I'm trying to programmatically submit a form on a web site that I do not own. I'm trying to simulate what I would manually do with a web browser. I am issuing an HTTP POST request using an HTTP library.
For a reason that I don't know I am getting a different result (an error, a different response, ...) when I programmatically submit the form compared to a manual submission in a web browser.
How can that be and how can I find out what mistake I have made?
This question is intentionally language and library agnostic. I'm asking for the general procedure for debugging such issues.
All instances of this problem are equivalent. Here is how to resolve all of them:
The web site you are posting to cannot tell different clients apart. It cannot find out whether you are using a web browser or an HTTP library. Therefore, only what you send matters for the decision of the server on how to react.
If you observe different responses from the server this means that you are sending different requests.
A few important things that you probably have to send correctly:
- URL
- Verb (
GET or POST)
- Headers:
Host, User-Agent, Content-Length
- Cookies (the
Cookie and Set-Cookie headers)
- The request body
Use an HTTP sniffer like Fiddler to capture what you are programmatically sending and what your browser is sending. Compare the requests for differences. Eliminate the differences one by one to see which one caused the problem. You can drag an HTTP request into the Composer Window to be able to modify and reissue it.
It is impossible to still get a different result if you truly have eliminated all differences between the manual and the programmatic requests.
