Wednesday, January 06, 2016

Configuring ASP.NET applications in IIS to accept requests with long URLs

When creating a RESTful web service with a GET method accepts a variable-length list of parameters, it can happen that URLs generated to call the service – including the query string containing the paremters – can end up being very long. 

I’m working on a REST method with a parameter that accepts a comma-delimited list of up to 2000 ID values.  With ID values being up to 7 characters in length, the URL for a request with the maximum 2000 7-character ID values, plus an 8th comma separator character after each ID value, ends up being 16000+ charaters long.  For example:,1000002,100003,…many more IDs here!…,1001999,1002000

After running into multiple obstacles trying to get my ASP.NET application running on IIS to successfully accept such incoming long request URLs without throwing errors, I’ve come to the conclusion that in situations like this, it’s better to have the API method accept HTTP POST instead of HTTP GET, and have the client pass the list of parameters in the message body intead of in the URL. This approach aligns with answers on Stackoverflow to the question Design RESTful GET API with a long list of query parameters.

However, I figured I’d go ahead and post the data I collected while troubleshooting the various errors that ASP.NET and IIS can return when a request with a long URL is submitted, in case I need this information again in the future, or in case it might help anyone else.

In all cases, the specific configured values (e.g. ”65535”) should be tailored to the specific needs of your application. Be aware that setting these values could have adverse security consequences for your application, as a large HTTP request submitted by an attacker won’t be rejected early in the pipeline as it would normally.

All of this investigation was done with an ASP.NET application targeting .NET Framework 4.5.1, running on IIS 10, on a Windows 10 64-bit PC.

Symptom 1

Response HTTP status code: HTTP 414

Response body: HTTP Error 414. The request URL is too long.

Relevant response header: Server: Microsoft-HTTPAPI/2.0

Fix 1

In the Windows Registry, at Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\HTTP\Parameters, create a DWORD-type value with name MaxFieldLength and value sufficiently large, e.g. 65535.

Note: This error is actually thrown by http.sys, before the request even gets passed along to IIS in the request-handling pipeline.  Thus, web.config settings aren’t able to address this particluar error.  See article Check the “Server” HTTP header for the source of an error returned from IIS.

If you decide to make this change, then obviously it’ll need to be made in all environments (including all production server(s)) -- not just on your local dev PC.  Also, whatever script and/or documentation your team uses to set up new server instances will need to be updated to include this registry setting, so that your team doesn’t forget to apply this setting 18 months from now when setting up a new production server. (This was a big reason that for the API I’m building, I opted to just scrap the long-GET-URL approach, and make my method a POST instead.)



Symptom 2

Response HTTP status code: HTTP 404

Response body: (Empty)

Relevant response headers:

Server: Microsoft-IIS/10.0
X-Powered-By: ASP.NET

Fix 2

In web.config, add the following configuration (modifying existing elements when they are present, otherwise adding new elements):

      <requestLimits maxQueryString="65535" /> 


Note: In my ASP.NET solution, I needed to make this change in my root project’s web config. This setting was ignored when I added the change in my API sub-project’s web.config.  (This however was not the case for the web.config change mentioned in “Fix 3” below.)  Related MSDN article: ASP.NET Configuration File Hierarchy and Inheritance


Symptom 3

Response HTTP status code: HTTP 400

Relevant response body text snippets:

System.Web.HttpException: The length of the query string for this request exceeds the configured maxQueryStringLength value.

[HttpException (0x80004005): The length of the query string for this request exceeds the configured maxQueryStringLength value.] System.Web.HttpRequest.ValidateInputIfRequiredByConfig() +492 System.Web.PipelineStepManager.ValidateHelper(HttpContext context) +55

Relevant response headers:

Server: Microsoft-IIS/10.0
X-AspNet-Version: 4.0.30319
X-Powered-By: ASP.NET

Fix 3

In web.config, add the following configuration (modifying existing elements when they are present, otherwise adding new elements):

  <httpRuntime maxRequestLength="65535" maxUrlLength="65535" maxQueryStringLength="65535" />

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