MapServer makes use of libcurl.dll to connect to web resources. The reason I’ve had to learn this is because libcurl.dll is vital when using MapServer as a WMS client, or “a cascading WMS server” – using MapServer to connect to another WMS server and then “forward” them as if they were any other map layer.
libcurl.dll is the library version of the command line tool cURL – a tool written in C (hence the name) that:
is a command line tool for transferring data with URL syntax, supporting FTP, FTPS, HTTP, HTTPS…curl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading…
..libcurl is most probably the most portable, most powerful and most often used C-based multi-platform file transfer library on this planet – be it open source or commercial. – http://curl.haxx.se/