Hanging cURL

I have just come across a nasty issue that eventually turned out to be caused by cURL, on Cygwin.

curl http://localhost:18182/....

hangs. Forever.
Turning verbosity on with --ascii-trace this is the output of the happy path:

* About to connect() to localhost port 18182 (#0)
*   Trying ::1...
* Timeout
*   Trying 127.0.0.1...
* connected
* Connected to localhost (127.0.0.1) port 18182 (#0)

When it hangs, it does it when Trying ::1...

With the little help from RawCap I noticed that the TCP stack never receives the ACK when in SYN SENT state (see here for a picture). And cURL sometimes isn't forcibly timing out.

By specifying --connect-timeout the problem is solved.