Build process *must* not depend on the Internet access. You must be able
to preliminary download all necessary source code needed for the skel.
And as in most package build systems, that retrievable source code is
called "distfile".

There are no requirements how you will get it. But in most cases that is
either downloadable tarball or archive created from specific VCS commit.
Modern DVCS'es commit is already self integrity checked tree of files.
But tarball is just a bunch of bytes. You have to provide the URL where
to get it, and checksum(s) to verify against, to be sure that it is not
tampered or altered somehow. Moreover many distribution sites also
provide a detached cryptographic signature of the tarball, which you can
verify against some know author's public key.

That is why
=> Metalink4
files are used to get tarballs. They also tend to contain checksum hash
present on the download website.

You can download all distfiles by invoking "redo distfiles/all"
target. An archive with all of them can be created with
"distfiles/pack >distfiles.tar" command.

.meta4 files can be processed by either of three programs:

* $META4_FETCHER=meta4ra-check
  Use "meta4ra-check -dl 0" command to download the first URL. This is
  by default, because meta4ra utilities are anyway installed already. It
  won't try to download other URLs, unlike other fetch options there!

* $META4_FETCHER=wget
  Use wget compiled with --with-metalink option. The only drawback is
  that most OS distributions contain Wget without that (--input-metalink)
  option.

* $META4_FETCHER=aria2c
  => Aria2
  Unfortunately it sometimes fails to deal with links on GitHub.com.