CODEWITHSUNDEEP
pythonrestnginxpostmanhttp-status-code-400
Matthew Moisen
Matthew Moisen

Reputation: 18299

How to intentionally cause a 400 Bad Request in Nginx?

I've split my previous question into two.

A consumer of my REST API says that on occasion I am returning a 400 Bad Request - The request sent by the client was syntactically incorrect. error.

My application (Python/Flask) logs don't seem to be capturing this, and neither do my webserver/Nginx logs.

I've changed the default 400 and 404 error message HTML's by adding the following line to the config (and adding the corresponding HTML pages in the proper directory):

    error_page  404              /404.html;
    location = /404.html {
        root   html;
    }
    error_page  400              /400.html;
    location = /400.html {
        root   html;
    }

This error message plainly states "Nginx" so that I know Nginx is giving the 400 and not my python/flask application (whose 400 has been changed to plainly state "Flask").

Is there a way that I can intentionally cause Nginx to return a 400 error, perhaps through python, CURL, or Postman? For example, I've also changed the 404 error page, and can intentionally get Nginx to return the corresponding error HTML by calling an invalid URL.

Upvotes: 1

Views: 5809

Answers (2)

Naftuli Kay
Naftuli Kay

Reputation: 91820

Create a location in NGINX's config to return 400 errors:

location /error { 
    return 400;
}

Presto.

Upvotes: 1

Joe Young
Joe Young

Reputation: 5895

Nginx will return status code 400 if it receives a header field larger than the configured large_client_header_buffers

A request header field cannot exceed the size of one buffer as well, or the 400 (Bad Request) error is returned to the client. Buffers are allocated only on demand. By default, the buffer size is equal to 8K bytes. If after the end of request processing a connection is transitioned into the keep-alive state, these buffers are released.

Source: http://nginx.org/en/docs/http/ngx_http_core_module.html#large_client_header_buffers

So you just need to create a curl request with a header larger than 8k. Here's an example using a bit of python to generate the header variable to pass into curl:

(nginx)macbook:nginx joeyoung$ myheader=$(python -c "print 'A'*9000")
(nginx)macbook:nginx joeyoung$ curl -vvv --header "X-MyHeader: $myheader" http://my.example.website.com

Results:

...
> 
< HTTP/1.1 400 Bad Request
< Server: nginx/1.4.7
< Date: Wed, 02 Sep 2015 22:37:29 GMT
< Content-Type: text/html
< Content-Length: 248
< Connection: close

Upvotes: 1

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