Requests to URL with length greater than 3776 characters return with status code 400

Petur 1

Hi,

whenever our request URL exceeds the length of 3776 characters, the Application Gateway returns HTTP status code 400 (we know it's the Application Gateway returning since the same requests return fine when not going through the Application Gateway). According to answer to this question, it should currently support up to 8k characters. Is there a settings we need to change to support these long URLs? Tried both versions of the Application Gateway, same result. Diagnostic logs of the Application Gateway don't show why the result was 400, only that it was 400.

Thanks.

GitaraniSharma-MSFT 47,931 Reputation points Microsoft Employee

2020-07-16T14:45:46.73+00:00

Hello @Petur-0337 ,

As per the updated limits, Azure application gateway supports a maximum URL length of 32KB. And this 32 KB includes URl, query string, request headers, cookies, etc. You may need to check if something else is high. Are you using HTTP/2? What about total request header size?
GitaraniSharma-MSFT 47,931 Reputation points Microsoft Employee

2020-07-20T08:12:20.24+00:00

Hello @Petur ,

Any update on this post?

Thanks,
Gita
Petur 1 Reputation point

2020-07-20T18:38:17.77+00:00

Hello @GitaraniSharmaMSFT-4262

We tried using HTTP/2, didn't help. As for the request header size - using method from answer on this post, curl (-w "%{size_request} %{size_upload}") says 5826 (bytes/chars?). Don't know, if there's any other way of measuring the total request header size.

For now, we deployed the application and application gateway as is, meaning there are some filters currently not working thanks to the limitations on the AppGw, which is not really ideal.
Saroj Kumar Pradhan 1 Reputation point

2020-07-21T05:22:54.923+00:00

Application Gateway returning 400 seems a little strange to me. Even if AAG tried to block a request, the response code should be 403 (Forbidden) in this case and AAG log would clearly show what rule was activated then. Are you sure 400 is not returned from any of the backend pool?
Petur 1 Reputation point

2020-07-22T05:58:15.997+00:00

This is a log entry from when AppGw blocks the request. (I redacted some properties but the requestQuery = "..." is what's actually in the log)

I had to upload the json into 3rd-party because this portal would let me submit this comment with it - https://pastebin.com/JYV5LN0f

In AppGw v2, the log entry contains the full query.

I mean, I'm quite sure it's the AppGw returning the 400. The same query works fine when sent directly to the application without AppGw. And all backend pools are displayed as healthy in the AppGw.

Small info about architecture - the application is located in AKS with internal load balancer, AppGw directs traffic there. Backend pool's target IP is the same as when I access the application directly without AppGw to try and test whether the request works without AppGw.

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Requests to URL with length greater than 3776 characters return with status code 400