When requesting data using POST from a certain server (IIS), then
randomly there is no response, not even after 40 seconds
(CURLOPT_TIMEOUT = 40, CURLOPT_CONNECTTIMEOUT = 40).

This *never* happened to me when doing the same request by using a web
browser (filling out a form and submitting it) or with Firefox's Poster
plugin. Unless when using cURL, the server responds in a couple of
seconds at most.

Any idea what could be the source of the problem? Any debugging
suggestions?

The problem occurs with cURL versions:

* 7.21.6 (Ubuntu 11.10)

* 7.19.7 (Amazon LINUX AMI on EC2)

Example server response header:

  HTTP/1.1 200 OK
  Date: Mon, 06 Feb 2012 15:27:57 GMT
  Server: Microsoft-IIS/6.0
  X-Powered-By: ASP.NET
  X-AspNet-Version: 4.0.30319
  Cache-Control: private, max-age=0
  Content-Type: application/json; charset=utf-8
  Content-Length: 110471

Log of a request without response (CURLOPT_VERBOSE = 1, anonymized):

  Feb 06 17:45:36 Data type: 0
  Feb 06 17:45:36 Data (64): Re-using existing connection! (#0) with
host example.com

  Feb 06 17:45:36 Data type: 0
  Feb 06 17:45:36 Data (61): Connected to example.com (1.2.3.4) port 80 (#0)

  Feb 06 17:45:36 Data type: 2
  Feb 06 17:45:36 Data (299): POST /XYZ.NET/Search.aspx/GetInfo HTTP/1.1
  Host: example.com
  Accept: */*
  Cookie: ASP.NET_SessionId=fj42wfbhs5si4hlfcs5pc3ve;
SOMETHING=2712629952.50480.0000
  Accept-Encoding: deflate,gzip
  Content-Type: application/json; charset=utf-8
  Content-Length: 123

  Feb 06 17:45:36 Data type: 4
  Feb 06 17:45:36 Data (269):
{"request":{"x":3,"y":4},"sessionKey":"o4or2764-0s6p-8923-nn98-p4p94p60s9p0"}
  Feb 06 17:46:16 Data type: 0
  Feb 06 17:46:16 Data (77): Operation timed out after 40042
milliseconds with 0 out of -1 bytes received

  Feb 06 17:46:16 Data type: 0
  Feb 06 17:46:16 Data (22): Closing connection #0
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Received on 2012-02-06