In the Server 2008 R2 IIS Manager UI Help file the information about the "Maximum allowed content length (Bytes)" setting is very wrong.
I have not checked if this bug is also present in "Windows Server 2012". It does apply to "Windows Server 2008 R2" with all IIS and core related updates as of today.
Specifically, open IIS Manager, go to your server node, click the "Request filtering" icon, on the "File name extensions" tab select "Edit feature settings...", then click the "?" icon in the dialog box title bar.
The help page that opens contains the following misleading and outright wrong text:
Maximum allowed content length (Bytes)
Specifies the maximum length, in bytes, for content.
Note
Specifying zero (0) for this value means that the length is unlimited.
This has 2 serious issues:
- Nothing in the description says that this is a limit on the "POST" (etc.) request payload, not the returned web site content, leading to the impression that this setting needs to be larger to allow download of big files (such as .iso images). This is just misleading however though clearly should be clarified when changing the text anyway because of the next issue.
- Specifying 0 apparently does not have the claimed effect, but seemingly causes IIS to refuse all POSTs etc. with more than 0 bytes of request data. This is a serious bug as it causes the server to appear correctly configured and up while all real requests are being denied.
I would suggest that Microsoft applies both the following fixes:
- Publish a knowledge base article warning administrators that the 0 setting has an effect different from what was previously documented, this is needed to protect administrators who read the help file before it was fixed and remember what they read.
- Include a corrected help file in a regular patch tuesday update. Fortunately replacing/patching the help file should not require a server reboot, since it is not usually kept locked by any running services.