https://issues.dlang.org/show_bug.cgi?id=20120
--- Comment #4 from Rainer Schuetze <r.sagita...@gmx.de> --- bisecting curl shows this commit as the offender: https://github.com/curl/curl/commit/72d5e144fbc6a9db264ae425bb788af218f25d9e defining DONT_USE_RECV_BEFORE_SEND_WORKAROUND when compiling curl lets the std.net.curl unittests pass. Not sure who's to blame, though. >From the discussions in the referenced issues, the curl developers were not too convinced about the patch. Maybe it is also Windows version specific. --