Karl M added the CURLOPT_CONNECT_ONLY and CURLINFO_LASTSOCKET options that

an app can use to let libcurl only connect to a remote host and then extract
the socket from libcurl. libcurl will then not attempt to do any transfer at
all after the connect is done.

lib/urldata.h

@@ -856,6 +856,7 @@ struct UrlState {
      set, it holds an allocated connection. */
   struct connectdata **connects;
   long numconnects; /* size of the 'connects' array */
+  int lastconnect;  /* index of most recent connect or -1 if undefined */
 
   char *headerbuff; /* allocated buffer to store headers in */
   size_t headersize;   /* size of the allocation */
@@ -1083,6 +1084,7 @@ struct UserDefined {
   bool ignorecl;         /* ignore content length */
   bool ftp_skip_ip;      /* skip the IP address the FTP server passes on to
                             us */
+  bool connect_only;     /* make connection, let application use the socket */
 };
 
 /*