytes would send it off the rails but 1500 worked. Same sort of thing was found for text files although I was able to send more than 1500 bytes – don’t remember how much but did find a point where a 1 byte difference caused a problem.

Test Load of Default IIS Website:

IIS log is not helpful – it reports the failed attempt as a 200 but says it took 19 seconds. It is returning a win32 status code of 121 though:

#Fields: date time s-ip cs-method cs-uri-stem cs-uri-query s-port cs-username c-ip cs(User-Agent) cs(Referer) sc-status sc-substatus sc-win32-status time-taken
2020-10-23 13:36:12 127.0.0.1 GET /iisstart.png - 80 - 127.0.0.1 Mozilla/5.0+(Windows+NT+10.0;+Win64;+x64;+rv:82.0)+Gecko/20100101+Firefox/82.0 http://localhost/ 200 0 121 18944

The tracing mostly doesn’t fire I guess because the 200 is eventually returned?

Fiddler gives me this for the image request:

[Fiddler] ReadResponse() failed: The server did not return a complete response for this request. Server returned 0 bytes.       

For this site the actual html loads fine – it’s only 696 bytes – the image it references does not though (97K).

If anybody can shed some light on this or give me a direction in troubleshooting I would greatly appreciate it.

Thanks

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