ignoreis set to 1, ignore the Content-Length header in the HTTP response and ignore asking for or relying on it for FTP transfers.
This is useful for HTTP with Apache 1.x (and similar servers) which will report incorrect content length for files over 2 gigabytes. If this option is used, curl will not be able to accurately report progress, and will simply stop the download when the server ends the connection.
It is also useful with FTP when for example the file is growing while the transfer is in progress which otherwise will unconditionally cause libcurl to report error.
Only use this option if strictly necessary.