Yesterday, when I deployed the server for my PocketPC-Application to an environment running lighttpd and PHP with FastCGI SAPI, I found out that the communication between the device and the server didn’t work.
All I got on the client was an Exception because the server sent back error 417: Precondition failed.
Of course there was nothing in lighttpd’s error log, which made this a job for
The response from the server had no body explaining what was going on, but in the request-header, something interesting was going on:
Additionally, the request body was empty.
It looks like HttpWebRequest, with the help of the compact framework’s ServicePointManager is doing something really intelligent which lighttpd doesn’t support:
By first sending the POST request with an empty body and that Expect: 100-continue-header, HttpWebRequest basically gives the server the chance to do some checks based on the request header (like: Is the client authorized to access the URL? Is there a resource available at that URL?) without the client having to transmit the whole request body first (which can be quite big).
The idea is that the server does the checks based on the header and then either sends a error response (like 401, 403 or 404) or it advises the client to go ahead and send the request body (code 100).
Lighttpd doesn’t support this, so it sends that 417 error back.
The fix is to set Expect100Continue of System.Net.ServicePointManager to false before getting a HttpWebRequest instance.
That way, the .NET Framework goes back to plain old POST and sends the complete request body.
In my case that’s no big disadvantage because if the server is actually reachable, the requested URL is guaranteed to be there and ready to accept the data on HTTP-level (of course there may be some errors on the application level, but there has to be a request body for them to be detected).