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A general description is here.
Provisions for the optional use of ‘libcurl’ (version 7.28.0 or later) for Internet access (including on Windows):
• ‘capabilities("libcurl")’ reports if this is available.
• ‘libcurlVersion()’ reports the version in use, and other details of the ‘"libcurl"’ build including which URL schemes it supports.
• ‘curlGetHeaders()’ retrieves the headers for ‘http://’, ‘https://’, ‘ftp://’ and ‘ftps://’ URLs: analysis of these headers can provide insights into the `existence' of a URL (it might for example be permanently redirected) and will be so used in ‘R CMD check’.
• ‘download.file()’ has a new optional method ‘"libcurl"’ which will handle more URL schemes, follow redirections, and allows simultaneous downloads of multiple URLs.
• ‘url()’ has a new method ‘"libcurl"’. The default method is controlled by a new option ‘url.method’, and applies also to the opening of URLs _via_ ‘file()’ (which happens implicitly in functions such as ‘read.table’.)
• When ‘file()’ or ‘url()’ is invoked with a ‘https://’ or ‘ftps://’ URL which the current method cannot handle, a suitable method is selected if one is available.